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below 1Hz, a TTL reference signal rather than a sine refenrence signal is recommended.
The SE1022D lock-in amplifier has two reference signal modes: internal reference mode and external reference mode.
In internal reference mode, the internal oscillator generates a digitally synthesized sine wave which is used to multiply
with the input signal. The phase-locked-loop (PLL) is not used since the lock-in reference provides the excitation. The
phase noise will not affect the internal reference signal. The phase noise is extremely low. This mode can work normally
from 1mHz to 102kHz.
In external reference mode, an external sine wave or TTL logic signal can be used as the external reference signal. PLL
will be used in this mode, but it will generate a little phase jitter which may cause measurement errors.
The phase jitter means that average phase shift is zero but the instantaneous phase shift has a few milli-degrees of noise.
The phase jitter makes the reference signal plus noise at different frequencies. According to the coherence principle of
PSD, the output is not a single frequency, but a distribution of frequencies about the true reference frequency.
In fact, phase noise in the SE1022D is very low and generally has no effect. In applications that requiring no phase jitter,
the internal reference mode should be chose. Since there is no PLL in internal mode. The internal oscillator and the
reference sine waves are directly linked and there is no jitter in the measured phase.
Additionally, channel B can be set to single reference mode while channel A is in either internal or external reference
mode, which takes signal of channel A as the reference signal for channel B. In this way, time clock of both channels will
be synchronous during the whole process of weak signal measurement.
1.4 Phase Sensitive Detectors
The PSD in the SE1022D acts as a digital multiplier as is shown in Fig.5. The input signal amplified and filtered is
converted to digital signal by a 24-bits A/D converter and then goes into the PSD. The reference sine wave is computed to
24 bit of accuracy, and the accuracy of the whole PSD is 48 bit.
The PSD module in lock-in amplifier is mainly used to implement the coherent modulation of the input signal and
reference signal. Generally, there are two kinds of phase-sensitive detectors (PSD's): digital PSD's and analog PSD's.
Traditional PSD's use an analog multiplier to multiply the input signal with the reference signal. There are many problems
associated with these, including harmonic rejection, output offsets, limited dynamic reserve and gain error. It will limit the
accuracy of PSD's and bring in various noises.
The digital PSD multiplies the digitized signal with a digitally computed reference sine wave. Because the reference sine
wave is computed to 24 bit of accuracy, the harmonics have -120 dB roll off. That is to say, the harmonics do not affect
the products of the PSD.
