Signal Input - Stanford Research Systems SR844 User Manual

3-8

Signal Input

Signal Input

The keys in this section operate on the RF signal input of the instrument,
before the signal is mixed down to the IF (Intermediate Frequency, 2-12 kHz).
Refer to the Chapter 2, The Functional SR844, for more information.
Sig Z-In
This key selects the input impedance of the SR844 Signal Input, either 50
The indicators above the key show the current selection.
Ω
In 50
setting for signal sources capable of driving a 50
50 ohm cables in this mode, since an impedance mismatch will cause a reflection of
power at the location of the mismatch, resulting in a discrepancy between the signal
emanating from the source and that measured by the SR844.
The 1 M
standard 10X scope probe is used to measure the voltage at a test point. In this setting,
the input signal is buffered by a pre-amplifier before going through the RF attenuator and
gain stages to the mixers.
Important!
The 1 M
The bandwidth of the 1 M
source impedance. The source impedance (R) and the input capacitance (30 pF) form a
simple low-pass filter at f
at the input and are not measured accurately by the SR844.
Wideband Dynamic Reserve or RF reserve, allocates the RF signal gain before the mixer.
Wide
The Wide Reserve should be set to accommodate all interfering signals within the 20
Dynamic
kHz - 200 MHz bandwidth of the RF input. High reserve applies minimum RF gain
Reserve
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 somewhere in between.
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,
Dynamic Reserve, for more information.
SR844 RF Lock-In Amplifier
mode, the SR844 input is matched to a 50
Ω
setting is appropriate for high-impedance sources, or for situations where a
Ω
input should only be used if the source impedance is much greater than 50
Ω
input is limited by its 30 pF input capacitance and the
π
= 1/2
c
Ω
source. This is the appropriate
Ω
load. It is highly recommended to use
RC. Signals at frequencies greater than f
Ω Ω
or 1 M .
Ω
.
are attenuated
c