Sensitivity - Stanford Research Systems SR844 User Manual

Sensitivity

The keys in this section select the overall sensitivity of the instrument
(Output/Input). The IF dynamic reserve is also selected in this section. Refer to
Chapter 2, What is Dynamic Reserve, for more information.
The overall analog gain (output/input) is 10 VDC output divided by the full scale AC

Sensitivity

signal input and ranges from 10 to 10
of RF signal 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.
Sens
These keys select the full scale sensitivity of the instrument. The full scale sensitivity
Up/Down
ranges from 100 nVrms (–127 dBm) to 1 Vrms (+13 dBm) in 1-3-10 steps (10 dB). The
sensitivity is indicated below the Up/Down keys. Note that the dBm measurements are
calculated assuming a 50
AUTO This key sequence automatically adjusts the sensitivity based on the detected signal
[Shift– magnitude, the instrument reserve settings and any overload conditions. This function
SensUp] executes once when the keys are pressed. A tone sounds when the function is complete.
The sensitivity will not continue to change even if there is a substantial change in the
input signal. In the case of a substantial signal change, it may be necessary to perform the
Auto Sensitivity function again, or adjust the sensitivity/reserve manually. It is common
for users to make changes in the reserve and/or sensitivity after the unit has completed
the Auto Sensitivity function. Auto Sensitivity takes more time to complete at larger time
constants. The AUTO indicator is on while Auto Sensitivity is in progress.
Auto Sensitivity will not execute if the time constant is greater than 1 s.
Close 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
Dynamic
reference frequency than the IF bandwidth (180 kHz). High reserve applies minimum IF
Reserve
gain preventing overloads before the DSP. Low Noise provides maximum IF gain and the
best output signal to noise. Normal is somewhere in between.
8
. The overall gain is achieved with a combination
Ω
source, and will be incorrect for the 1 M
Sensitivity 3-13
Ω
input.
SR844 RF Lock-In Amplifier