Dynamic Reserve - Stanford Research Systems SR810 Manual

SR810 Basics

DYNAMIC RESERVE

We've mentioned dynamic reserve quite a bit in
the preceding discussions. It's time to clarify
dynamic reserve a bit.
What is dynamic reserve really?
Suppose the lock-in input consists of a full scale
signal at fref plus noise at some other frequency.
The traditional definition of dynamic reserve is the
ratio of the largest tolerable noise signal to the full
scale signal, expressed in dB. For example, if full
scale is 1 µV, then a dynamic reserve of 60 dB
means noise as large as 1 mV (60 dB greater than
full scale) can be tolerated at the input without
overload.
The problem with this definition is the word
'tolerable'. Clearly the noise at the dynamic
reserve limit should not cause an overload
anywhere in the instrument - not in the input signal
amplifier, PSD, low pass filter or DC amplifier. This
is accomplished by adjusting the distribution of the
gain. To achieve high reserve, the input signal
gain is set very low so the noise is not likely to
overload. This means that the signal at the PSD is
also very small. The low pass filter then removes
the large noise components from the PSD output
which allows the remaining DC component to be
amplified (a lot) to reach 10 V full scale. There is
no problem running the input amplifier at low gain.
However, as we have discussed previously,
analog lock-ins have a problem with high reserve
because of the linearity of the PSD and the DC
offsets of the PSD and DC amplifier. In an analog
lock-in, large noise signals almost always disturb
the measurement in some way.
The most common problem is a DC output error
caused by the noise signal. This can appear as an
offset or as a gain error. Since both effects are
dependent upon the
frequency, they can not be offset to zero in all
cases and will limit the measurement accuracy.
Because the errors are DC in nature, increasing
the time constant does not help. Most lock-ins
define tolerable noise as noise levels which do not
affect the output more than a few percent of full
scale. This is more severe than simply not
overloading.
Another effect of high dynamic reserve is to
generate noise and drift at the output. This comes
about because the DC output amplifier is running
noise amplitude and
at very high gain and low frequency noise and
offset drift at the PSD output or the DC amplifier
input will be amplified and appear large at the
output. The noise is more tolerable than the DC
drift errors since increasing the time constant will
attenuate the noise. The DC drift in an analog
lock-in is usually on the order of 1000ppm/°C
when using 60 dB of dynamic reserve. This means
that the zero point moves 1% of full scale over
10°C temperature change. This is generally
considered the limit of tolerable.
Lastly, dynamic reserve depends on the noise
frequency. Clearly
frequency will make its way to the output without
attenuation. So the dynamic reserve at fref is 0dB.
As the noise frequency moves away from the
reference frequency,
increases. Why? Because the low pass filter after
the PSD attenuates the noise components.
Remember, the PSD outputs are at a frequency of
|fnoise-fref|. The rate at which the reserve
increases depends upon the low pass filter time
constant and roll off. The reserve increases at the
rate at which the filter rolls off. This is why
24 dB/oct filters are better than 6 or 12 dB/oct
filters. When the noise frequency is far away, the
reserve is limited by the gain distribution and
overload level of each gain element. This reserve
level is the dynamic reserve referred to in the
specifications.
actual reserve
60 dB
40 dB
20 dB
0 dB
The above graph shows the actual reserve vs the
frequency of the noise. In some instruments, the
signal input attenuates frequencies far outside the
lock-in's operating range (f
cases, the reserve can be higher at these
3-12
noise at the reference
the dynamic reserve
60 dB specified reserve
low pass filter
bandwidth
f f
ref noise
>>100 kHz). In these
noise