Can A Band Pass Filter Speed Up Lock In Amplification?
in [Basics]
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From: Bret Cahill on 1 Jul 2010 20:45 Is prefiltering or filtering before the phase sensitive detection step of lock in a common practice? If the signal frequency is in a fairly narrow band, say one octave, can a band pass filter speed up the aquistion time of a lock in? Bret Cahill
From: Bob Masta on 2 Jul 2010 08:34 On Thu, 1 Jul 2010 17:45:07 -0700 (PDT), Bret Cahill <BretCahill(a)peoplepc.com> wrote: >Is prefiltering or filtering before the phase sensitive detection step >of lock in a common practice? > >If the signal frequency is in a fairly narrow band, say one octave, >can a band pass filter speed up the aquistion time of a lock in? > I think you may be conflating "lock-in amplifier" with "phase locked loop". Lock-in amps typically use PLLs to acquire their own internal copy of an external reference signal. Modern lock-ins then multiply sine and cosine versions of this reference by the input signal to be measured. In many applications, where the reference is under your control (you are generating it), the PLL is totally superfluous and is actually detrimental, due to the long lock time. If you generate sine and cosine versions of the reference and feed them to the multipliers directly, there is no "lock" time. Then the only time lag is due to the low-pass output filter that follows each multipler... the narrower the ultimate bandwidth, the longer the lag. In that respect, it's just the same as if you had (somehow) built up a super-duper narrowband bandpass filter from conventional circuitry instead of going the multiplier/low-pass route: The ultimate bandwidth determines the lag. Typically, lock-ins are used to get ultra-narrow bandwidths (1 Hz or less, often *way* less), which you couldn't approach with a conventional analog bandpass filter due to impossible Q (and hence stability) requirements. Preceding the signal input with a filter will only add the delay of that filter to the lock-in process. It won't improve the response of the overall output. (Not to mention that in most situations the external pre-filter will be orders of magnitude wider than the ultimate lock-in bandwidth anyway.) However, the "acquisition time" that lock-in specs mention has to do with the PLL lock time. Nothing you put on the signal input will help that, and anything you put on the reference input will most likely degrade it. Best regards, Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!
From: George Herold on 2 Jul 2010 09:33 On Jul 1, 8:45 pm, Bret Cahill <BretCah...(a)peoplepc.com> wrote: > Is prefiltering or filtering before the phase sensitive detection step > of lock in a common practice? > > If the signal frequency is in a fairly narrow band, say one octave, > can a band pass filter speed up the aquistion time of a lock in? > > Bret Cahill I have no idea what you mean by 'speed up'. The old EG&G 124A http://www.kandelelectronics.com/products/6980/ had a band pass filter on the front end. This was to increase the dynamic range. (Filter and then amplify some more!) Why do you say your signal frequency is in a narrow band? Typically the lockin has a single frequency. (The modulation frequency.) If you are changing the modulation frequency then the phase shift of the band pass filter might cause some problems. George H.
From: Bret Cahill on 2 Jul 2010 10:30 > >Is prefiltering or filtering before the phase sensitive detection step > >of lock in a common practice? > > >If the signal frequency is in a fairly narrow band, say one octave, > >can a band pass filter speed up the aquistion time of a lock in? > > I think you may be conflating "lock-in amplifier" > with "phase locked loop". Lock-in amps typically > use PLLs to acquire their own internal copy of an > external reference signal. Modern lock-ins then > multiply sine and cosine versions of this > reference by the input signal to be measured. The original intent was for the reference to be generated externally. > In many applications, where the reference is under > your control (you are generating it), the PLL is > totally superfluous and is actually detrimental, > due to the long lock time. If you generate sine > and cosine versions of the reference and feed them > to the multipliers directly, there is no "lock" > time. Then the only time lag is due to the > low-pass output filter that follows each > multipler... That still takes time. > the narrower the ultimate bandwidth, > the longer the lag. Which could be years . . > In that respect, it's just the same as if you had > (somehow) built up a super-duper narrowband > bandpass filter from conventional circuitry > instead of going the multiplier/low-pass route: > The ultimate bandwidth determines the lag. The low pass filtering operation of band pass should always take more time than the high pass step. It doesn't take any time to eliminate dc. > Typically, lock-ins are used to get ultra-narrow > bandwidths (1 Hz or less, often *way* less), > which you couldn't approach with a conventional > analog bandpass filter due to impossible Q (and > hence stability) requirements. It depends on the how much time you have to eliminate how much noise. > Preceding the signal input with a filter will only > add the delay of that filter to the lock-in > process. It won't improve the response of the > overall output. (Not to mention that in most > situations the external pre-filter will be orders > of magnitude wider than the ultimate lock-in > bandwidth anyway.) > > However, the "acquisition time" that lock-in specs > mention has to do with the PLL lock time. Nothing > you put on the signal input will help that, and > anything you put on the reference input will most > likely degrade it. If you have a good in-phase reference then adding on more "blind" forms of filtering, even adaptive filtering using a reference of unknown phase angle filtering, isn't going to save any time. Bret Cahill > Best regards, > > Bob Masta > > DAQARTA v5.10 > Data AcQuisition And Real-Time Analysis > www.daqarta.com > Scope, Spectrum, Spectrogram, Sound Level Meter > Frequency Counter, FREE Signal Generator > Pitch Track, Pitch-to-MIDI > DaqMusic - FREE MUSIC, Forever! > (Some assembly required) > Science (and fun!) with your sound card!
From: Bret Cahill on 2 Jul 2010 20:25
> > Is prefiltering or filtering before the phase sensitive detection step > > of lock in a common practice? > > > If the signal frequency is in a fairly narrow band, say one octave, > > can a band pass filter speed up the aquistion time of a lock in? > > > Bret Cahill > > I have no idea what you mean by 'speed up'. It takes a certain amount of time for the ac signal + ac noise, after it is converted to a dc signal + ac noise, to integrate and overwhelm the ac noise. > The old EG&G 124A > > http://www.kandelelectronics.com/products/6980/ > > had a band pass filter on the front end. This was to increase the > dynamic range. (Filter and then amplify some more!) > > Why do you say your signal frequency is in a narrow band? Typically > the lockin has a single frequency. (The modulation frequency.) If > you are changing the modulation frequency then the phase shift of the > band pass filter might cause some problems. This was originally about the multiplication of a noisy signal by a good clean reference. Both the signal and reference always have the same phase angle, 0, but the frequency of both change [together] over a narrow frequency range. Supposing you cannot get a good clean reference, just another noisy signal where the second signal is in phase with the first? The product of two noisy signals is a rectified signal plus ac noise -- just like in conventional phase sensitive detection except the magnitude of the rectified signal has no use. If the product of the two signals isn't desired the only thing the product could be used for is the frequency which would need to be picked out by tuning another circuit to that frequency. Bret Cahill |