0
\$\begingroup\$

In this white paper of Zurich instruments two ways of explaining a lock-in amplifier are shown: low-pass filtering and time-averaging. Whereas the explanation with time-averaging can easily be done with complex numbers, the low-pass filtering seems to be more complicated (since it involves Fourier analysis to get from the time to the frequency domain?): https://www.zhinst.com/others/en/lock-amplifiers

A larger bandwidth in the frequency domain seems to correspond to a smaller time interval for time-averaging in the time domain. Therefore, there is a trade-off between good time resolution and high SNR. The signal buried in noise can vary over time, therefore it is important to set the right bandwidth. Most modern lock-in amplifiers work digitally by applying a low-pass filter, right? But couldn't they also perform the mathematical operation in the time domain (multiplying the sin and cos / complex numbers) ?

My question is if somebody can explain this relationship between the time and frequency domain more exactly. I couldn't really find any information about that. In the white paper it is also not explictly explained. And what exactly does the lock-in amplifier do when it applies a digital low-pass filter?

\$\endgroup\$
3
  • \$\begingroup\$ Most (if not all) lock-in amplifier do the maths using sin and cos so I'm not sure how that affects your question. \$\endgroup\$
    – Andy aka
    Commented Jan 27, 2021 at 15:51
  • \$\begingroup\$ Essentially, low pass filtering IS time domain averaging. You use averaging to reduce short-term fluctuations to see long term trends. That is precisely the same as reducing high frequency content. The simple un-weighted average, (sum of N samples / N) is a rectangular impulse response, and will have a sin(x)/x frequency response (aka sinc filter). \$\endgroup\$
    – user16324
    Commented Jan 27, 2021 at 16:33
  • \$\begingroup\$ I followed the link you provided, scrolled down to the white paper on the principles of operation, and figure 5 therein seems to address your question. The white paper is quite good: I downloaded it for careful reading later. \$\endgroup\$
    – Ed V
    Commented Jan 27, 2021 at 18:03

1 Answer 1

3
\$\begingroup\$

Time- and frequency-domain representations of lock-in amplifiers are equivalent. The signal processing algorithms to implement a lock-in amplifier are carried out in the time domain. However, its functionality becomes more intuitive when described in the frequency domain.

A lock-in amplifier receives a periodic signal at a known frequency and outputs the amplitude and phase of that signal with respect to a reference at the same frequency. This is done by a dual-phase (0° and 90°) demodulator as shown in the figure below. Each branch of the demodulator includes a mixer followed by a low-pass filter (LPF).

Dual-phase demodulator as the core of a lock-in amplifier

When mixing two signals with the same frequency ω, you generate two components at frequency difference and frequency sum i.e., 0 and 2ω. The LPF is supposed to remove the 2ω component from the signal and only allow the DC component to pass through. Therefore, the filter bandwidth must be much less than 2ω in order to effectively suppress the undesired frequency component. Moreover, the LPF rejects the out-of-band noise to improve the signal-to-noise ratio (SNR) of the measurement. The lower the bandwidth is, the higher the SNR. In a nutshell, the LPF of a lock-in amplifier has two tasks: 1) suppressing the unwanted frequency components and, 2) rejecting the out-of-band noise.

In the time-domain, a low-pass filter is represented and implemented by a weighted integrator in which the signal is multiplied by a weight function and then the outcome is integrated over the time continuously. The weight function is determined by the filter type. For instance, a sinc filter has a uniform weight function which results in a simple moving averager; while, for a Butterworth filter, the weight function is exponential and thus the integrator is like an exponential moving averager. The time-constant associated with the exponential weight function is inversely proportional to the filter bandwidth as shown in Table 1 of the White Paper. Standard digital lock-in amplifiers use the Butterworth filter which can be efficiently implemented by IIR filters in the time domain. The temporal response of the Butterworth filter of several orders is provided here.

In summary, the digital signal processing algorithms used in lock-in amplifiers including mixers and low-pass filters are implemented in the time domain, while their functionality is better explained, interpreted, and understood by the users in the frequency domain.

\$\endgroup\$
1
  • \$\begingroup\$ There is always a trade between the temporal response and the frequency attenuation. They describe a distributed RC filter, which might converge to a Gaussian, but a Bessel filter might be better because of slightly better attenuation for approximately the same effect but better group delay (as N goes up). Other solutions involve adaptive moving averages, or other non-LTI filters. (edit) Just watching the video, where they do mention the trade-off. \$\endgroup\$ Commented Feb 9, 2021 at 20:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.