SRS Labs SR510 - Manual (Page 34)

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Circuit Description
Introduction
The SR510 Lock-in amplifier is an integrated
instrument combining state of the art analog
design with advanced microprocessor based
control and interfaces. This discussion is intended
to aid the advanced user in gaining a better
understanding of the instrument.
The SR510 has 8 main circuit areas: the signal
amplifier, the reference oscillator, the demod-
ulator, the analog output and controls, the front
panel, the microprocessor, the computer inter-
faces, and the power supplies. With the exception
of the front panel and a few pieces of hardware,
the entire lock-in is built on a single printed circuit
board. Each section is isolated from the others as
much as possible to prevent spurious signal
pickup. To aid in the location of individual
components, the first digit of each part number
generally refers to the schematic sheet number on
which it occurs. To help find the part on the circuit
board, the parts list includes a location on the
circuit board for each component.
Signal Amplifier
Assuming the input selector switch is set to a
voltage input, the signal is coupled in through
capacitors C101 and C102. The input impedance
is set by the 100 M
Ω
resistors R101 and R102
over the operating frequency range. Note that
R103 isolates the signal shields from the
instrument ground forcing the return signal current
back along the cable shields. The signal is then
applied differentially to the gates of Q101. Q101 is
a low noise dual JFET. The drain current through
R109 is kept constant by 2/2 U101. The other half
of U101 maintains a virtual null between the drains
of the two transistors and thus an identical current
flows through R110. Any input that would cause a
differential between the two drains is amplified by
1/2 U101 and fed back via R112 in such a way as
to reduce that differential. Since the two
transistors are at equal and constant currents,
their gate-source potentials are constant. Thus,
the fed back signal which appears at the source of
the right hand transistor exactly matches the input.
Likewise, this signal will match the input to the left
hand transistor but with the opposite sign.
Resistors R112 and R110 attenuate the fed back
signal from the output of U101 resulting in a
differential input, single ended output, fixed gain of
10 amplifier. P101 adjusts the current balance
between the two transistors and therefore their
gain match and common mode rejection.
The output of the pre-amp is scaled by resistors
R119-R122 and analog switch U103 which make
up a 1-2-5-10 attenuator. The signal is then
amplified by 2/2 U102. Input overload is sensed
through diodes D101-D104.
Current Amplifier
When the input selector is set to current, the input
to the pre-amp comes from the output of the
current to voltage converter, 1/2 U102. U102 is a
low voltage-noise bipolar op amp. Q102 serves as
an input buffer to provide low current-noise to the
input. The op amp always maintains a null at the
gates of Q102 thus providing an input impedance
of 1K
Ω
(R128). The input current is converted to a
voltage by R135 and the op amp. Q103
bootstraps out the summing junction capacitance
of Q102.
Notch Filters
U107 is a high Q, line frequency, notch filter which
can be switched in and out by analog switch 1/4
U106. The frequency and depth of the filter can
be adjusted with P102 and P103. Resistors R146-
R149 and switch U108 make up a selectable
attenuator. U118 is a line frequency 2nd harmonic
notch filter selected by 2/4 U106. P104 and P105
adjust the frequency and depth. The second
notch filter has a gain of 3 and its output is scaled
by U110 and resistors R156-R159. The signal
then takes two paths; to inverting amplifier U111
and to the input of the tracking bandpass filter.
U111 has the same gain as the bandpass filter.
The output of either U111 or the bandpass filter is
selected by 3/4 U112 and 4/4 U106 and amplified
by U113. U114 and U115 provide a last stage of
gain and scaling and the final output is ac coupled
and buffered by 4/4 U208.
Bandpass Filter
The bandpass filter is a three op amp state-
variable active filter. 3/4 of U201 make up the
three op amps of the standard filter. U203, U204,
and U205 are analog switches which select the
feedback capacitors for the 5 decades of
operation. The two halves of U202 are matched
transconductance amplifiers operating as
programmable, voltage controlled, current sources
which take the place of the normal, frequency
setting, resistors. A voltage proportional to the