Stanford Research Systems SR844, User Manual

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Circuit Descriptions 6-13
SR844 RF Lock-In Amplifier
The output is the conditioned signal (after attenuation, filtering and gain) that goes to the
mixers on 84CMX.
Relay S102 selects the signal path corresponding to the chosen input impedance. The
50
Ω
signal path goes through a 2dB input matching attenuator, R102–104. The 1M
Ω
signal path goes through a preamplifier consisting of an AC-coupled JFET Q108
followed by an amplifier U120. P108 is used to adjust the bias of Q108 for minimum
distortion.
Between S102 and S140 is a 5-pole Cauer passive low-pass filter with a cutoff frequency
of about 220 MHz. The filter components are C132–138 and L132–133.
Relay S140 switches the signal between a straight-through path and one containing 20
dB attenuation (R140–142). Note that S140 and S150 are not under direct user control,
rather they are set by the instrument depending on the user–selected wide reserve, close
reserve and sensitivity.
Between S140 and S150 is a 3-pole passive high-pass filter with a cutoff of about 20kHz,
using C144–147 and L144–145. The legs with R187, R188 provide compensation at low
and high frequencies respectively.
Relay S150 switches the signal between a straight-through path and one containing 20
dB gain (U150). The components R150–151 and C150 provide impedance matching to
the amplifier.
The output signal goes to the mixers on 84CMX via connector J011.
There is also overload detection on this board, with two sense inputs. One input senses
the preamplifier output via R164 and C164, the other senses the mixer input via R160
and C160. The two sense inputs share peak–detecting capacitors and comparators.
D172, D173 charge C171 to the maximum of either sense signal, while D170, D171
charge C170 to the minimum of either sense signal. All signals and the comparator
thresholds are referred to +7.5VDC nominal, which is generated by U156. C170 and
C171 are monitored by the two halves of U170, a FET-input dual comparator. U180
combines the comparator outputs so that
OVLD2
goes low when either comparator’s
threshold is exceeded.
OVLD2
goes to the platform interface on the motherboard 84MBD; this is also where the
relay control bits
SI0–SI2
come from.
84CMX: Chop and Mix Board
The 84CMX board is located midway across the front of the instrument. It contains the
Chop and Mix sections.
Document Number Sheet Schematic
CMXM 1 Mixers
CMXC 2 Chop Circuit