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42
Basics
The Reference Oscillator
A lock-in amplifier requires a reference oscillator phase-locked to the signal frequency.
In general, this is accomplished by phase-locking an internal oscillator to an externally
provided reference signal. This reference signal usually comes from the signal source that
is providing the excitation to the experiment.
Reference Input
The SR865A reference input can trigger on an analog signal (like a sine wave) or a TTL
logic signal. The first case is called External Sine. The input is ac coupled (above 1 Hz)
and the input impedance is 1 MΩ. A sine wave input greater than 200 mV pk is required.
Positive zero crossings are detected and considered to be the zero for the reference phase
shift.
TTL reference signals can be used at all frequencies up to 4 MHz. For frequencies
below 1 Hz, a TTL reference signal is required. Many function generators provide a
TTL SYNC output which can be used as the reference. This is convenient since the
generator's sine output might be smaller than 200 mV or be varied in amplitude. The
SYNC signal will provide a stable reference regardless of the sine amplitude.
When using a TTL reference, the reference input trigger can be set to Pos TTL (detect
rising edges) or Neg TTL (detect falling edges). In each case, the internal oscillator is
locked (at zero phase) to the detected edge.
Internal Oscillator
The internal oscillator in the SR865A is basically a 4 MHz function generator with sine
and sync outputs. The oscillator generates a digitally synthesized sine wave. The internal
oscillator sine wave is output differentially at the SINE OUT BNC's on the front panel.
An attenuator sets the amplitude of the output to a value between 1 nV and 2 V (rms).
When an external reference is used, this internal oscillator sine wave is phase-locked to
the reference. The rising zero crossing is locked to the detected reference zero crossing or
edge. In this mode, the SINE OUT provides a sine wave phase-locked to the external
reference. Phase locking is accomplished digitally by the SR865A.
The internal oscillator may be used without an external reference. In the Internal
Reference mode the frequency is set in the lock-in and the SINE OUT provides the
excitation for the experiment. The phase-locked-loop is not used in this mode.
The BlazeX output on the rear panel can be configured to provide the sync output. The
internal oscillator's rising zero crossings are detected and the output is a square wave.
Reference Oscillators and Phase
The internal oscillator sine wave is not the reference signal to the phase sensitive
detectors. The SR865A computes a second sine wave, phase shifted by θ
internal oscillator (and thus from an external reference), as the reference input to the X
phase sensitive detector. This waveform is sin(ω
adjustable in 0.001° increments.
SR865A DSP Lock-in Amplifier
t + θ
). The reference phase shift is
r
ref
Chapter 2
from the
ref
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