SRS Labs SR530, Lock-In Amplifier manual Reference Oscillator, Demodulator and Low Pass Amplifier

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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