Tel: 886. 909 602 109 Email: [email protected]
www.salukitec.com
9
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 SE1201 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 SE1201 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.
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
drift. This will cause errors in the reference and greater errors in the results of coherent modulation.
The dynamic reserve of an analog PSD's is limited to about 60 dB, because there are always many background noises.
When there is a large noise signal present, 1000 times or 60dB greater than the full-scale signal, the analog PSD measures
the signal with an error. Because the lock-in amplifier is mainly used to detect weak signals, when the amplitude of
background noise is similar to or larger than the signal amplitude, the results of coherent modulation will be wrong.