Stanford Research Systems SR844, User Manual

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2-20 SR844 Basics
SR844 RF Lock-In Amplifier
consists of multiplying a signal by a square wave of amplitude 1 and frequency
ω
C
. Half
of the time the output equals the input. The other half of the time the output is the
negative of the input. Assuming that all chopping operations are properly synchronized,
we can take a signal, chop it, and chop it again and recover the original signal.
Now let’s take a reference signal at
ω
1
(
ω
1
≈ ω
C
), chop it and put it into the mixer
reference input, where it gets multiplied by the signal input. Without chopping, the mixer
output would have a DC output proportional to the signal input. With chopping, the DC
output is multiplied by
±
1 at the chopping frequency. If we now chop this output we can
recover the DC output. However, the chopping operations are not ideal and signals at
frequencies other than the reference can cause DC outputs from the final chopping
operation. The spurious responses are typically –10 dB at a frequency offset of
±
2f
C
,
dropping to -30 dB at
±
6f
C
and -42 dB at
±
12f
C
where f
C
is in the range of 2-12 kHz.
Coherent Pickup
At the high reference frequencies used by the SR844, a small amount of reference signal
pickup occurs in the RF signal path. This is called coherent pickup. Since the pickup is
phase coherent with the reference frequency it is detected by the SR844 as if it was a real
signal input. Measuring signals which are smaller than the instrument’s own coherent
pickup requires care and the use of offsets.
The typical amount of coherent pickup in the SR844 is shown below.
The level of coherent pickup (three curves above) is dependent on the RF input gain. The
choice of Wide (RF) reserve and Sensitivity determines the RF gain and thus, the level of
coherent pickup. The following table shows which curve to use.