Stanford Research Systems SR844, User Manual

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Circuit Descriptions 6-15
SR844 RF Lock-In Amplifier
The local oscillator signals
XDRV
±
are a balanced
Q,-Q
pair of signals at ECL levels.
They are attenuated by R764, Y764 and R765, Y765 before being AC coupled to the
Local Oscillator inputs U760.10 and U760.11.
The conditioned signal from 84SIG is split between the two mixers and attenuated by
R740 and R760. The split lines are AC-coupled to the mixer’s RF inputs, U760.6 and
U760.7. R761 is the signal termination; in addition the tantalum capacitors F760–761
are necessary for proper signal termination at low frequencies. R790–791 are also part
of the matched attenuator, the DC blocking capacitor C790 is required because the
amplifier U150 on 84SIG adds a DC bias to the signal.
The mixer generates the product of the signal on its RF inputs and the signal on its LO
inputs. In general the low frequency component of the product is the desired I.F. signal,
while the high frequency components at 2
×
F
REF
and other multiples are undesired and
need to be filtered out. The capacitor network Z762, C762 and Z763, C763 provide a
small amount of filtering between the mixer front end and its built-in preamplifier. R762
squelches a resonance between the self-resonant frequencies of Z762 and C762. The
built-in preamplifier is set to a gain of 2 by X769 and Y769, the Common node for the
preamplifier is biased to ½ of the supply voltage by R769 and R770, and the capacitors
Z769 – E769 hold the common node stable. R768 controls the operating current of the
mixer.
The amplified mixer output is filtered before going to the I.F. board 84IFN. L770–L771
and C770–C774 constitute a 2-stage low-pass filter, the first stage being a 3rd order
Cauer filter with cutoff at 180 kHz, while the second stage is a 2nd order filter with
cutoff at 2 MHz. Together the filter provides good rejection of the 2
×
F
REF
mixer outputs
out to 400 MHz and beyond. F770 provides a DC block since the mixer is not ground-
referenced.
84IFN: I.F. and Noise Board
This board is mounted just above the left rear portion of the motherboard 84MBD. This
board contains the I.F. amplifiers, anti-aliasing filters and A-to-D converters. There are
two each of these signal path circuits, one for the X or In-Phase channel, one for the Y or
Quadrature channel. The X channel is described below; operation of the Y channel is
identical. There is also a noise generator and overload detector on this board.
Document Number Sheet Schematic
IFN1 2 IF Amplifiers and Anti-Aliasing Filters
IFN2 3 Noise, ADC and Overload
IFN1: I.F. Amplifiers
The inputs to this circuit are
MIX
.
X
, the filtered I.F. output from the mixer on 84CMX,
and
GND.X
, the ground on 84CMX to which
MIX
.
X
is referenced. U400 is a differential
amplifier with
×
4 gain that converts the differential inputs to a single-ended signal
referenced to local ground and also boosts the signal above the noise level of succeeding