Main Analog-To-Digital Converter; Reference Channel Inputs; Reference Channel - Ametek 7230 Instruction Manual

avoided when setting the reference frequency.
A buffered version of the analog signal just prior to the main ADC is available at the
signal monitor (SIG MON) connector on the rear panel of the instrument; it may be
viewed on an oscilloscope to monitor the effect of the line frequency rejection and
anti-aliasing filters and signal-channel amplifiers.

3.3.06 Main Analog-to-Digital Converter

The analog signal 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 for
direct user use by downloading the data to a computer.
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.

3.3.07 Reference Channel Inputs

External reference signals are normally applied to the model 7230 via the front panel
REF IN connector. Internally this can be switched to function as a general-purpose
input, designed to accept virtually any periodic waveform with a 50:50 mark-space
ratio and of suitable amplitude, or specifically set to accept TTL-logic level signals.
Following the trigger buffering circuitry the selected reference signal is routed to the

reference channel.

In dual reference mode where two external reference inputs can be used, one
reference is applied to the front panel REF IN connector, while the second, which
must be of 3.0 kHz or lower frequency, is applied to the rear-panel TRIG IN
connector.
3.3.08 Reference Channel
The reference channel circuitry 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 can each be derived from the internal
oscillator, the front panel REF IN reference input, or the rear panel TRIG IN input.
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
Chapter 3, TECHNICAL DESCRIPTION
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