Internal Oscillator - Voltage Control; Demodulators - Dual Phase Multipliers; Demodulators - Output Filters - Ametek 7230 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION

3.3.13 Internal Oscillator - Voltage Control

3.3.14 Demodulators - Dual Phase Multipliers

3.3.15 Demodulators - Output Filters

3-10
The auxiliary ADC 1 input can be used to modulate the internal oscillator output
frequency or amplitude. Controls allow a quiescent amplitude and/or frequency to be
set, and a translation function (i.e. frequency and/or amplitude change per volt
change at the input) to be specified.
The function of each of the two demodulators is to multiply the digitized output of
the signal channel by digital representations of cosine and sine waves at the
demodulation frequency, to generate respectively the X and Y channel outputs. In
normal operation the demodulation frequency is at the internal or external reference
frequency, but when detecting at a harmonic of this then it is at some multiple, n (the
reference harmonic number) of it.
The demodulator outputs are digitally scaled to provide the demodulator gain
control. As discussed earlier in section 3.3.04 this gain is adjusted as the AC Gain is
adjusted to maintain the selected full-scale sensitivities.
In normal single reference mode the Demodulator 2 function is inactive, but it is
brought into operation when dual reference or dual harmonic modes are selected.
The outputs from the X channel and Y channel multipliers feed the X channel and Y
channel output low-pass filters, implemented as Finite Impulse Response (FIR)
stages with selectable 6 or 12 dB/octave slope (roll-off). Further filtering can be
carried out within the main output processor, to allow 18 and 24 dB/octave slopes.
In traditional audio terminology, a first-order low-pass filter is described as having a
slope of 6 dB per octave. This is 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 7230 to apply to the envelope of the frequency response function of the
digital finite impulse response (FIR) output filters. Accordingly the web control
panel control which selects the configuration of the output filters is labeled SLOPE
and the options are labeled 6, 12, 18, 24 dB/octave. Note that at the shorter time
constant settings the filter slope options are limited to 6 or 12 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.
The filters are of the finite impulse response type with the averaging time of each
section being equal to double the nominal time constant. This in turn defines the
settling time following a step change in input signal as being 2  TC  n, where TC
is the time constant and n = 1 for 6 dB, 2 for 12 dB, 3 for 18 dB and 4 for 24 dB
slope settings. Hence, for example, the settling time after a step change at the input
when the TC is 100 ms and the slope is 12 dB/octave will be 400 ms.