SRS Labs Lock-In Amplifier Circuit Description, Introduction, Signal Amplifier, Current Amplifier

Circuit Description

Introduction

The SR530 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 SR530 has eight main circuit areas: the signal amplifier, the reference oscillator, the demodulator, the analog output and controls, the front panel, the microprocessor, the computer interfaces, and the power supplies. With the exception of the front panel, the quadrature oscillator and demodulator, 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 (or first two digits of a four digit part number) 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. Parts with a four-digit part number beginning with 10,11, or 12 are found on the quadrature detector plug-in board located in the center of the main circuit board. Part numbers beginning with 6 refer to parts on the front panel.

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 MΩ 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 1KΩ (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,

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