Phase Sensitive Detectors - Stanford Research Systems SR810 Manual

THE PHASE SENSITIVE DETECTORS (PSD's)
The SR810 multiplies the signal with the reference
sine waves digitally. The amplified signal is
converted to digital form using a 16 bit A/D
converter sampling at 256 kHz. The A/D converter
is preceded by a 102 kHz anti-aliasing filter to
prevent higher frequency inputs from aliasing
below 102 kHz. The signal amplifier and filters will
be discussed later.
This input data stream is multiplied, a point at a
time, with the computed reference sine waves
described previously. Every 4 µs, the input signal
is sampled and the result is multiplied by the two
reference sine waves (90° apart).
Digital PSD vs Analog PSD
The phase sensitive detectors (PSD's) in the
SR810 act as linear multipliers, that is, they
multiply the signal with a reference sine wave.
Analog PSD's (both square wave and linear) have
many problems associated with them. The main
problems are harmonic rejection, output offsets,
limited dynamic reserve and gain error.
The digital PSD multiplies the digitized signal with
a digitally computed
Because the reference sine waves are computed
to 20 bits of accuracy, they have very low
harmonic content. In fact, the harmonics are at the
-120 dB level! This means that the signal is
multiplied by a single reference sine wave (instead
of a reference and its many harmonics) and only
the signal at this single reference frequency is
detected. The SR810 is completely insensitive to
signals at harmonics of the reference. In contrast,
a square wave multiplying lock-in will detect at all
of the odd harmonics of the reference (a square
wave contains many large odd harmonics).
Output offset is a problem because the signal of
interest is a DC output from the PSD and an
output offset contributes to error and zero drift.
The offset problems of analog PSD's are
eliminated using the digital multiplier. There are no
erroneous DC output offsets from the digital
multiplication of the signal and reference. In fact,
the actual multiplication is totally free from errors.
The dynamic reserve of an analog PSD is limited
to about 60 dB. When there is a large noise signal
reference sine wave.
present, 1000 times or 60 dB greater than the full
scale signal, the analog PSD measures the signal
with an error. The error is caused by non-linearity
in the multiplication (the error at the output
depends upon the amplitude of the input). This
error can be quite large (10 % of full scale) and
depends upon the noise amplitude, frequency,
and waveform. Since noise generally varies quite
a bit in these parameters, the PSD error causes
quite a bit of output uncertainty.
In the digital lock-in, the dynamic reserve is limited
by the quality of the A/D conversion. Once the
input signal is digitized, no further errors are
introduced. Certainly
multiplication does not depend on the size of the
numbers. The A/D converter used in the SR810 is
extremely linear, meaning that the presence of
large noise signals does not impair its ability to
correctly digitize a small signal. In fact, the
dynamic reserve of the SR810 can exceed 100 dB
without any problems. We'll talk more about
dynamic reserve a little later.
An analog linear PSD multiplies the signal by an
analog reference sine wave. Any amplitude
variation in the reference amplitude shows up
directly as a variation in the overall gain. Analog
sine wave generators are susceptible to amplitude
drift, especially as a function of temperature. The
digital reference sine wave has a precise
amplitude and never changes. This eliminates a
major source of gain error in a linear analog lock-
in.
The overall performance of a lock-in amplifier is
largely determined by the performance of its
phase sensitive detectors. In virtually all respects,
the digital PSD
counterparts.
We've discussed how the digital signal processor
in the SR810 computes the internal oscillator and
two reference sine waves and handles both phase
sensitive detectors. In the next section, we'll see
the same DSP perform the low pass filtering and
DC amplification required at the output of the
PSD's. Here again, the digital technique eliminates
many of the problems associated with analog lock-
in amplifiers.
3-7
SR810 Basics
the accuracy of
outperforms its
the
analog