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the input limit and at the reference frequency will give a one percent output error
when the dynamic reserve is set to 60 dB, or a ten percent error when the dynamic
reserve is set to 80 dB.
In a typical low-level signal recovery situation, many unwanted inputs need to be
dealt with and it is normal practice to make small adjustments to the reference
frequency until a clear point on the frequency spectrum is reached. In this context an
unwanted alias is treated as just another interfering signal and its frequency is
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
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.
3.3.07 Reference Channel Inputs
The 7270 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 routed to the reference
channel.
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 are selected from the three possible
options of internal, external front panel, and external rear panel inputs. The reference
circuit generates new phase values for each individual channel and sends these to the
Chapter 3, TECHNICAL DESCRIPTION
3-7
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