Phase-Shifter - Ametek 7280 Instruction Manual

Chapter 3, TECHNICAL DESCRIPTION

3.3.08 Phase-Shifter

3-8
waveform at the selected harmonic of the reference frequency. Dual harmonic mode
operates in a similar way to dual reference mode, but in this case the reference circuit
generates phase values for both of the selected harmonics of the reference frequency.
Dual harmonic mode may therefore be used with either internal or external
references.
External Reference Mode
In external reference mode the reference source may be applied to either a general
purpose input, designed to accept virtually any periodic waveform with a 50:50
mark-space ratio and of suitable amplitude, or to a TTL-logic level input. Following
the trigger buffering circuitry the reference signal is passed to a digital phase-locked
loop (PLL) implemented in the reference circuit. This measures the period of the
applied reference waveform and from this generates the phase values.
Internal Reference Mode
With internal reference operation the reference circuit is free-running at the selected
reference frequency and is not dependent on a phase-locked loop (PLL), as is the case
in most other lock-in amplifiers. Consequently, the phase noise is extremely low, and
because no time is required for a PLL to acquire lock, reference acquisition is
immediate.
Both the signal channel and the reference channel contain calibration parameters
which are dependent on the reference frequency. These include corrections to the
anti-alias filter and to the analog circuits in the reference channel. In external
reference operation the processor uses a reference frequency meter to monitor the
reference frequency and updates these parameters when a change of about 2 percent
has been detected.
A TTL logic signal at the present reference frequency is provided at the REF MON
connector on the rear panel.
The reference circuit also implements a digital reference phase-shifter, allowing the
phase values being sent to the demodulator DSP to be adjusted to the required value.
If the reference input is a sinusoid applied to the REF IN socket, the reference phase
is defined as the phase of the X demodulation function with respect to the reference
input.
This means that when the reference phase is zero and the signal input to the
demodulator is a full-scale sinusoid in phase with the reference input sinusoid, the X
channel output of the demodulator is a full-scale positive value and the Y channel
output is zero.
The circuits connected to the REF IN socket detect a positive-going crossing of the
mean value of the applied reference voltage. Therefore when the reference input is
not sinusoidal, its effective phase is the phase of a sinusoid with a positive-going zero
crossing at the same point in time, and accordingly the reference phase is defined
with respect to this waveform. Similarly, the effective phase of a reference input to
the TTL REF IN socket is that of a sinusoid with a positive-going zero crossing at
the same point in time.