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Chapter 3, TECHNICAL DESCRIPTION
3.3.08 Main Analog-to-Digital Converter
3.3.09 Reference Channel Inputs - 7124 RCU
3.3.10 Reference Channel Inputs- Main Console
3.3.11 Reference Channel
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The main analog signal arriving via the fiber link at the main console, or the signal
from its AUX input, is then routed to the main analog-to-digital converter, which
runs at a sampling rate of 1.0 MHz. The output from the converter feeds one of the
two demodulators, which uses DSP techniques to implement the digital multipliers
and output low-pass filters for each of the X and Y channels.
The ADC output also passes to the fast output curve buffer where it can be stored
both for direct user use (by downloading the data to a computer, or viewing it on the
user graphs), or to allow the power spectral density of the input signal to be
calculated. This uses a discrete Fourier transform, which in many ways is similar to a
fast Fourier transform (FFT), and the results of this calculation are shown on the
Spectral Display menu.
In dual reference and dual harmonic mode a second demodulator is active, and in
normal operation the input to this is also taken from the main ADC output.
Before discussing the demodulators and the output stages of the lock-in amplifier,
the reference channel, which provides the other input to the demodulators, will be
described.
The 7124 RCU provides two signal inputs for an external reference signal. The front
panel REF IN is a general-purpose input, designed to accept virtually any periodic
waveform with a 50:50 mark-space ratio and of suitable amplitude, while the rear-
panel TTL REF IN is suitable for TTL-logic level input signals. Following the
trigger buffering circuitry the selected reference signal is passed via a fiber optic link
to the main console.
The main console provides the same two options for connecting an external
reference signal as the 7124 RCU.
The external reference signal arriving via the fiber link at the main console, or one of
the signals from its own reference connectors, is then routed to the reference channel
circuitry. This is responsible for implementing a phase-locked loop to lock onto the
selected external reference signal (when in external reference mode), or processing
signals from the internal oscillator (when in internal reference mode). The reference
channel generates a series of phase values, output at a rate of one every 1 µs, which
are used to drive the reference channel inputs of the two demodulators.
In dual reference mode, the two references are selected from the five possible
options of internal, external front panel, and external rear panel inputs on the main
console, and external front panel, and external rear panel inputs on the 7124 RCU.
The reference circuit generates new phase values for each individual channel and
sends these to the demodulators.
In single harmonic mode, the reference circuit generates the phase values of a
waveform at the selected harmonic of the reference frequency. Dual harmonic mode
operates in a similar way to dual reference mode, but in this case the reference
circuit generates phase values for both of the selected harmonics of the reference
frequency. Dual harmonic mode may therefore be used with either internal or
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