Rf Signal Input Path; Input Impedance; 200 Mhz Low Pass Filter; 20 Db Attenuator - Stanford Research Systems SR844 User Manual

RF Signal Input Path

The path the input signal takes from the front panel input to the two mixers depends on the
chosen input impedance and wide (RF) reserve. The SR844 accepts input signals in the
range 25 kHz to 200 MHz, with signal levels up to 1 Vrms (+13 dBm). (The damage
threshold is 5 V DC+AC.)

Input Impedance

For signal sources with 50
it also provides lower input noise. When using 50
Ω
the 50
If the 1 M
preamp with a nominal gain of x2 (+6 dB). The 1 M
source impedance is much greater than 50
Important!
The bandwidth of the 1 M
impedance. The source impedance (R) and the input capacitance (30 pF) form a simple
low-pass filter at f
input and not measured accurately by the SR844. Even a 50
106 MHz filter at the 1 M

200 MHz Low Pass Filter

This passive filter removes signal components above 200 MHz that could interfere with
the operation of the SR844.

20 dB Attenuator

This attenuator provides 20 dB (x10) of input signal attenuation. This is useful in cases
where the real or interfering signals are large. At high sensitivities (near 1 V), the
attenuator is required to scale the actual signal to prevent mixer overload. At lower
sensitivities (100 mV and below), the attenuator is used to provide wide (RF) dynamic
reserve by preventing overloads later in the signal path. While using the attenuator
deteriorates the noise performance of the instrument, it improves the dynamic reserve.

20 kHz High Pass Filter

This filter provides a block to DC and line frequency signals that could interfere with
signal measurement.

20 dB Gain

This gain stage can be used to boost low-level signals above the mixer noise floor in
situations where the interfering signals are not too strong. This gain is required for
sensitive measurements (below 100
reserve is needed.
Ω
source impedance, the 50
input is preferred.
Ω
|| 30 pF input impedance is selected, the signal is buffered by a FET-input
Ω
input is limited by its 30 pF input capacitance and the source
π
= 1/2 RC. Signals at frequencies greater than f
c
Ω
input!
Ω
input provides input matching;
Ω
or other small source impedances,
Ω
input should only be used if the
Ω
.
µ
V). It is also used when less wide (RF) dynamic
SR844 RF Lock-In Amplifier
SR844 Basics
are attenuated at the
c
Ω
source impedance forms a
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