Stanford Research Systems SR810 Manual page 38

frequencies than within the operating range. While
this may be a nice specification, removing noise at
frequencies very far from the reference does not
require a lock-in amplifier. Lock-ins are used when
there is noise at frequencies near the signal. Thus,
the dynamic reserve for noise within the operating
range is more important.
Dynamic reserve in the SR810
The SR810, with its digital phase sensitive
detectors, does not suffer from DC output errors
caused by large noise signals. The dynamic
reserve can be increased to above 100 dB without
measurement error. Large noise signals do not
cause output errors from the PSD. The large DC
gain does not result in increased output drift.
In fact, the only drawback to using ultra high
dynamic reserves (>60 dB) is the increased output
noise due to the noise of the A/D converter. This
increase in output noise is only present when the
dynamic reserve is above 60 dB AND set to High
Reserve or Normal. However, the Low Noise
reserve can be very high as we'll see shortly.
To set a scale, the SR810's output noise at 100
dB dynamic reserve is only measurable when the
signal input is grounded. Let's do a simple
experiment. If the lock-in reference is at 1 kHz and
a large signal is applied at 9.5 kHz, what will the
lock-in output be? If the signal is increased to the
dynamic reserve limit (100 dB greater than full
scale), the output will reflect the noise of the signal
at 1 kHz. The spectrum of any pure sine generator
always has a noise floor, i.e. there is some noise
at all frequencies. So even though the applied
signal is at 9.5 kHz, there will be noise at all other
frequencies, including the 1 kHz lock-in reference.
This noise will be detected by the lock-in and
appear as noise at the output. This output noise
will typically be greater than the SR810's own
output noise. In fact, virtually all signal sources will
have a noise floor which will dominate the lock-in
output noise. Of course, noise signals are
generally much noisier than pure sine generators
and will have much higher broadband noise floors.
If the noise does not reach the reserve limit, the
SR810's own output
detectable at ultra high reserves. In this case,
simply lower the dynamic reserve and the DC gain
will decrease and the output noise will decrease
also. In general, do not run with more reserve than
noise may become
necessary. Certainly don't use High Reserve when
there is virtually no noise at all.
The frequency dependence of dynamic reserve is
inherent in the lock-in detection technique. The
SR810, by providing more low pass filter stages,
can increase the dynamic reserve close to the
reference frequency.
applies to noise signals within the operating range
of the lock-in, i.e. frequencies below 100 kHz. The
reserve at higher frequencies is actually higher but
is generally not that useful.
Minimum dynamic reserve (Low Noise)
The SR810 always has a minimum amount of
dynamic reserve. This minimum reserve is the
Low Noise reserve setting. The minimum reserve
changes with
instrument. At high gains (full scale sensitivity of
50 µV and below), the minimum dynamic reserve
increases from 37 dB at the same rate as the
sensitivity increases. For example, the minimum
reserve at 5 µV sensitivity is 57 dB. In many
analog lock-ins, the reserve can be lower. Why
can't the SR810 run with lower reserve at this
sensitivity?
The answer to this question is - Why would you
want lower reserve? In an analog lock-in, lower
reserve means less output error and drift. In the
SR810, more reserve does not increase the output
error or drift. More reserve can increase the output
noise though. However, if the analog signal gain
before the A/D converter is high enough, the
5 nV/√Hz noise of the signal input will be amplified
to a level greater than the input noise of the A/D
converter. At this point, the detected noise will
reflect the actual noise at the signal input and not
the A/D converter's noise. Increasing the analog
gain (decreasing the reserve) will not decrease the
output noise. Thus, there is no reason to decrease
the reserve. At a sensitivity of 5 µV, the analog
gain is sufficiently high so that A/D converter noise
is not a problem. Sensitivities below 5 µV do not
require any more gain since the signal to noise
ratio will not be improved (the front end noise
dominates). The SR810 does not increase the
gain below the 5 µV sensitivity, instead, the
minimum reserve increases. Of course, the input
gain can be decreased and the reserve increased,
in which case the A/D converter noise might be
detected in the absence of any signal input.
3-13
SR810 Basics
The specified
the sensitivity (gain)
reserve
of the