Reference Oscillator; Psd, Lp Filters And Dc Amplifier - Stanford Research Systems SR530 Manual

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
reference frequency is converted into a current by
1/4 U208 and Q201. This current programs the
effective "resistance" of the two transconductance
amplifiers and thus, tunes the center frequency of
the filter to follow the reference. The output of the
filter is buffered by 4/4 U201. The two remaining op
amps in U208 are used to detect signal overloads
throughout the amplifier chain.

Reference Oscillator

The reference input signal is ac coupled and
buffered by U301. R378 isolates the reference
shield from the lock-in ground to prevent ground loop
currents. 1/2 U303 switches the polarity of the
reference reaching comparator U304. U305 is a
retriggerable one-shot whose output indicates a no
reference condition if no comparator pulses are
generated for three seconds.
U309 is a dual transconductance amplifier in a
triangle VCO configuration. U310 selects the
integrating capacitor depending on the frequency
range. The VCO frequency is determined by the
programming current through R318 and therefore by
the output voltage of U308. C306 is the phase-
locked loop low pass filter which is buffered by
U308. U307 is a programmable current source used
to charge and discharge C306. The amount of
current available to U307 is determined by the VCO
control voltage, thus, the tracking rate of the VCO is
proportional to the VCO frequency. The triangle
output is compared to a constant voltage by U314.
1/2 U313 and 1/2 U312 select f or 2f operation. This
signal is fed back to the phase detector U306 to be
compared with the reference output of U304. U315
compares the triangle output with a variable voltage
to generate a square-wave signal phase-shifted from
the reference. The range of this fine phase shift
control is -5 to 95 degrees.
The output of U315 serves as the reference to a
second phase-locked loop. This second PLL uses a
similar proportional tracking triangle VCO.
Comparator U329 looks at the square wave output
of the VCO while comparator U328 detects the zero
crossings of the triangle output. 1/2 U327 selects
one these comparators to feed back to the phase
detector, U316. Since the square and triangle
outputs are in quadrature, U327 selects either an in-
phase or quadrature relationship between the
two VCO's. Thus, the output of the second VCO
can be shifted from -5 to 185 deg from the
reference.
The triangle output is divided by R363 and R362
before reaching transconductance amplifier 2/2
U322. The amplitude of the triangle input to this
amplifier is enough to just saturate the input and
provide a sine wave output. 2/2 U325 then
amplifies the sine wave before it goes to the
demodulator. U324 is a comparator which
generates a square wave in-phase with the sine
output. U326 divides the frequency of the
square wave by eight and 2/2 U327 selects the
frequency of the square wave chopper.
The square wave output of U322 serves as the
reference to the quadrature oscillator PLL. This
PLL is identical to the triangle oscillator, sine
wave shaper described above. U1004 detects
the zero-crossings of the triangle wave to feed
back to the phase comparator, U1002. This
ensures that the quadrature triangle wave is 90
deg out of phase from the first sine wave. The
quadrature triangle is shaped into a sine wave
by 2/2 U1009 and amplified by 2/2 U1014.
U1012 is a comparator which generates a
square wave in-phase with the quadrature sine
wave. U1013 divides the frequency of the
square wave by eight and 1/2 U1011 selects the
frequency of the square wave chopper.
Demodulator and Low Pass
Amplifier
Amplifier U402 and switch U401 select the
polarity of the reference sine wave. This allows
phase shifts up to 360 degrees from the
reference input. The sine wave is ac coupled by
U403 and inverted by U404. U405 selects
alternating polarities of the sine wave at the
chopper frequency, f/2 or f/16. This chopped
sine wave is then multiplied by the output of the
signal amplifiers by the analog multiplier U406.
The synchronous output of the multiplier that
corresponds to the in-phase signal is a square
wave at the chopper frequency. The output is
ac coupled by U407 to remove the dc offset of
the multiplier. U408 inverts the signal and U405
chops the square wave to recover a dc output.
U409 buffers the chopper output before the first
low pass time constant. Op amps U416 and 2/2
U402 make up the first low pass amplifier with
relays U411-U415 and U417 selecting the time
constant. The second low pass amplifier is
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