Stanford Research Systems SR844, User Manual

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2-18 SR844 Basics
SR844 RF Lock-In Amplifier
Recognizing that different experimental situations call for different gain-allocation
strategies, the SR844 provides multiple dynamic reserve modes separately for both the
RF signal gain (before the mixer) and the IF gain (after the mixer).
Wide Reserve or RF reserve, allocates the RF signal gain before the mixer. See Chapter 3,
Signal Input , for a table of RF gain vs Wide Reserve. The Wide Reserve should be set to
accommodate all interfering signals within the 20 kHz - 200 MHz bandwidth of the RF
input. High reserve applies minimum RF gain preventing large interfering signals from
causing amplifier overloads. Low Noise provides maximum RF gain and the best output
signal-to-noise and is less susceptible to coherent pick-up. Normal is in between.
Close Reserve or IF reserve, allocates the IF gain after the mixer and before the DSP. The
Close Reserve should be set to accommodate interfering signals closer to the reference
frequency than the IF bandwidth (180 kHz). High reserve applies the minimum IF gain
preventing overloads before the DSP. Low Noise provides the maximum IF gain and the
best output signal to noise. Normal is once again somewhere in between. The maximum
allowable IF gain is proportional to (sensitivity
x RF gain)
-1
- up to a maximum of 50 dB.
Choosing the Wide Reserve sets the RF gain (see Chapter 3, Signal Input ) and thus
determines the maximum allowable IF gain. The minimum IF gain is 1.
After selecting the Wide (RF) and Close (IF) reserve modes, the DSP supplies the
remainder of the gain required for the correct output scaling.
Important!
As a general rule, try to use Low Noise reserve modes if possible. Only increase the
reserve if overloads occur. This will provide the best output signal-to-noise and have the
least coherent pickup (see below).
Some sensitivity settings do not have three different gain allocations available. For
example, the 1 V sensitivity can only be achieved by a single gain allocation. Sensitivity
settings below 10
µ
V require all of the available gain. In these cases two, and sometimes
all three, of the reserve modes actually use the same gain allocation. The dynamic reserve
of these identical gain allocations is, of course, the same.
Note that the 1 Vrms input specification should never be exceeded in a measurement
situation. This means that when the sensitivity is 1 Vrms, there is no room left for
interfering signals, and the dynamic reserve is zero!