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In the case of internal reference mode, the internal oscillator can generate a digitally synthesized sine wave which can
multiply with input signal. The phase-locked-loop(PLL) is not used since the lock-in reference is providing the excitation.
The phase noise will not affect the internal reference signal. This mode can work normally on from 1mHz to 3MHz.
In the case of external reference mode, an external sine wave signal or TTL logic signal can be used as the external
reference signal. PLL will be used in this mode, but will generate a little phase jitter and this 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 causes the reference signal is doped with 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 SE2031 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.
1.4 Phase Sensitive Detectors
The PSD in the SE2031 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 32 bit of accuracy, the harmonics have -120 dB roll off. That is to say, the harmonics do not affect
the products of the PSD.
Fig.5 PSD diagram
Because the PSD based on analog method has temperature drift, there are always some deviation between the output and
actual result that is the uncertain system error. While the PSD based on digital method has a precise amplitude and never
change, so it will not generate any system errors. This eliminates a major source of gain error in a linear analog lock-in.
Considering that the inputs of analog multiplier are analog quantity, the reference signal will be affected by temperature
