Ametek 7230 Instruction Manual page 56

Chapter 5, WEB CONTROL PANEL OPERATION
5-14
and provides detection either at the reference frequency or one harmonic of it.
Virtual Reference
Virtual reference mode is an extension of internal single reference mode operation,
where the Y channel output is used to make continuous adjustments to the internal
oscillator frequency and phase. The effect is to achieve phase-lock with the applied
signal such that the X channel output is maximized and the Y channel output is
zeroed. Virtual reference mode operation is only possible with signals at frequencies
between 100 Hz and 250 kHz.
Dual Reference
In dual reference mode the model 7230 can make simultaneous measurements at two
different reference frequencies, internal or external. If using two external references,
one of them is restricted to being no greater than 3.0 kHz.
Tandem Demodulation
Some experiments, such as pump-probe optical work, generate an amplitude
modulated carrier signal. The carrier signal is at one, typically higher, frequency, and
the modulation at another, lower, frequency. The required measurement is the
amplitude and/or phase of the modulation.
Conventionally, lock-in detection of this signal would require two instruments. The
first would be run at the carrier frequency with its output filters set to a sufficiently
short time constant to allow the amplitude modulation to pass. Hence the X-channel
analog output would be the modulation that was to be measured; to achieve this, the
signal would then be passed to the signal input of a second lock-in amplifier running
at the modulation frequency.
The built-in Tandem Demodulation mode, which is essentially the same as dual
reference mode but with the input to the second set of demodulators taken as the X-
channel output from the first, allows the 7230 to make this measurement in a single
instrument.
Dual Harmonic
Dual harmonic mode allows the simultaneous measurement of two different
harmonics of the input signal.