Reference Channel - Stanford Research Systems SR810 Manual

REFERENCE CHANNEL

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 which is providing the excitation
to the experiment.
Reference Input
The SR810 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 will trigger the input discriminator.
Positive zero crossings
considered to be the zero for the reference phase
shift.
TTL reference signals can be used at all
frequencies up to 102 kHz. 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 Edge (detect rising
edges) or Neg Edge (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 SR810 is basically a
102 kHz function generator with sine and TTL
sync outputs. The oscillator can be phase-locked
to the external reference.
The oscillator generates a digitally synthesized
sine wave. The digital signal processor, or DSP,
sends computed sine values to a 16 bit digital-to-
analog converter every 4 µs (256 kHz). An anti-
aliasing filter converts this sampled signal into a
low distortion sine wave. The internal oscillator
sine wave is output at the SINE OUT BNC on the
front panel. The amplitude of this output may be
set from 4 mV to 5 V.
are detected and
3-5
SR810 Basics
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
frequencies (below 10 Hz), the phase locking is
accomplished digitally by the DSP. At higher
frequencies, a discrete phase comparator is used.
The internal oscillator may be used without an
external reference. In the Internal Reference
mode, the SINE OUT provides the excitation for
the experiment. The phase-locked-loop is not
used in this mode since the lock-in reference is
providing the excitation signal.
The TTL OUT on the rear panel provides a TTL
sync output. The internal oscillator's rising zero
crossings are detected and translated to TTL
levels. This 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 DSP computes a second sine wave, phase
shifted by θ
from the internal oscillator (and thus
ref
from an external reference), as the reference input
to the X phase sensitive detector. This waveform
t + θ
is sin(ω ). The reference phase shift is
r ref
adjustable in .01° increments.
The input to the Y PSD is a third sine wave,
computed by the DSP, shifted by 90° from the
second sine wave. This waveform is sin(ω
90°).
Both reference sine waves are calculated to 20
bits of accuracy and a new point is calculated
every 4 µs (256 kHz). The phase shifts (θ
the 90° shift) are also exact numbers and accurate
to better than .001°. Neither waveform is actually
output in analog form since the phase sensitive
detectors are actually multiply instructions inside
the DSP.
Phase Jitter
When an external reference is used, the phase-
locked loop adds a little phase jitter. The internal
oscillator is supposed to be locked with zero
phase shift relative the external reference. Phase
jitter means that the average phase shift is zero
reference. At low
t + θ
+
r ref
and
ref