The Signal Channel

The instrument has both current and voltage inputs. The current input is a virtual ground, and the 100 MΩ voltage inputs can be used as single- ended or true differential inputs.

There are three signal filters. Each of these filters may be switched 'in' or 'out' by the user. The first filter is a line notch filter. Set to either 50 or 60 Hz, this filter provides 50 dB of rejection at the line frequency. The second filter provides 50 dB of rejection at the first harmonic of the line frequency. The third filter is an auto-tracking bandpass filter with a center frequency tuned by the microprocessor to the frequency of the signal. These three filters eliminate most of the noise from the signal input before the signal is amplified.

A high-gain ac amplifier is used to amplify the signal before entering the phase sensitive detector. The high gain which is available from this programmable amplifier allows the lock-in to operate with a lower gain in its dc amplifier. This arrangement allows high stability operation even when used on the most sensitive ranges.

Reference Channel

The processor controlled reference input discriminator can lock the instrument's PLL to a variety of reference signals. The PLL can lock to sine waves or to logic pulses with virtually no phase error. The PLL output is phase shifted and shaped to provide a precision sine wave to the phase sensitive detector.

Phase Sensitive Detector

The Phase Sensitive Detector is a linear multiplier which mixes the amplified and filtered signal with the reference sine wave. The difference frequency component of the multiplier's output is a dc signal that is proportional to the amplitude of the signal. The low-pass filter which follows can reject any frequency components which are more than a fraction of a Hertz away from the signal frequency.

DC Amplifier and System Gain

Adc amplifier amplifies the output of the low pass filters. The total system gain is the product of the ac and dc amplifier gains. The partitioning of the system gain between these two amplifiers will affect the stability and dynamic reserve of the

instrument. The output is most stable when most of the gain is in the ac amplifier, however, high ac gain reduces the dynamic reserve.

For the most demanding applications, the user may specify how the system gain is partitioned. However, with prefilters that are able to provide up to 100 dB of dynamic reserve, and with chopper stabilized dc amplifiers, most users will not be concerned with just how the system gain is allocated.

A Microprocessor Based Design

The instrument was designed to take full advantage of its microprocessor controller. This approach provides several key advantages...

The instrument may be interfaced to a laboratory computer over the RS-232 and IEEE-488 interfaces. In addition to simply reading data from the lock-in, the computer can control all of the instrument settings with simple ASCII commands.

A key feature of the instrument is its four A/D inputs and two D/A outputs. These analog I/O ports may be used to read and supply analog voltages to an experiment or measurement. All of the input and output ports have a full scale range of ±10.24VDC with 2.5 mV resolution and 0.05% accuracy.

Computer control can eliminate set-up errors, reduce tedium, and allow more complete data recording and post measurement analysis. Also, the computer can play an active role in the data acquisition by adjusting gains, etc., in response to changing measurement conditions.

The microprocessor based design eliminates many analog components to improve performance, reliability, and reduce cost. Each unit is computer calibrated at the factory, and calibration constants are placed in the instrument's read-only memory. The SR510 has only one-fifth of the analog trimming components that are found in older designs.

Creative programming on the user's part can extend the instrument's capabilities. For example, the lab computer can instruct the lock-in to measure the signal at zero and ninety degrees of phase. Doing so allows both the amplitude and phase of the signal of interest to be measured.

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