Circuit
Descriptions
6-17
SR844 RF Lock-In Amplifier
IFN2: Noise Generator
The noise of a 90.9K resistor (R470) at room temperature is 38nV/
√
Hz. U470 and U471
amplify this to about 0.8mV/
√
Hz. U472 and U473 constitute a 5th order Cauer low-pass
filter with cutoff around 1.0kHz. C479 is the termination for this filter. The filter output
is buffered by U474.
This narrow-band noise at DC is converted to narrow-band at F
S
/2 (that is, half the
sampling frequency) by multiplying the DC noise by
±
1 at frequency F
S
/2. To achieve
this U475A and U475B generate [DC noise
×
1] and [DC noise
×
–1] respectively.
U476A switches back and forth between the two at F
S
/2; its output is buffered by
U474B.
NOISE
is summed with the output of the anti-aliasing filter as mentioned
above.
The inputs
24K49D
±
and
NOI.CLK
are required to generate the F
S
/2 clock.
24K49D
±
are low-level signals at F
S
/2 coming from the divider chain; they are converted to a TTL
level signals by comparator U591. N591 sets the nominal operating voltage of the
comparator inputs to 2.5V, while Z591–592 allow the inputs to be shifted to this level.
NOI.CLK
is a short TTL pulse, also at F
S
, coming from the DSP board. U477A
synchronizes the edges of
24K–
(the comparator output) to the
NOI.CLK
pulse. The
output of U477A drives switch U476A mentioned above, so that it switches between
[
±
1
×
DC noise] at F
S
/2.
The RC networks (R497–498, C480–481) and (R574–575, C482–483) provide additional
roll-off at both high and low frequencies.
84XRF: External Reference Board
The 84XRF board is located on the right-hand side front of the instrument. The circuitry
associated with the external reference input is found on this board. There are four
functional blocks found on this board :
Document Number
Sheet
Schematic
XREFI 2
Reference
Input
XPHSC 3
Phase
Comparator
XPLLF
4
Phase-Locked Loop Filter
XVCO
5
Voltage Controlled Oscillator
XREFI: Reference Input
The input to this section is the raw signal provided by the user on the front panel external
reference input. The output is a matched pair of signals (
Q
and
-Q
) at the same
frequency and phase, but converted to a digital ECL square wave.
R213 is the optional 50
Ω
termination, which is switched into the circuit when relay S210
is closed. The coil for S210 is driven by Q210, whose input
-REF50
comes from the
Содержание SR844
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