Can A Band Pass Filter Speed Up Lock In Amplification?
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From: Bob Masta on 3 Jul 2010 08:07 On Fri, 2 Jul 2010 17:25:23 -0700 (PDT), Bret Cahill <Bret_E_Cahill(a)yahoo.com> wrote: <snip> > >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. If the two signals are in phase (implying that they have the same frequency) then when you multiply them together you will get terms at 0 Hz and twice the frequency. Sure, you could tune another circuit to 2f, but then what was the point of the multiplication in the first place? How are you any farther ahead than if you had just tuned your circuit to the original frequency? 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 3 Jul 2010 12:28 > >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. > > If the two signals are in phase (implying that > they have the same frequency) then when you > multiply them together you will get terms at 0 Hz > and twice the frequency. Sure, you could tune > another circuit to 2f, but then what was the point > of the multiplication in the first place? Wouldn't the multiplication increase the [product signal] SNR? The SNR of the original is somewhere between 0.3 - 3. > How are > you any farther ahead than if you had just tuned > your circuit to the original frequency? The goal was to use the product signal to somehow get a clean reference for the original noisy signals, but if it's just as easy to glean the phase and frequency from the original signals, that step is unnecessary at best. Bret Cahill > 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 3 Jul 2010 15:04 > 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? Instead of prefiltering the signal for the signal input, prefilter the signal to get something that would trigger a clean reference signal. Bret Cahill
From: Bob Masta on 4 Jul 2010 08:59 On Sat, 3 Jul 2010 09:28:23 -0700 (PDT), Bret Cahill <Bret_E_Cahill(a)yahoo.com> wrote: >> >Supposing you cannot get a good clean reference, just another noisy >> >signal where the second signal is in phase with the first? =A0The >> >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. =A0If 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. >> >> If the two signals are in phase (implying that >> they have the same frequency) then when you >> multiply them together you will get terms at 0 Hz >> and twice the frequency. Sure, you could tune >> another circuit to 2f, but then what was the point >> of the multiplication in the first place? > >Wouldn't the multiplication increase the [product signal] SNR? It might... I'll have to think about this (and maybe run some experiments with Daqarta). I think the best you could hope for would be a 3 dB improvement. That's what you'd get if you just added them together and divided by 2 (synchronous averaging), assuming that the noise in each signal is uncorrelated with the other's noise, while the underlying desired waves are identical. But synchronous averaging assumes that the desired portions of each signal are identical in shape, frequency, and amplitude. I don't think you intended to assume identical amplitudes, just shapes and frequencies. So if the multiplier idea can deal with different amplitudes, it might be useful. I'll report back tomorrow... 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: Bob Masta on 4 Jul 2010 09:07 On Sat, 3 Jul 2010 12:04:12 -0700 (PDT), Bret Cahill <Bret_E_Cahill(a)yahoo.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? > >Instead of prefiltering the signal for the signal input, prefilter the >signal to get something that would trigger a clean reference signal. > That's what the PLL is there for... it "filters" the signal you provide to extract the reference. The question is whether there are conditions where a pre-filter on the PLL would improve overall lock time, as opposed to changes in the PLL itself. Dunno about that, but note that since this implies that you know the desired frequency pretty well, you might instead choose to apply that knowledge to the PLL oscillator control. such that in the absence of a signal it runs at the desired center frequency, and has an overall frequency range that matches the known signal range. That might speed up lock time. Just a thought. 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!
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