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Chapter 2

Basics 59

SR865A DSP Lock-in Amplifier

The SR865A measures input signal noise AT the reference frequency. Many noise
sources have a frequency dependence which the lock-in can measure.

How Is Noise Measured?

Remember that the lock-in detects signals close to the reference frequency. How close?
Input signals within the detection bandwidth set by the low pass filter time constant and
roll-off appear at the output at a frequency f = f

sig

−

f

ref

. Input noise near f

ref

appears as

noise at the output with a bandwidth of dc to the detection bandwidth.

For Gaussian noise, the equivalent noise bandwidth (ENBW) of a low pass filter is the
bandwidth of the perfect rectangular filter which passes the same amount of noise as the
real filter.

The ENBW is determined by the time constant (T) and slope as shown below (for normal
RC type filters).

Slope

ENBW

6

dB/oct 1/(4T)

12

dB/oct

1/(8T)

18

dB/oct

3/(32T)

24

dB/oct

5/(64T)

The noise is simply the standard deviation (root of the mean of the squared deviations) of
the measured X or Y values averaged over a period of time. This involves computing the
average of (X

value

−

X

mean

)

2

where X

value

is the current X output and X

mean

is the mean X

output. The noise result is the square root of the average. The averaging time is roughly
200 time constants; for example, if the time constant is 100 mS, the noise measurement
would take about 20 seconds to settle.

If the Sync filter is enabled, that also affects the noise averaging time. The averaging time
due to the Sync filter is 100 Sync filter periods. The final noise averaging time is the
longer of these two averaging times, or 0.5 seconds.

Shorter averaging times yield a very poor estimate of the noise (the mean varies rapidly
and the deviations are not averaged well). Longer averaging times, while yielding better
results, take a long time to settle to a steady answer.

Noise and Sensitivity

Beginning with firmware version 1.20, noise is calculated using floating-point math, and
it is independent of the Sensitivity setting.

However, earlier firmware versions used data from the scaled output values of X and Y.
In order to accurately compute the noise on instruments running earlier firmware, the
Sensitivity must be set appropriately so that the displayed values of X or Y have enough
resolution. If the displayed values of X or Y are unchanging, then the computed noise
will be almost zero.

Noise Display

To display a noise measurement, choose X or Y noise (Xn or Yn) as one of the displayed
parameters in the [Config] screen. The SR865A is calculating the noise all of the time,
whether or not X or Y noise are being displayed. Thus, as soon as noise is displayed, the

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Содержание SR865A

Страница 1: ...Revision 2 04 Operation Manual SR865A 4 MHz DSP Lock in Amplifier www lambdaphoto co uk Distribution in the UK Ireland...

Страница 2: ...d to a Stanford Research Systems authorized service facility Contact Stanford Research Systems or an authorized representative before returning this product for repair Information in this document is...

Страница 3: ...e cord For 100V 120V use a 1 Amp fuse and for 220V 240V use a 1 2 Amp fuse See Appendix F page 177 for detailed fuse installation instructions Line Cord The SR865A has a detachable three wire power co...

Страница 4: ...Warning Regarding Use With Photomultipliers and Other Detectors The front end amplifier of this instrument is easily damaged if a photomultiplier is used improperly with the amplifier When left comple...

Страница 5: ...Safety and Preparation For Use iii SR865A DSP Lock in Amplifier...

Страница 6: ...iv Safety and Preparation For Use SR865A DSP Lock in Amplifier...

Страница 7: ...uts 28 Scanning 30 Chapter 2 Lock in Amplifier Basics 37 What is a Lock in Amplifier 37 What Does a Lock in Measure 40 Block diagram 41 The Reference Oscillator 42 The Phase Sensitive Detectors 43 Tim...

Страница 8: ...nds 120 FFT Screen Commands 125 Scan Commands 128 Data Transfer Commands 132 Data Capture Commands 134 Data Streaming Commands 140 System Commands 143 Interface Commands 146 Status Reporting Commands...

Страница 9: ...ms whichever is greater Phase Setting Resolution 360 232 deg Phase Noise Ext TTL reference 0 001 rms at 1 kHz 100 ms 12 dB oct typical Internal reference 0 0001 rms at 1 kHz 100 ms 12 dB oct Phase Dri...

Страница 10: ...ratioed by Aux In 4 Expand X Y and R may be expanded by 10 or 100 Capture Buffer 1 Mpoints internal data storage Store X X and Y R and or X Y R and at sample rates up to 1 25 MHz This is in addition...

Страница 11: ...Ethernet VXI 11 and telnet USB Flash Front panel slot for USB flash storage of screen shots and data and firmware upgrades Preamp Power 9 pin D connector to power SRS preamps Power 60 Watts 100 120 22...

Страница 12: ...109 Set sine output dc mode RSRC INT EXT DUAL CHOP i 109 Set reference mode RTRG SIN POSttl NEGttl i 109 Set external reference trigger REFZ 50ohms 1Meg i 109 Set external reference input impedance PS...

Страница 13: ...DAT1 DAT2 DAT3 DAT4 j OFF ON i 119 Turn data channel j strip graph off on GETSCREEN 119 Download screen capture image Strip Chart Commands page description GSPD i 120 Set horizontal scale from 0 5 s...

Страница 14: ...et the dc attenuator mode for scanning SCNINRVL i 129 Set the scan parameter update interval SCNENBL OFF ON i 129 Turn the scan off on SCNRUN 130 Start or resume the scan SCNPAUSE 130 Pause the scan S...

Страница 15: ...size to 1024 512 256 or 128 bytes 0 3 STREAMPORT i 142 Sets the Ethernet port to i 1024 65535 STREAMOPTION i 142 Sets big little endianness and integrity checking STREAM OFF ON i 142 Turn streaming o...

Страница 16: ...e standard event status byte SRE j i 148 Set the serial poll enable register STB j 148 Query the serial poll status byte PSC i 149 Set the Power On Status Clear bit ERRE j i 149 Set the error status e...

Страница 17: ...E A command cannot execute correctly or a parameter is out of range 5 CMD An illegal command is received 6 URQ Set by any user front panel action 7 PON Set by power on LIA Status Word bit name usage 0...

Страница 18: ...yte bit name usage 0 CLK External 10 MHz clock input error 1 BACKUP Battery backup failed 2 unused 3 unused 4 VXI VXI 11 error 5 GPIB GPIB fast data transfer mode aborted 6 USBDEV USB device error int...

Страница 19: ...attempting to perform an actual experiment Keys Knobs and Touch Buttons Key Front panel keys are referred to in square brackets Some keys have a second italicized label Press and hold these keys for 2...

Страница 20: ...dicated by the signal strength LEDs The sensitivity determines the scale factor for the analog outputs CH1 and CH2 as well as the numeric readouts and bar graphs The sensitivity does not affect the me...

Страница 21: ...h differential and common mode Use either sine out for single ended excitation Outputs The CH1 output can be proportional to either X or R while the CH2 output can be proportional to Y or Output funct...

Страница 22: ...ta displays as well as enter certain lock in parameters Screen Layout Press Screen Layout to cycle through the different screen layouts Trend Graph Full Screen Strip Chart Half Screen Strip Chart Full...

Страница 23: ...cy entry screen Numeric entry is straightforward Close will return to the data screen The buttons F1 F2 F3 and F4 are frequency presets Touching a preset will load the preset value immediately Touch a...

Страница 24: ...at the right edge There is no zooming in about a point in the distant past All parameters which may be assigned to a data channel are continuously recorded even when they are not displayed This means...

Страница 25: ...ragging on the screen while the vertical scale palette is displayed Each graph can also be turned off Touch the scale tile to turn the graph back on All changes to the graphs are non destructive They...

Страница 26: ...s paused Zooming in and out preserves the right hand edge of the graph at the point in time when the graph was paused Data storage continues in the background while the graph is paused When live scrol...

Страница 27: ...reference frequency The default data screen is the Trend Graph The 4 displayed parameters default to X Y R and Each parameter has a numeric display a bar graph and a trend graph The trend graph is a c...

Страница 28: ...hange the frequency to 10 kHz Use the Frequency knob to adjust the frequency to 1 00000 kHz The knob is very useful for making small adjustments or optimizing a setting Large changes are better left t...

Страница 29: ...A D converter without overload The Input Range should change to 100 mV 11 Press Input Range Down to select 30 mV Settings which have many options such as Input Range Time Constant and Sensitivity are...

Страница 30: ...re less noisy with 2 poles of filtering Press Slope twice more to select 24 dB oct With 4 poles of low pass filtering even this short time constant attenuates the 2f component reasonably well and prov...

Страница 31: ...ut sine wave from the synthesized function generator to the A input using a BNC cable and appropriate terminator The input impedance of the lock in is 10 M The generator may require a terminator Many...

Страница 32: ...dge and older data scrolls left The scroll rate is determined by the horizontal scale time per division The fastest rate is 0 5 s div and the shows 5 s of history The info bar is at the top of the scr...

Страница 33: ...ale tile Y to display its scale palette Touch to auto scale the blue Y trace The X and Y outputs are 0 2 Hz sine waves with 500 mV amplitudes 8 Touch the yellow scale tile R to display its scale palet...

Страница 34: ...ve an external monitor or TV Users who are looking at the monitor can see the lock in front panel settings Touch anywhere within the graph area to dismiss the status display The status display is dism...

Страница 35: ...n goes away 13 Touch the white scale tile Time to display the horizontal scale palette Touch repeatedly to zoom out Touch repeatedly to zoom back in Zoom out on the horizontal time scale to show more...

Страница 36: ...e vertical and horizontal scales Avgs sets the amount of averages and Live toggles to Paused Use the Cursor knob to move the cursor 16 Press Screen Layout again to change the display to the half scree...

Страница 37: ...Amplifier 18 Press Screen Layout again to cycle back to the trend graph Use Screen Layout to cycle through the various display screens This concludes the measurement example You should have a feeling...

Страница 38: ...input using a BNC cable The lock in defaults to the internal oscillator reference set at 100 000 kHz The default data screen is the Trend Graph The 4 displayed parameters default to X Y R and Each par...

Страница 39: ...reen Layout twice to show the half screen strip chart Now let s look at how the Sensitivity affects the different displays 6 Touch the green scale tile X to display its scale palette Touch to auto sca...

Страница 40: ...ar graphs Auto Scale will adjust the Sensitivity automatically 9 Press and hold the CH1 Offset knob above the CH1 BNC to display the offset keypad X Y and R may all be offset ratioed and expanded sepa...

Страница 41: ...nter This is because the offset affects the value of X unlike the sensitivity 12 Press the CH1 Offset knob briefly once to turn the X offset off The offset for CH1 can be turned on and off without cha...

Страница 42: ...V CH1 output The X graph is a line at 100 mV 2 divisions above center With offset and expand the output voltage gain and offset can be programmed to provide control of feedback signals with the proper...

Страница 43: ...trument parameters not measurement parameters are set This includes file numbering interface settings and software updates 3 Touch the h button in the Time section to highlight the hours setting Use t...

Страница 44: ...for this setup using the keypad Touch Confirm to commit the current setup to location 1 Setups are numbered 1 through 8 Setups should be named so they are easily distinguished 7 Now change the Sensiti...

Страница 45: ...l for Location 1 A summary of settings which will change upon recall is shown Simply touch Cancel to skip recalling this setup Touch Confirm to recall the setup and dismiss this screen The Sensitivity...

Страница 46: ...ocal pressed the lock in returns to its standard default settings See the Standard Settings list in the Operation section for a complete listing of the settings 2 Connect Aux Out 1 on the rear panel t...

Страница 47: ...hannels The Aux Inputs are always scaled to 10 V The Aux Inputs can read 4 analog voltages These inputs are useful for monitoring and measuring other parameters in an experiment such as pressure tempe...

Страница 48: ...mpedance of the lock in is 10 M The generator may require a terminator Many generators have either a 50 or 600 output impedance Use the appropriate feedthrough or T termination if necessary In general...

Страница 49: ...at the top is shown in orange indicating that it is under scan control At this time the frequency is the start value or 99 9900 kHz 8 Press Play Pause reset briefly don t hold it to start the scan Th...

Страница 50: ...R shows the lock in response as the internal frequency scans through the signal frequency at this time constant and filter Reduce the phase trace to see the resonance at 100 000 kHz 12 Press Config t...

Страница 51: ...nd Mode is set to repeat we see the scan repeat over and over in the history When scanning it is convenient to pause the strip chart at the end of the scan to review the results 15 Press and hold Scan...

Страница 52: ...he frequency will hold at the end value 100 010 kHz as displayed in the info bar The cursor is active when the strip chart is paused 20 Touch the white scale tile Time to display the horizontal scale...

Страница 53: ...DSP Lock in Amplifier While the scan is in progress press Scan setup briefly to turn scanning off Turn the scan off before the end and the internal frequency returns to its original value 100 000 kHz...

Страница 54: ...36 Getting Started Chapter 1 SR865A DSP Lock in Amplifier...

Страница 55: ...100 a very good filter centered at 10 kHz any signal in a 100 Hz bandwidth will be detected 10 kHz Q The noise in the filter pass band will be 50 V 5 nV Hz 100 Hz 1000 and the signal will still be 10...

Страница 56: ...dc signal In this case the filtered PSD output will be Vpsd 1 2 Vsig cos sig ref This is a very nice signal it is a dc signal proportional to the signal amplitude Narrow band detection Now suppose th...

Страница 57: ...al reference source In many situations the lock in s internal oscillator may be used instead The internal oscillator is just like a function generator with variable sine output and a TTL sync which is...

Страница 58: ...signal proportional to the component of the signal whose frequency is exactly locked to the reference frequency The low pass filter which follows the multiplier provides the averaging which removes th...

Страница 59: ...in cycles per second and math works best in radians For purposes of measurement frequencies as measured in a lock in amplifier are in Hz The equations used to explain the actual calculations are somet...

Страница 60: ...zero phase to the detected edge Internal Oscillator The internal oscillator in the SR865A is basically a 4 MHz function generator with sine and sync outputs The oscillator generates a digitally synth...

Страница 61: ...SR865A is very low and generally causes no problems In applications requiring no phase jitter the internal reference mode should be used Since there is no PLL the internal oscillator and the referenc...

Страница 62: ...the fact that the actual output is supposed to be a dc signal In fact when there is noise at the input there is noise on the output By increasing the time constant the output becomes more steady and...

Страница 63: ...output BNC The Sensitivity also sets the scale for the displayed bar graphs and numerical readouts of X Y and R Note that this is a numerical output conversion Output overloads do not affect the actua...

Страница 64: ...n the rear panel and Channel 1 and 2 CH1 and CH2 outputs on the front panel X and Y Rear Panel Outputs The X and Y rear panel outputs are the outputs from the two phase sensitive detectors with low pa...

Страница 65: ...ch is only 10 of sensitivity can be expanded to provide 10 V of output rather than only 1 V The normal use for expand is to expand the measurement resolution around some value which is not zero For ex...

Страница 66: ...value minus the offset Any display which is showing a quantity which is affected by a non unity expand will display a highlighted Expand indicator within its display Output expands affect the bar grap...

Страница 67: ...aller value increases the analog gain The signal strength LEDs on the front panel indicate how much of the A D range is being used The general rule is to decrease the Input Range as much as possible w...

Страница 68: ...Gaussian noise the peak to peak noise is about 5 times the rms noise Thus the output will have about 20 nV pk pk of noise Remember that the SR865A reports its measurements in Volts referred to the inp...

Страница 69: ...c coupled with 1 MHz bandwidth Input Connections In order to achieve the best accuracy for a given measurement care must be taken to minimize the various noise sources that can be found in the laborat...

Страница 70: ...s are susceptible to magnetic pickup Common Mode Signals Common mode signals are those signals which appear equally on both center and shield A or on the center of both A and B A B With either connect...

Страница 71: ...1 k 10 nA range Currents from 3 A down to 1 fA full scale can be measured The impedance of the signal source is the most important factor to consider in deciding between voltage and current measureme...

Страница 72: ...se bandwidth of the measurement Since the input signal amplifier in the SR865A has a bandwidth of approximately 4 MHz the effective noise at the amplifier input is Vnoise 250 R nVrms or 1 3 R V pk pk...

Страница 73: ...oise in a current measurement The shot noise or current noise is given by f I 2 rms noise q I where q is the electron charge 1 6 10 19 coulomb I is the RMS ac current or dc current depending upon the...

Страница 74: ...Hz and Vnoise 120 V Cstray can be estimated using a parallel plate equivalent capacitor If the capacitance is roughly an area of 1 cm2 separated by 10 cm then Cstray is 0 009 pF The resulting noise c...

Страница 75: ...Using magnetic shielding to prevent the magnetic field from crossing the area of the experiment 4 Measuring currents not voltages from high impedance detectors Resistive coupling or ground loops Curr...

Страница 76: ...ble This current affects the detector and the measured signal Some ways to minimize microphonic signals are 1 Eliminate mechanical vibrations near the experiment 2 Tie down cables carrying sensitive s...

Страница 77: ...the average The averaging time is roughly 200 time constants for example if the time constant is 100 mS the noise measurement would take about 20 seconds to settle If the Sync filter is enabled that...

Страница 78: ...time is required If the time constant is changed then the noise measurement will need to settle to the new value X and Y noise are displayed in units of V rms The ENBW of the time constant is NOT fac...

Страница 79: ...key with square brackets such as Key A complete description of the keys follows in this section Knobs Knobs are used to adjust the internal reference frequency reference phase shift sine output ampli...

Страница 80: ...ling edge The input impedance is either 1 M or 50 The Sine Trig is ac coupled 1 Hz For low frequencies 1 Hz it is necessary to use a TTL reference signal The TTL input provides the best overall perfor...

Страница 81: ...u Touch Sounds to toggle the key click on and off Display Off Operation Press the Blank key to operate with the front panel display and LEDs off The SR865A is still operating the outputs are active da...

Страница 82: ...0 Reference Source Internal Internal Frequency 100 000 kHz Harmonic 1 Sine Amplitude 0 00 Vrms DC Level 0 0 V Ext Reference Trigger Sine Ext Reference Impedance 50 Signal Input Voltage A B A Couple AC...

Страница 83: ...ound The largest allowable current before overload is around 3 A 1 A range or 30 nA 10 nA range No current larger than 10 mA should ever be applied to this input Voltage A B The voltage input can be e...

Страница 84: ...creasing the gain In general use the smallest input range possible without overload Remember the largest signal whether it s at fref or just noise will be the first to overload This setting has no eff...

Страница 85: ...opriate units This time constant sets the bandwidth of the low pass filter after the phase sensitive detectors for X and Y This is the filter that removes signals at frequencies other than fref In gen...

Страница 86: ...with RC filters They often yield better results in less time See Appendix A for more information about the advanced filters Filter Sync Pressing this key turns synchronous filtering on or off Synchron...

Страница 87: ...lter usually 1 fref Since the synchronous filter follows the phase sensitive detectors the time constant filters and output scaling any change in the signal amplitude reference frequency phase time co...

Страница 88: ...e or range When CH2 outputs a voltage proportional to the output scale is 18 Volt or 180 10 V The phase bar graph and numeric readout scales are also unaffected by the Sensitivity Synchronous Filters...

Страница 89: ...Aux Input voltage of 1 000 V corresponds to unity Outputs which would exceed 10 V generate an Output Overload and the red output Overload LED will light Any corresponding displayed numeric value bar...

Страница 90: ...ill show an increased resolution but will continue to display the original value minus the offset Any display which is showing a quantity which is affected by a non unity expand will display a highlig...

Страница 91: ...ts can also be set using the keypad or by turning the Offset knob Turning the knob ALWAYS adjusts the offset even if the offset is toggled off and the keypad is not shown Offsets are set as a percenta...

Страница 92: ...fo bar across the top This bar always shows tiles displaying the reference phase frequency detected harmonic sine out amplitude and dc level Each of these parameters can be adjusted using the knobs an...

Страница 93: ...to adjust the frequency In this mode the Sine Out is at the internal frequency External When the source is External the SR865A will phase lock to the external reference provided at the Ref In BNC The...

Страница 94: ...in detection is at the measured external frequency The SR540 is driven to phase lock with this internal frequency but is subject to the mechanical limitations of its motor and slew rate The internal f...

Страница 95: ...s the external reference input trigger mode TTL When either Pos TTL or Neg TTL is selected the SR865A locks to the selected edge of a TTL square wave or pulse train For reliable operation the TTL sign...

Страница 96: ...ile to show the numeric entry screen When using an external reference the reference phase shift is the phase between the external reference and the digital sine wave which is multiplying the signal in...

Страница 97: ...by the harmonic detection Its frequency is always the fundamental reference frequency Internal and Chop Reference When the reference mode is Internal or Chop these keys change the internal harmonic n...

Страница 98: ...nal can provide a trigger or sync signal to the experiment when the internal reference source is used This signal is also available when the reference is externally provided In this case the sync outp...

Страница 99: ...the different screen layouts Trend Graph Full Screen Strip Chart Half Screen Strip Chart Full Screen FFT Half Screen FFT Big Numbers Screen Shot Pressing Screen Shot saves a screen shot to the USB mem...

Страница 100: ...To change a setting press Blank to return to normal operation change the desired parameter then press Blank again The Blank LED is lit while the rest of the display is off Config The SR865A displays u...

Страница 101: ...data graph for wide cursors since the marker shows the min max or mean of all the data within the cursor width Relative Chart Cursor To read chart data relative to a reference point within the chart...

Страница 102: ...3 SR865A DSP Lock in Amplifier Touch in the vertical scale paleete to change the width of the cursor In the FFT display the wide cursors always find the peak within the cursor region This makes readin...

Страница 103: ...that there is sufficient gain to drive the A D converter Remember that the signal input is both the signal at fref plus noise which may be much larger The largest component of the signal will determi...

Страница 104: ...the keypad Touch Confirm to commit the current setup or Cancel to quit the operation Locations which already contain a saved setup will show the setup name and the time and date it was saved Touch Re...

Страница 105: ...tments may be made Local When a host computer places the unit in the REMOTE state the keys knobs and touchscreen are locked out The Remote indicator is on above the Local key To return to front panel...

Страница 106: ...ime spent at each scan step along the way The shorter the time the more steps in the scan and the smoother the parameter varies The longer the time the fewer steps With experiments that take time to s...

Страница 107: ...ess and hold Play Pause reset to reset the scan back to the Begin Value This leaves the Ready LED on and turns the Done LED off Press Play Pause reset briefly to start the scan again To Stop Scanning...

Страница 108: ...perator follows all of its operands For more information ask your friends or check the internet The 2nd button switches the functions of the operator keys Note that the reference phase frequency and a...

Страница 109: ...rocess for the date The time and date are used to label data files and screen shots as well as saved settings 10MHz Timebase The SR865A can accept an external 10 MHz timebase input on the rear panel T...

Страница 110: ...list of subdirectories Touch a subdirectory to select it as the destination Touch to go up one directory level Repeat until the desired directory is selected Prefix and Suffix File names are generated...

Страница 111: ...to select a baud rate Select a terminator and parity VXI 11 and Web Settings The SR865A allows computers to connect via VXI 11 from a range of Trusted IP addresses or Disabled altogether Set the Trust...

Страница 112: ...X Y R and and the desired Format 32 bit Float or 16 bit Integer Integers are scaled to the sensitivity The number of bits per sample point can range from 16 X integer to 128 XYR float This has an impa...

Страница 113: ...ck in Amplifier The SR865A s firmware can be updated via the USB memory stick Follow the directions that accompany the update to prepare the memory stick Insert the memory stick into the front panel a...

Страница 114: ...tem key RS 232 Port The RS 232 interface port is configured as a DCE The baud rate and parity are set in the system menu hold the Calc system key The number of data bits is always 8 USB Port The USB p...

Страница 115: ...5 V the resolution is 1 mV and the inputs are 1 M These inputs can be displayed on the screen read over the computer interfaces and graphed on the strip charts Aux Out 1 4 These are auxiliary analog...

Страница 116: ...put zero crossings This is useful when the sine output amplitude is small and a synchronous trigger is required to a scope for example This output is active even when the SR865A is locked to an extern...

Страница 117: ...A Use A B as the input configuration Be sure to twist the A and B cables so that there is no differential noise pickup between the cables The signal coupling and input impedance is now determined by t...

Страница 118: ......

Страница 119: ...RS 232 word length is always 8 bits The transmit terminator character s may also be specified Communicating With USB The SR865A is a Test and Measurement Class instrument and easily interfaces with La...

Страница 120: ...er does not need to continually check the Interface Ready bit When using the GPIB interface serial polling may be used to check the Interface Ready bit in the Serial Poll Byte while an operation is in...

Страница 121: ...the command mnemonic and omitting the desired parameter s from the command No space is allowed between the command and the Values returned by the SR865A are sent as a string of ASCII characters If mu...

Страница 122: ...23456 KHZ Queries of f p or v always return the value in the base units of Hz degrees or Volts Enumerated Strings Many commands have an enumerated list of allowed settings It is possible to specify th...

Страница 123: ...5A DSP Lock in Amplifier Example Commands RSRC 1 Set reference source to external RSRC EXT RSRC Query the reference source FREQ 10E3 Set the internal reference frequency to 10000 Hz FREQ 10 KHZ FREQ 1...

Страница 124: ...may be specified in degrees default or millidegrees microdegrees radians milliradians or microradians Example PHAS 12 34 DEG Set the reference phase to 12 34 deg PHAS 1 234E1 PHAS 12340 MDEG PHAS Retu...

Страница 125: ...l reference frequency Example FREQEXT Returns the external frequency in Hz FREQDET The FREQDET query returns the actual detection frequency This is helpful in dual reference mode or harmonic detection...

Страница 126: ...effect since the SR865A will follow the chopper If multiple SR865A SR540 systems are being used in a single experiment BLADEPHASE can be used to modify the relative phase of the choppers Blade phase...

Страница 127: ...rnal i 1 dual i 2 or chop i 3 Example RSRC EXT Set the reference mode to external RSRC 1 RSRC Returns the reference mode i RTRG SIN POSttl NEGttl i The RTRG i command sets the external reference trigg...

Страница 128: ...are available whenever the sine out amplitude keypad is displayed The parameter j 0 3 selects a preset A1 A4 The value of v will be rounded to 3 digits or 1 nV whichever is greater The value of v is...

Страница 129: ...e ICPL i command sets the voltage input coupling to ac i 0 or dc i 1 Example ICPL DC Set the voltage input coupling to dc ICPL 1 ICPL Returns the voltage input coupling mode i IGND FLOat GROund i The...

Страница 130: ...Example ILVL Returns the signal strength indication SCAL i The SCAL i command sets the sensitivity according to the table below i sensitivity i sensitivity 0 1 V A 15 10 V pA 1 500 mV nA 16 5 V pA 2 2...

Страница 131: ...t i 1 18 dB oct i 2 or 24 dB oct i 3 Example OFSL 1 Set the filter slope to 12 dB oct OFSL Returns the filter slope i SYNC OFF ON i The SYNC i command turns the synchronous filter off i 0 or on i 1 Ex...

Страница 132: ...XP 1 2 CEXP 1 Returns the Y expand mode i CEXP Y COFA X Y R j OFF ON i The COFA j i command turns the output offset for X j 0 Y j 1 or R j 2 to off i 0 or on i 1 Setting an offset for phase is not all...

Страница 133: ...ase is not allowed The ratio function for X and Y are tied to aux input voltage 3 following the equation V 10 Expand V 1 000 3 In Aux Offset y Sensitivit orY X Output u u The ratio function for R is s...

Страница 134: ...l Example OAUX 0 Returns the aux in1 voltage v OAUX 1 Returns the aux in2 voltage v AUXV j v NV UV MV V The AUXV j v command sets an aux output to voltage v The parameter j 0 3 selects aux output 1 4...

Страница 135: ...priate amount of time ARNG The ARNG command performs the Auto Range function This command is the same as pressing the Auto Range key The outputs may take many time constants to return to their steady...

Страница 136: ...USB memory stick must be present in the front panel USB port CDSP DAT1 DAT2 DAT3 DAT4 j parameter i The CDSP j param command assigns a parameter to data channel j This is the same parameter assignmen...

Страница 137: ...s written to a USB memory stick upon pressing the Screen Shot key After sending GETSCREEN the user s remote program should repeatedly perform a Status Byte query either by serial poll or through the V...

Страница 138: ...j to x div The scale will be set to the nearest allowed scale in a 1 2 5 sequence The value of j 0 3 corresponds to the DAT1 green DAT2 blue DAT3 yellow and DAT4 orange data channels Example GSCL DAT2...

Страница 139: ...yellow and DAT4 orange data channels Example GACT DAT1 Auto Scale zero center data channel 1 green GACT 0 GAUF DAT1 DAT2 DAT3 DAT4 j The GAUF j command performs an Auto Find on data channel j This co...

Страница 140: ...If the chart is live then the marker is set to the most current bin location corresponding to the right and edge of the chart the marker however will not become visible until the chart is paused Examp...

Страница 141: ...j 4 returns 0 if there is no overload or unlock at the cursor positon The cursor is only available when the strip chart is paused The status response encodes three distinct sources that cause the vio...

Страница 142: ...00 In this example the cursor position is 2 days 16 hours 7 minutes and 30 seconds before the chart was paused corresponding to PCUR 171 on the 1 day division GSPD 15 chart When the cursor is in rela...

Страница 143: ...i 1 or post time constant filter i 2 Example FFTR FILT Set the FFT source to post time constant filter FFTR 2 FFTR Returns the FFT source i FFTS i The FFTS i command sets the FFT vertical scale dB div...

Страница 144: ...span as returned by FFTMAXSPAN The FFT span will be set to the nearest allowed span max span divided by powers of 2 Example FFTSPAN 305 Set the FFT span to 305 2 Hz div FFTSPAN Returns the FFT span i...

Страница 145: ...in Hz FCRX is a query only command Example FCRX Returns the FFT cursor frequency value FCRY The FCRY query returns the amplitude value of the FFT cursor readout in dB FCRY is a query only command Note...

Страница 146: ...the scan end mode to repeat SCNLOG 1 SCNLOG Returns the scan end mode i SCNSEC x The SCNSEC x command sets the scan time to x seconds The scan time is limited to 20 days 1728000 seconds Example SCNSE...

Страница 147: ...transients in the sine output Example SCNDCATTN 1 Set the scan dc level attenuator mode to fixed SCNDCATTN Returns the mode for dc level output attenuators SCNINRVL i The SCNINRVL i command sets the...

Страница 148: ...end j 1 frequency to f The value of f will be rounded to 6 digits or 0 1 mHz whichever is greater The value of f is limited to 1 mHz f 4 MHz Example SCNFREQ BEG 1234 Set the begin frequency to 1 234...

Страница 149: ...0 5 V with 1 mV resolution Example SCNAUX1 BEG 4 567 Set the begin AuxOut1 value to 4 567 V SCNAUX1 0 4567 MV SCNAUX1 0 Returns the begin AuxOut1 value SCNAUX2 BEGin END j v NV UV MV V The SCNAUX2 j v...

Страница 150: ...output 12 PHAse Reference Phase 4 IN1 Aux In1 13 SAMp Sine Out Amplitude 5 IN2 Aux In2 14 LEVel DC Level 6 IN3 Aux In3 15 FInt Int Ref Frequency 7 IN4 Aux In4 16 FExt Ext Ref Frequency 8 XNOise Xnoise...

Страница 151: ...the values of the 4 parameters presently displayed as Data 1 through Data 4 at a single instant This is equivalent to the SNAP command described above with four parameters returned The four values ret...

Страница 152: ...ided by factors of 2 Note that if the time constant is modified during a capture the sampling rate will change mid capture This will likely create confusing results and should be avoided For the case...

Страница 153: ...ger In both cases a falling edge at the BNC input triggers the start or stop of capture The TRIG IN BNC has a 10 k internal pull up resistor to 3 3 V giving users flexibility around interfacing In one...

Страница 154: ...a given buffer length total data points capturing more parameters results in fewer points of each parameter and a shorter total capture time The capture rate is unaffected by CAPTURECFG Example CAPTUR...

Страница 155: ...er is a falling edge For hardware triggered start or stop a triggered start requires using OneShot acquisition while a triggered stop required using Continuous acquisition Both parameters i and j are...

Страница 156: ...uous captures that are overwriting old data the number of captured bytes decreases by 2048 bytes before increasing again After capture has stopped this query returns the actual number of bytes of non...

Страница 157: ...turns wrapped data The maximum length j is 64 corresponding to 64 kbytes Both parameters i and j must be provided Capture must be stopped when executing CAPTUREGET or a range error is generated The bi...

Страница 158: ...tive Shorter time constants allow faster streaming rates up to a limit of 1 25 MHz The actual streaming rate can be set to the maximum allowed rate divided by factors of 2 Starting and Stopping The ST...

Страница 159: ...ximum rate divided by 2n The value of n is limited to 0 n 20 Set n 0 for the maximum streaming rate For example if STREAMRATEMAX returns 78125 then STREAMRATE 4 will set the streaming rate to 78125 24...

Страница 160: ...ta in little endian format while setting bit 0 false sends data in big endian format Setting bit 1 true enables data integrity checking while setting bit 1 false disables data integrity checking By de...

Страница 161: ...0 21 DATE 2 21 DATE YEAR Returns the system clock year last 2 digits TBMODE AUTO INternal i The TBMODE i command sets the external 10 MHz timebase mode to auto i 0 or internal i 1 Example TBMODE AUTO...

Страница 162: ...Example SDFM CSV Set the data file format to csv SDFM 0 SDFM Returns the data file format i FBAS s The FBAS s command sets the file name prefix to the string s The string s must be within quotes All l...

Страница 163: ...AP The DCAP command is the same as pressing the Screen Shot key The file save will take some time to complete A USB memory stick must already be in the front panel USB port SVDT The SVDT command is th...

Страница 164: ...register see page 152 Querying OPC Always returns a 1 but does not affect the Standard Event Status register LOCL i The LOCL command sets the local remote function If i 0 the SR865A is set to LOCAL i...

Страница 165: ...r On i 1 When Overide Remote is On then the front panel is NOT locked out when the unit is in the REMOTE state The REMOTE indicator will still be on and the LOCAL key will still return the unit to the...

Страница 166: ...ber from 0 to 255 The ESR j command queries the value 0 or 1 of bit j 0 7 Reading the entire byte will clear it while reading bit j will clear just bit j Example ESR Returns decimal value of the stand...

Страница 167: ...mple ERRE 48 Set bits 4 and 5 in the error status enable register ERRE 4 1 Set bit 4 in the error status enable register ERRE Returns decimal value of the error status enable register ERRE 4 Returns b...

Страница 168: ...CUROVLDSTAT The CUROVLDSTAT command queries the present overload states of the lock in These overloads reflect the state of the lock in at the time the command is processed They are NOT set until read...

Страница 169: ...T cleared by reading the status byte using STB The bit stays set as long as the status condition exists This is true even for SRQ SRQ will be set whenever the same bit in the serial poll status byte A...

Страница 170: ...s set Since bit 3 in the LIA Status byte AND Enable register is set this ALSO sets bit 3 LIA in the Serial Poll Status byte Since bit 3 in the Serial Poll Status byte AND Enable register is set an SRQ...

Страница 171: ...nel 2 scale overload 10 DAT3OV Data Channel 3 scale overload 11 DAT4OV Data Channel 4 scale overload 12 DCAPFIN Display capture to USB stick completed 13 SCNST Scan started 14 SCNFIN Scan completed Th...

Страница 172: ......

Страница 173: ...evertheless there are filters other than simple RC filters which offer quantifiable benefits to lock in users What are these benefits For two filters with the same noise bandwidth i e whose outputs wo...

Страница 174: ...simply adding identical poles in succession Because the Gaussian filter is computationally intensive it is not available for time constants longer than 3s Linear Phase Filter For time constants of 10...

Страница 175: ...f the different filters also see Step Response graphs on the following pages The chart below summarizes the noise bandwidth of the different filters Note that for a single stage all 3 filters are desi...

Страница 176: ...e stage of the RC filter Each stage of the Linear Phase filter has the same roll off as a single stage of the RC filter 80 60 40 20 0 Response dB 0 1 1 10 100 Frequency fc 12dB oct RC Gaussian FIR Lin...

Страница 177: ...have increasing latency with more stages Linear Phase filters have 1 overshoot 1 0 0 8 0 6 0 4 0 2 0 0 Response V 7 6 5 4 3 2 1 0 Time tc 12dB oct RC Gaussian FIR Linear Phase Time to reach 1 of fina...

Страница 178: ...when using the BlazeX output In this example the input signal is a 2 MHz sinewave that abruptly changes from 10 mVrms to 30 mVrms The output filter is configured as RC 12 dB oct with a 1 s time consta...

Страница 179: ...king separate measurements at fref 2 fref 3 fref etc the FFT can display the harmonic frequencies and amplitudes with amazing clarity in a single measurement Another example is noise analysis In an FF...

Страница 180: ...eplacement for a full FFT spectrum analyzer instrument It is intended to give the user an overview of the spectrum of the input signal or the output of the PSD and time constant filters The length of...

Страница 181: ...A input using a BNC cable and appropriate terminator The input impedance of the lock in is 10 M The generator may require a terminator Many generators have either a 50 or 600 output impedance Use the...

Страница 182: ...sor In the FFT display the wide cursors always find the peak within the cursor region This makes reading an FFT much easier Touch the highlighted dB div tile again to dismiss the scale palette Notice...

Страница 183: ...e the number of averages to 10 FFT graphs can be averaged to reduce fluctuations Averaging is exponential in the number of averages Touch twice to decrease the number of averages to 1 Turn the averagi...

Страница 184: ...Use the and buttons to set the Hz div to 305 2 Hz The total span is about 1 kHz At a 1 ms time constant the maximum span is about 50 kHz 4 883 kHz div At a 1 s time constant the maximum span is 1 6 k...

Страница 185: ...ess the Filter Slope key once to increase the filter slope to 12 dB oct The peak amplitude at 99 000 kHz is diminished even further This is the effect of the steeper time constant filter At the lock i...

Страница 186: ...168 The FFT Display Appendix B SR865A DSP Lock in Amplifier...

Страница 187: ...unit s IP address into the browser bar of a computer which can access the same subnet Note that typical installations do not allow the unit to be seen on the internet only on the internal subnet For...

Страница 188: ...170 Using the Webserver Appendix C SR865A DSP Lock in Amplifier Click the Monitor Output tab to display a simple readout of the 4 data channels...

Страница 189: ...data matters In big endian format a 32 bit 4 byte word is stored as B3 B2 B1 B0 where B3 is the most significant byte of the word and B0 is the least significant byte Little endian format reverses the...

Страница 190: ...n 0 and 31 inclusive This sample rate already takes into account the subsampling requested in the command STREAMRATE Status bit 24 is high when an input overload sync filter overload if sync filter is...

Страница 191: ...te is 1 25MHz If floating point XYR data is streamed then the data rate can exceed 1 25 MHz 4 channels 4 bytes 20 MBytes s This data rate requires Gigabit ethernet to work Fast 100Mb s ethernet is not...

Страница 192: ......

Страница 193: ...modulation of the signal This results from the mixing of 2 frequencies in the experiment Often one of the frequencies is much lower than the other Let s call these frequencies car and mod for carrier...

Страница 194: ...he two frequencies and will always be lower than the greater frequency The original experimental signal contains components at the sum and difference frequencies cos cos 2 mod car mod car mod car t t...

Страница 195: ...ates both North American single fuse or metric dual fuse operation This appendix provides detailed instructions for modifying the input voltage selection and replacing the line fuse Power Entry Module...

Страница 196: ...for detailed illustrations 3 Pull the voltage selector card straight out of the housing using small needle nose plyers to grip the plastic indicator pin body 4 Orient the selector card so that the de...

Страница 197: ...w to install or replace the fuse s 1 Disconnect and remove the power cord 2 Insert a small screwdriver at the point X as shown 3 Gently lift the entire door back away from the rear panel of the SR865A...

Страница 198: ...ether at the same time Also note that when installing the North American AG fuse the fuse is not centered on the plastic retaining hook but rather shifted towards the right hand side see image above l...

Страница 199: ...own default state To do this press Save Recall then touch Recall default Touch Confirm to reset the SR865A to its default settings Each test generally starts with a preset This procedure will be refer...

Страница 200: ...ests are performed It is necessary to turn the unit off and on to preset it As long as the unit is powered on immediately this will not affect the test results Necessary Equipment The following equipm...

Страница 201: ...its operation using a DVM scope or some other piece of test equipment After checking the setup repeat the test from the beginning to make sure that the test was performed correctly If the test continu...

Страница 202: ...llowing adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 00 Hz Press Input Range Down multiple times Set the input range to 10 mV Press Sensitivity Down multiple...

Страница 203: ...touch Recall default Touch Confirm 2 Use the front panel to make the following adjustments Touch Fint to display the internal frequency keypad Enter a frequency of 1 0 kHz Touch Ampl to display the s...

Страница 204: ...be configured as i Reset press blue shift key then Reset ii SETACV RNDM press blue S then 3 times then then 2 times then Enter iii ACV press ACV iv a for RANGE 1 press blue R then 1 then Enter iv b f...

Страница 205: ...Frequency response is checked at frequencies above 1 kHz The sine amplitude is set to 100 mV for all frequencies The test frequencies are listed below Test Frequencies 10 kHz 100 kHz 1 0 MHz a Use th...

Страница 206: ...ed to set performance Lower Limits and Upper Limits for Amplitude Accuracy and Flatness iii On the Performance Test Record for Lower and Upper limits perform the multiplication indicated with the refe...

Страница 207: ...ed below Test Frequencies 1 0 kHz 10 kHz 100 kHz 1 0 MHz a Use the front panel to make the following adjustments Touch Ampl to display the amplitude keypad Enter the 100 mV Press Input Range Up Down S...

Страница 208: ...Sine Out BNC This means that each measurement will nominally be 50 of the programmed sine amplitude Procedure 1 PRESET press Save Recall then touch Recall default Touch Confirm 2 Use the front panel...

Страница 209: ...ress External Trig Select Pos TTL Press Config Touch Data1 Display to highlight the green data source Touch Fext to choose external reference frequency for Data1 Touch Close Press Screen Layout multip...

Страница 210: ...of the specified 24 inch cable will introduce an additional 6 9 ns of propagation delay between sine output and signal input at 4 MHz this corresponds to 10 of additional phase delay Procedure 1 PRES...

Страница 211: ...h Fint to display the internal frequency keypad Enter the frequency from the table b Wait for the readings to stabilize Record the value of orange c Repeat steps 4a and 4b for all frequencies in the t...

Страница 212: ...uts repeat steps 2a through 2e a Connect the CH1 or CH2 output to the DVM Set the DVM to a 20 V range b Use the front panel to make the following adjustments Press the CH1 or CH2 Output Offset knob Tu...

Страница 213: ...reen Layout multiple times Set the screen to show 8 big numerical bar graphs Press Config Touch Data1 Display to highlight the green data source Touch In 1 to choose Aux input 1 for Data1 Repeat this...

Страница 214: ...ency of 1 0 kHz Press Input Range Down multiple times Set the input range to 10 mV Press Sensitivity Down multiple times Set the sensitivity to 100 nV Press Filter Slope adv Select 12 dB oct Press Cou...

Страница 215: ...00 mV DC _______ 0 500 mV Common Mode Rejection Frequency Reading Upper Limit 1 kHz _______ 30 V Sine Output Amplitude and Flatness Sine Output Ampl Lower Limit Reading Upper Limit 1 00 V 0 490 V A __...

Страница 216: ...H ______ Amplitude Linearity Input Range Sine Out Ampl Lower Limit Reading Upper Limit 1 V 1 0000 Vrms 0 99 A ______ _______ 1 01 A ______ 100 00 mVrms 0 99 C ______ _______ 1 01 C ______ 10 000 mVrm...

Страница 217: ...______ 10 10 V 5 000 4 95 V _______ 5 05 V 0 000 0 010 V _______ 0 010 V 5 000 5 05 V _______ 4 95 V 10 000 10 10 V _______ 9 90 V Output Voltage Lower Limit Reading Upper Limit AUX OUT 3 10 000 9 90...

Страница 218: ...0 V _______ 9 90 V 5 000 5 05 V _______ 4 95 V 0 000 0 010 V _______ 0 010 V 5 000 4 95 V _______ 5 05 V 10 000 9 90 V _______ 10 10 V Input Voltage Lower Limit Reading Upper Limit AUX IN 4 10 000 10...

Страница 219: ...ng for the voltage preamp front end Programmable gain blocks around U402 and U404 implement the SR865A Input Range setting with U406 driving the rear panel Signal Mon after all programmable gain is ad...

Страница 220: ...ulators Auxiliary I O The rear panel auxiliary I O circuit supports the 4 AUX OUT and 4 AUX IN analog signals Outputs are received differentially referenced to system ground from the motherboard on he...

Страница 221: ...ls has a separate ground network all of these grounds are loosely tied to chassis ground through resistors R31 R36 10 k each to avoid any supplies drifting away during testing before connection to the...

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