Output Processor - Output Filters - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION

3.3.13 Output Processor - Output Filters

Although shown on the block diagram as a separate entity, the output processor is in
fact part of the instrument's main microprocessor. It provides more digital filtering of
the X channel and Y channel signals if this is needed in addition to that already
performed by the demodulators. As with most lock-in amplifiers, the output filter
configuration in the model 7280 is controlled by the slope control. This may seem
somewhat strange, and a few words of explanation may be helpful.
In traditional audio terminology, a first-order low-pass filter is described as having a
slope of 6 dB per octave because in the high frequency limit its gain is inversely
proportional to frequency (6 dB is approximately a factor of 2 in amplitude and an
octave is a factor of 2 in frequency); similarly a second-order low-pass filter is
described as having a slope of 12 dB per octave. These terms have become part of the
accepted terminology relating to lock-in amplifier output filters and are used in the
model 7280 to apply to the envelope of the frequency response function of the digital
finite impulse response (FIR) output filters. Accordingly the front-panel control
which selects the configuration of the output filters is labeled SLOPE and the options
are labeled 6, 12, 18, 24 dB/octave.
The 6 dB/octave filters are not satisfactory for most purposes because they do not
give good rejection of non-random interfering signals which can cause aliasing
problems as a result of the sampling process in the main ADC. However, the
6 dB/octave filter finds use where the lock-in amplifier is incorporated in a feedback
control loop, and in some situations where the form of the time-domain response is
critical. The user is recommended to use 12 dB/octave unless there is some definite
reason for not doing so.
Note that at short time constant settings the filter slope options are limited to 6 or
12 dB/octave.
The output time constant can be varied between 1 µs and 100 ks. When set to a value
between 1 µs and 1 ms or 4 ms, X and Y or X and Magnitude outputs are available at
the CH 1 and CH 2 outputs. At longer time constant settings, all outputs are valid
and available at the CH 1 and CH 2 outputs and as the internal digital values
reported to a remote computer or stored to the internal curve buffer. The large digital
displays and bar-graph indicators on the front panel have an effective minimum time
constant limit imposed by their update rates, which are 512 ms and 64 ms
respectively. As noted in section 3.3.12 above, in dual reference and dual harmonic
modes the analog outputs at time constants shorter than 1 ms are limited to X and
1
X .
2
The filters are of the finite impulse response type with the averaging time of each
section being equal to double the nominal time constant. These filters offer a
substantial advantage in response time compared with analog filters or digital infinite
impulse response (IIR) filters.
When the reference frequency is below 10 Hz the synchronous filter option is
available. This means that the actual time constant of the filter is not generally the
selected value T, but a value which is equal to an integer number of reference cycles.
If T is greater than 1 reference cycle, then the time constant is between T/2 and T.
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