Wtb: lock-in amplifier (found one!) - Page 1
Forum section: electronics / general

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 03, 2020, 12:27:46 pm
content: Hi! Looking for a lock-in amplifier. Preferably in europe, but anything goes.Can be fairly vintage, doesnt need to be digital.Anyone?Thanks!--Chris

author: precaud
author_rank: Frequent Contributor
timestamp: September 03, 2020, 12:30:55 pm
content: How many channels? What bandwidth?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 03, 2020, 02:28:38 pm
content: one, and ideally 100 khz - but my requirements are pretty flexible.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 03, 2020, 03:32:57 pm
content: If you don't need crazy low level noise and not in a big hurry, just tinker and try to DIY one. I suspect you need it for a specific measurement, not a generic one with many amplifications, filters, etc.Saying this because I was looking for one a few months ago and it wasn't cheap (in US). In EU there was none for sale at that time.

author: _Wim_
author_rank: Super Contributor
timestamp: September 03, 2020, 04:39:16 pm
content: For the older analog units, they can be bought for sub 100€ on Ebay if you are a bit patient. Look for Princeton Research, EG&G or Ithaco. Do not look for "lock-in amplifier", as the best deals can be got from sellers who do not know what they are selling.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 03, 2020, 05:11:30 pm
content: That's what I'm currently doing, but one could get lucky that somebody had one laying arondBrookdeal is also a common brand that turns up.

author: _Wim_
author_rank: Super Contributor
timestamp: September 03, 2020, 07:16:32 pm
content: Quote from: ChristofferB on September 03, 2020, 05:11:30 pmI have a few lying around, but every now and then I use them also...

author: gnavigator1007
author_rank: Unknown
timestamp: September 03, 2020, 08:26:50 pm
content: So I have a very basic understanding of what a lock-in amplifier is, but I'm curious what scenarios one might be used. I came very close to ending up with several that were included in an auction of several pallets of various equipment that didn't all seem related.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 03, 2020, 08:28:46 pm
content: Quote from: gnavigator1007 on September 03, 2020, 08:26:50 pm

author: DaJMasta
author_rank: Super Contributor
timestamp: September 03, 2020, 08:31:51 pm
content: If you're just looking for deals that come around, I'd look for a dual phase version. Having the single output is usable, but it requires more complex setup and analysis, having the extra channel built in for X/Y outputs also lets you output magnitude and phase directly, among other things, so it becomes a fair bit more powerful/usable as a standalone unit (though I guess you're always going to have it in between the pickup and the digitizer/display).I've seen working SR530s go for as little as $400 or so, and since SRS equipment usually commands a premium, it's a pretty nice price, and the analog displays aren't bad for this sort of thing, you're usually going to be monitoring the output externally for any useful application. If you don't mind taking a chance, a lot of lock ins are sold for parts just because people don't know how to test them, so while a frontend or other bit may need replacing sometimes, there are working ones that can be found as deals.

author: lukier
author_rank: Unknown
timestamp: September 03, 2020, 08:39:44 pm
content: Would you be interested in Stanford Research Systems SR850? I don't have much use for it. I don't have any formal calibration, but I can run performance tests to show it is working correctly.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 03, 2020, 08:46:14 pm
content: Quote from: gnavigator1007 on Today at 08:26:50 pmSo I have a very basic understanding of what a lock-in amplifier is, but I'm curious what scenarios one might be used. I came very close to ending up with several that were included in an auction of several pallets of various equipment that didn't all seem related.You feed it a reference frequency/phase and a signal to be analyzed, you get the magnitute of JUST that frequency at that phase. Like an extremely narrowband radio receiver.It's useful because low-level high-noise signals can be modulated, and the modulation freq. monitored by the lock-in. Personally I'm going to use it for DIY EPR/NMRQuote from: DaJMasta on Today at 08:31:51 pmIf you're just looking for deals that come around, I'd look for a dual phase version. Having the single output is usable, but it requires more complex setup and analysis, having the extra channel built in for X/Y outputs also lets you output magnitude and phase directly, among other things, so it becomes a fair bit more powerful/usable as a standalone unit (though I guess you're always going to have it in between the pickup and the digitizer/display).I've seen working SR530s go for as little as $400 or so, and since SRS equipment usually commands a premium, it's a pretty nice price, and the analog displays aren't bad for this sort of thing, you're usually going to be monitoring the output externally for any useful application. If you don't mind taking a chance, a lot of lock ins are sold for parts just because people don't know how to test them, so while a frontend or other bit may need replacing sometimes, there are working ones that can be found as deals.Roger. That sounds like good advice. I'm also considering this deal, though the shipping is pretty expensive:https://www.ebay.com/itm/Pair-Physical-Electronics-32-010-Lockin-Amplifier-for-parts-repair/254667649965?hash=item3b4b5ff3ad:g:eEIAAOSwdAVfH4q5Few features, but a whole unit in spare parts

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 03, 2020, 08:49:34 pm
content: Quote from: lukier on September 03, 2020, 08:39:44 pmI'm afraid that's way too high-end for my budget. I'm kind of looking for something crusty and 1970's. Thanks, though. Looks like a very nice instrument!

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2020, 09:05:52 pm
content: I second the recommendation to get a two-phase unit.There are two types: both have the capability of outputting a DC voltage for "I" (in-phase) and "Q" (quadrature) components of the input signal, as well as displaying the two voltages on panel meters.To get magnitude and phase, one type uses analog (or digital in newer units) computation to compute magnitude and phase from those two voltages. This can be useful for obtaining a null signal (e.g., from a bridge) through a very narrow bandwidth.The other type servos the reference phase angle to null the Q signal, thereby obtaining the magnitude in the I channel and reporting the phase shift as the phase angle. This may be ok, but it does not work for the null case above, since there is no signal to activate the phase servo at null. With my PAR 129A (the second type), I connect two 3.5-digit DVMs to the I and Q output, and calculate magnitude and phase in a simple spreadsheet.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 03, 2020, 09:11:36 pm
content: So the second type can not operate in the simpler mode? that might be inconvenient, as I plan on driving an X-Y plotter directly.How difficult would it be to make an adapter box for the simpler, single output operation?

author: TimFox
author_rank: Super Contributor
timestamp: September 03, 2020, 09:51:45 pm
content: The second type, as my PAR 129A for example, can be set up to generate magnitude and phase signals, it just doesn't work down to very low magnitudes, as needed for nulling. The "I" meter and output reads magnitude, and the "Q" meter and output are re-purposed for phase.I generally use the I and Q outputs directly, but if you are using an analog X-Y plotter you can use the magnitude/phase outputs. On the PAR 129A, you need to set the front-panel switches to "red" values to use magnitude/phase.A single-phase unit reads only the "I" output, but you can manually adjust the reference phase to maximize it. A two-phase unit is better for variable frequency use. All lock-in amplifiers feature switchable averaging times (up to very long times) which is great for noise reduction, but limits how fast you can change the frequency.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 03, 2020, 09:55:52 pm
content: That clears things up, thanks! I've never actually used one.

author: Brumby
author_rank: Unknown
timestamp: September 04, 2020, 12:42:17 am
content: I have this:- but I do not know it's operational state .... and it's in Sydney.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2020, 07:39:00 am
content: A two channel version can be very convenient. For sending the data to a computer or the like it is usually better to look at I and Q and let the computer do the conversion to things like phase and magnitude of really needed. I would expect the 2 channel versions to all support this mode. There can be differences how (and if) the magnitude and phase more is implemented.Many of the older units are 1 phase only. For the very common use in optics experiments a little like in the TSP video shown a single phase version is sufficient. It needs an extra trimming of the phase setting once in a while, but than it is better to just look at one output. Using 2 channels an let the lock-in calculate the magnitude and phase in real time adds some noise from the 2 nd channel, if the signal is noisy. It is OK if the noise level is relatively low.For a truly complex signal with information in phase an amplitude the 2 phase version is a real advantage.Older lock-ins are also often slow, e.g. limited to a few kHz like typical for mechanical choppers. I remember a old type with fixed internal filters - so need to change a small daughter board to change the frequency.A more complicated part of the lockin is the handling of the reference signal input. Many lock-ins include some basic generator to also generate the reference signal, to drive an experiment. This case would be the easier one and one may be able to do a DIY solution for this with no that much effort. However it looks like used Lock-ins go for a relatively low price.

author: precaud
author_rank: Frequent Contributor
timestamp: September 04, 2020, 01:43:49 pm
content: Quote from: ChristofferB on September 03, 2020, 02:28:38 pmI have 2-3 that will do it, but they're in the US. You can PM me for details.

author: _Wim_
author_rank: Super Contributor
timestamp: September 05, 2020, 06:46:00 am
content: Quote from: ChristofferB on September 03, 2020, 08:46:14 pmThat seems expensive for a single phase unit. I would more likely consider this one, but would offer max 100$ for it. Full service manual is available for this unit. I am maybe biased off course, because I own a 5204https://www.ebay.com/itm/Princeton-Applied-Research-5204-Lock-In-Analyzer-FOR-PARTS/133011025905?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D20200818142525%26meid%3D0eac80c223654fc0a4c830ed41ad5abd%26pid%3D101195%26rk%3D2%26rkt%3D12%26mehot%3Dpf%26sd%3D153777546812%26itm%3D133011025905%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithBBEV2bDemotion&_trksid=p2047675.c101195.m1851

author: precaud
author_rank: Frequent Contributor
timestamp: September 05, 2020, 02:18:34 pm
content: I have:EG&G/PAR 5104 and 5204, I believe they both go to 20kHz.EG&G/PAR 5209 (120kHz) with GPIB. Same as Signal Recovery 5209ITHACO 3961 2-channel, 100kHz with GPIBPAR 200 2-channel, 100kHz with GPIB

author: _Wim_
author_rank: Super Contributor
timestamp: September 05, 2020, 06:28:55 pm
content: Quote from: precaud on September 05, 2020, 02:18:34 pmThe 5204 also goes to 100kHz, I do not know for the 5104

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 09, 2020, 08:34:38 pm
content: Thanks for all the advice and offers. I found a small single output (50 KHz?) unit very close by. Brookdeal/Ortec 9402 for 45$!Pretty low-end I guess, but postage was almost free. No inter-EU import charges either.eBay auction: #284005680205

author: TimFox
author_rank: Super Contributor
timestamp: September 09, 2020, 09:43:43 pm
content: When you use this one, you need to connect the generator appropriately to the reference input, then adjust the phase knob to maximize the received signal from your gadget through the input, to get the magnitude of the signal. If the phase angle between reference and input changes, you need to re-adjust the phase knob. If you want to switch from in-phase to quadrature component, there are 90 and 180 deg push-buttons above the phase knob.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 09, 2020, 10:35:46 pm
content: Thanks for the advice. If I modulate whatever I measure with the same generator I use as reference, won't any drift in phase just cancel out?

author: TimFox
author_rank: Super Contributor
timestamp: September 10, 2020, 01:11:37 am
content: It depends on whether the gadget being measured has a variable phase shift at the reference frequency, such as a tuned circuit or AC amplifier. I use mine to characterize amplifiers.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 12, 2020, 04:52:06 pm
content: Slightly offtopic question, please:What amplitude are the noise and the expected signal in your setup, and for how long can the signal be averaged in order to get something meaningful out of it?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 14, 2020, 02:42:04 pm
content: No idea, I haven't built my instrument yet!It's supposed to be an EPR instrument, which means a fairly slow magnetic sweep over an x-band microwave cavity.See my other thread on the matter:https://www.eevblog.com/forum/rf-microwave/x-band-cavity-design-and-microwave-safety/The way I manage advanced projects like this is usually to scrounge for expenisve/critical parts and then design the best instrument/setup based on that.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 14, 2020, 04:59:10 pm
content: Quote from: Brumby on September 04, 2020, 12:42:17 amBrumby, this ended up being the model I got locally.Do you, or anybody else, for that matter have the manual for it? It can be called either ortec/brookdeal 9402 or brookdeal 402? Thanks!

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2020, 06:22:42 pm
content: The lockin does not look that complicated. Most of the controls are pretty obvious. The only function not clear is the LF filter. The zero check is very likely to zero the low pass filter input.Otherwise it's the basic lock-in functions.It would be still nice to have specs and if possibly a schematics in case repairs are needed.The rechargeable batteries would be one of the points to check. They look like lead acid, so avoid to run them empty.For the specs, one can check quite a few points, like input impedance, noise levels, dynamic range (max signal before clipping) and filter order (the output time constant could be 1st order or maybe 2 nd order).

author: _Wim_
author_rank: Super Contributor
timestamp: September 14, 2020, 06:55:37 pm
content: Quote from: ChristofferB on September 14, 2020, 04:59:10 pmYou can download the manual for the 401a (which seems very similar) from here:https://elektrotanya.com/brookdeal_electronics_model_401a_lock-in_amplifier.pdf/download.html#dl

author: _Wim_
author_rank: Super Contributor
timestamp: September 14, 2020, 07:06:50 pm
content: Quote from: _Wim_ on September 14, 2020, 06:55:37 pmI guess the 401a is almost identical, just one filter board less

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 14, 2020, 08:01:59 pm
content: It's actually very complex in construction! there are something like 8 vertical boards mounted on a main board with different functions! there is almost NO air in the unit iself!Also mine is missing the battery, it's been desoldered from the psu, so I assume it has once been leaky/faulty - or perhaps there was an option without battery.I guess battery operation would be nice in reducing 50 Hz hum, but if all the other units are run off of the mains, I wonder how big the benefit is.Great find with the manual! thank you!!

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2020, 08:55:08 pm
content: The mains suppression of the lock-in can be quite good. The supply to the lock-in should in most cases not be a problem. I remember using some 49.7 Hz with no problem. So I don't think the battery operation would be essential.The reference part can be quite complicated - though it is usually not handled with much detail in the usual simple theory. This is kind of the hidden half of the circuit.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 15, 2020, 10:23:58 am
content: That's true but it's quite nice to have the manual when it's equipment that is new to you. I've never used a lock-in amp before.I also find it easier to verify correct operation when I can copy a setup from a manual. This particular manual has a multitude of interesting application examples as well, which is quite nice.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 15, 2020, 11:05:20 am
content: As a first time user, apart from the mandatory user manual, this video is highly recommended, a must see (if not yet seen), as a 101 in lock-in amplifiers.Quote from: PA0PBZ on September 03, 2020, 08:28:46 pm

author: Brumby
author_rank: Unknown
timestamp: September 20, 2020, 09:25:23 am
content: Quote from: ChristofferB on September 14, 2020, 04:59:10 pmSorry. I only have the unit. It was a rescue from a probable ewaste fate taken sight unseen, described as "some sort of amplifier". That was enough to intrigue me ... and the price was right.Not sure what to do with it, but my thanks for the Shahriar video.

author: guenthert
author_rank: Frequent Contributor
timestamp: September 20, 2020, 05:48:39 pm
content: There's plenty of information regarding lock-in amplifiers on the 'net, they're a mainstay of physics labs for quite some time. imho fascinating little buggers.https://authors.library.caltech.edu/12641/1/LIBajp03.pdfis a fairly short and sweet hands-on introduction.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 20, 2020, 10:46:36 pm
content: Great resource! Thanks!--What would be the quickest way to test out the basic functionality of a lock-in amp?If I connect a sig. gen to both ref and input, wouldn't the read-out always be proportional to the input peak voltage?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2020, 06:21:19 am
content: For a quick basic test, use a voltage divider to create a small (e.g. 1 mV range) signal from a function generator, and read the signal with the Locking. The Fgen signal would also be the reference. It is about the stability of the amplitude when in phase and also adjusting the phase to a zero reading and check how stable that zero is, to see phase stability. The signal should be same amplitude just opposite sign with a 180 deg. phase shift.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 21, 2020, 08:57:57 am
content: Sometimes I use the oscilloscope as a lock-in amplifier.Rigol DS1054Z has an averaging acquire mode. It can average up to 1024 traces. Many times a big and stable in sync reference is available somewhere anyway, either from some other place in the signal path, or as a separate channel from the signal generator. So, when the sync is from a stable reference and the acquire mode is set to average, the oscilloscope can display very small signals that otherwise would be invisible in the noise.In sync triggering and averaging mode is exactly what a lock-in amplifier is doing, except this time you can also visualize the signal, not only measure it's amplitude and phase.Even with a humble DS1054Z, the technique is incredibly effective.

author: TimFox
author_rank: Super Contributor
timestamp: September 21, 2020, 12:43:09 pm
content: There is a range of voltage at the reference input that gives satisfactory lock: 1 V rms sine-wave is a typical recommendation. My PAR 129A is rated up to 5 V rms max input.If you get lock (shown by an indicator light, mine is "out of lock"), then the meter indication is proportional to the magnitude of the in-phase component of the signal input, including the sensitivity switch setting, down to a very low level. Again, my PAR 129A indication is calibrated in rms volts, and goes from negative to positive, depending on the phase angle.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2020, 03:55:44 pm
content: The averaging mode on DSOs is not so much like an lock-in amplifier, but more like the related box car integrator. One can get the same result as with an Lock-in if one would do low res. FFT or sine / cosine fit on the averaged data. The lock-in does the mixing down to 1 or 2 fourier components first and averages later, but one gets the same result from averaging first and do the fourier part later. The averaged scope trace is often more helpful. However the limited vertical resolution of scopes limits the dynamic range. Digital lock ins usually use much higher resolution ADCs (e.g. 18 bit). Analog solution also usually have quite some dynamic reserve - so signal peaks can be quite a bit higher than the nominal full scale.The scale factor is a point to check. Usually it is for a sine wave and phase tuned to maximum signal (zero with 90 deg. shift). Internally quite a few lock-in actually don't do a true multiply with a sine and integrate, but multiply with a +-1 square wave function and optionally filter out the higher harmonics of the signal first. The square wave reference is often more stable and it gets more signal when used with a more square wave like signal (e.g. from a mechanical chopper).With no instructions / detailed specs, one of the first points may be testing the actual performance / limits to get some of the parameters ideally found in the specs.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 21, 2020, 04:22:45 pm
content: Well I do have the manual for the Brookdeal/ortec 401 which is almost identical save from the 402 having the option of diff. input.I think they're internally identical, they have the same board construction, panel layout etc.Either way, I may have fried the input buffer/preamp. When I got the amplifier, it behaved erratically, panel meter shooting up and down, flashing the 'overload' indicator - signal was at the most in the 100 mV area, and the input can withstand 3VP-P, so I don't buy that. Now there's no meter movement.Any signal on the ref port will make the 'unlocked' LED extinguish after a settling period, so I think (hope!) the ref channel and phase detector is ok.It would also make sense that it's the input that sees the load of ESD and overvoltages. There's a 'signal monitor' out, giving the amplifierd signal after the input amp, before the PSD, and it shows no voltage with any signal in.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 21, 2020, 06:44:17 pm
content: whoops false alarm, a coax solder joint had broken off! Thanks for the advice, people! With a 200 mV ref signal and the same signal fed through a huge (x5000) attenuator i can now vary the output amplitude with input amplitude- and offsetting the REF phase 90* nulls the output!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 21, 2020, 07:01:12 pm
content: The unit by the way, is very compactly built.The boards in the black box are the phase sensitive detector, I assume they want to keep the details a trade secret, since schematic and board layout for only those two are missing from the manual.

author: SilverSolder
author_rank: Super Contributor
timestamp: September 21, 2020, 08:10:33 pm
content: Quote from: RoGeorge on September 21, 2020, 08:57:57 am+1 for this method. The one caveat is that a scope is not as sensitive as a lock-in amplifier, so you need some kind of pre-amp to make this work for really low level signals.

author: SilverSolder
author_rank: Super Contributor
timestamp: September 21, 2020, 08:17:11 pm
content: Quote from: Kleinstein on September 21, 2020, 03:55:44 pmI use the crude technique of piping the reference sine into the scope's Channel A, then displaying the averaged sample signal on Channel B, via a pre-amplifier. This way, it is easy to see the phase shift as well as amplitude changes. It works well enough that it has (so far) kept my impulse to purchase a lock-in amplifier suppressed!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 22, 2020, 11:39:45 am
content: This is turning more and more into a general lock-in amplifier thread, which I don't mind the slightest, it seems silly to open a new thread with the exact same discussuion.I found an article in Review of Scientific Instruments that extends the lock-in amplifier range to hundreds of MHz by making a downconverter for each channel,mixer, common LO, IF amp+filter. The IF filter being in the LIA's freq. range. This would be a very good starting point for a very very good radioastronomy receiver!And also a very easy project with chinese ebay RF modules (I have a million of the AD831 mixer boards, they're super handy!) and a cheapo RF synthesizer.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2020, 11:50:19 am
content: The usual lock in amplifier are not that special. Normal SDR can get comparable performance and cover a wider frequency band at a time. The noise and mirror images would be set by the input and mixer part anyway.