Stanford Research Systems SR844, Руководство пользователя

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SR844 Basics 2-9
SR844 RF Lock-In Amplifier
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
Ω
source impedance, the 50
Ω
input provides input matching;
it also provides lower input noise. When using 50
Ω
or other small source impedances,
the 50
Ω
input is preferred.
If the 1 M
Ω
|| 30 pF input impedance is selected, the signal is buffered by a FET-input
preamp with a nominal gain of x2 (+6 dB). The 1 M
Ω
input should only be used if the
source impedance is much greater than 50
Ω
.
Important!
The bandwidth of the 1 M
Ω
input is limited by its 30 pF input capacitance and the source
impedance. The source impedance (R) and the input capacitance (30 pF) form a simple
low-pass filter at f
c
= 1/2
π
RC. Signals at frequencies greater than f
c
are attenuated at the
input and not measured accurately by the SR844. Even a 50
Ω
source impedance forms a
106 MHz filter at the 1 M
Ω
input!
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
µ
V). It is also used when less wide (RF) dynamic
reserve is needed.