Stanford Research Systems SR844 User Manual page 72

3-14 Sensitivity
The overall gain is achieved with a combination of RF gain (before the mixers), IF gain
(after the mixers) and DSP gain (in the output filters). Changing the sensitivity changes
the overall gain while changing the dynamic reserves (Wide and Close) affects the
allocation of gain between RF, IF and DSP gains. See the discussion in Chapter 2, What
is Dynamic Reserve, for more information.
Close Resrv
This key cycles through the three Close (IF) Dynamic Reserve modes, High, Normal or
Low Noise.
The Low Noise mode selects the maximum IF gain allowed at the current sensitivity and
wide reserve. Low Noise provides the best possible output signal-to-noise and should be
used whenever possible.
AUTO
This key sequence automatically selects the Close Dynamic Reserve mode. This
[Shift–
function will execute once when the keys are pressed. A tone sounds when the function is
Close
complete. The reserve will not continue to change even if the input signal changes
Reserve]
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.
IF OVLD The IF OVLD indicator shows that the IF section is overloaded. This overload occurs
after the mixers and is caused by input signals close to the reference frequency (within
∼
180 kHz with No Filter and within
on, try a higher close reserve or a larger sensitivity.
SR844 RF Lock-In Amplifier
∼
18 kHz with 6-24 dB/oct filtering). If IF OVLD is