Stanford Research Systems SR844, User Manual

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Signal Input 3-9
SR844 RF Lock-In Amplifier
Important!
The Wide Reserve setting and the sensitivity determine the amount of internal coherent
pickup. See the discussion in Chapter 2 on
Dynamic Reserve and Coherent Pickup for
more information.
Wide Resrv
Up/Down
These keys set the Wideband Dynamic Reserve mode to either High, Normal or Low
Noise. The current setting is indicated by the LEDs above the keys.
The Low Noise mode selects the maximum RF gain allowed at the current sensitivity.
Low Noise provides the best possible signal-to-noise and the least coherent pickup and
should be used whenever possible.
The instrument selects RF attenuation or gain depending on the Wide Reserve mode and
the instrument sensitivity (see below).
AUTO
[Shift–Wide
ResrvDown]
This key sequence selects the Wideband Dynamic Reserve mode automatically. This
function will execute
once when the keys are pressed. A tone sounds when the function is
complete. The reserve will not continue to change even if the input signal changes
substantially. To adjust for the changed conditions, it may be necessary to perform the
Auto function again, or make manual changes. The
AUTO indicator is on while this
function executes.
RF OVLD The RF OVLD indicator shows that the RF input is overloaded. This overload occurs in
the RF signal path before the mixers. If
RF OVLD
is on, try a higher wide reserve or a
larger sensitivity.
RF Attenuation (–20 dB) or Gain (+20 dB) for different combinations of Wide Dynamic Reserve and
Sensitivity is shown in the table below. 0 dB means that the signal goes straight into the mixer with neither
attenuation nor gain. Note that at sensitivities below 30
µ
V, the full dynamic reserve of the instrument is
available even at +20 dB gain, so there is no reason to switch in attenuation. Also, at minimum sensitivity
(1 V rms) attenuation is always required to prevent the mixer from overloading.
Sig Z-In 50
Ω Ω Ω Ω
1 M
Ω Ω Ω Ω
Wide
Reserve
HIGH NORMAL LOW NOISE HIGH NORMAL LOW NOISE
Sensitivity
1 V –20 dB –20 dB –20 dB –20 dB –20 dB –20 dB
300 mV –20 dB –20 dB –20 dB –20 dB –20 dB –20 dB
100 mV –20 dB 0 dB 0 dB –20 dB –20 dB 0 dB
30 mV –20 dB 0 dB 0 dB –20 dB 0 dB 0 dB
10 mV –20 dB 0 dB +20 dB 0 dB 0 dB +20 dB
3 mV –20 dB 0 dB +20 dB 0 dB 0 dB +20 dB
1 mV -20 dB 0 dB +20 dB 0 dB +20 dB +20 dB
300
µ
V
0 dB +20 dB +20 dB 0 dB +20 dB +20 dB
100
µ
V 0 dB +20 dB +20 dB 0 dB +20 dB +20 dB
30
µ
V
0 dB +20 dB +20 dB +20 dB +20 dB +20 dB
10
µ
V
+20 dB +20 dB +20 dB +20 dB +20 dB +20 dB
3
µ
V
+20 dB +20 dB +20 dB +20 dB +20 dB +20 dB