How to accurately measure 50Hz (60 Hz) in the field? - Page 1
Forum section: electronics / projects

author: Ron Werner
author_rank: Contributor
timestamp: May 28, 2025, 07:28:13 pm
content: Any VLF listener will have noticed how extremely far the 50 Hz (for me, for US readers 60 Hz) mains signals propagate, easily over distances of several kilometers. In addition, there are the harmonics up to several kHz.I want to make a portable receiver/meter that fits into my backpack, which I can bring with me on my many hikes into the hills/mountains in the area where I live.I have started experimenting with LC tuned circuits and filters with opamps, but my scope tells me that these are not really working too well. Too low selectivity, very ugly output signals.My initial questions would be, keeping in mind that the equipment must easily fit into a backpack, and that I focus only on 50 Hz signals in this phase:-what kind of antenna: loop or ferrite?-Would ground electrodes be an alterantive?-how to achieve selectivity: LC or opamp filters? (or a combination..)(No microprocessors etc. I am strictly old-school!)All suggestions and tips are most welcome. Maybe someone has been working on a similar project?Looking forward to hearing from anyone...Ron WernerNorway

author: ejeffrey
author_rank: Super Contributor
timestamp: May 28, 2025, 07:57:24 pm
content: You won't make a sensible LC bandpass filter for 50 Hz, and it would weigh too much for your backpack if you did. However that is relatively easily acheivable with active filters using opamps. Look into the multiple feedback filter architecture for bandpass filters of moderate Q and gain.

author: BrianHG
author_rank: Super Contributor
timestamp: May 28, 2025, 08:52:19 pm
content: Quote from: ejeffrey on May 28, 2025, 07:57:24 pmHow so?Yes, the antenna will need to be large, but you can get truly thin coiled inductors in the mH and even H range.We used to make use of such filters for the DC power supply in vacuum tube amps. (Ok, those were iron core and weighed around 1-2pounds for the inductor. But those were power inductors, some like 1 to 5 Henry, and I don't believe the OP needs to carry current in his circuit.)However, the antenna will need a super low capacitance bridge to the LC filter to get a narrow response.Such a weak connection means you will need op-amp amplification with protection at the opamp inputs. If you go near to any 50/60hz mains, the coil filter may dish out more than a few volts AC.

author: TimFox
author_rank: Super Contributor
timestamp: May 28, 2025, 09:06:50 pm
content: Quote from: BrianHG on May 28, 2025, 08:52:19 pmAt 50 Hz, a100 mHinductor has about 31 ohms of reactance, and would resonate with10 uF(available in polypropylene with highQ. A higher inductance is still practical for low power levels.With a loaded totalQ= 10, the bandwidth would be roughly 5 Hz. The impedance levels required are very reasonable with bipolar op amps.Correction: 1 H and 10 uF or 100 mH and 100 uF.

author: IanB
author_rank: Super Contributor
timestamp: May 28, 2025, 09:26:54 pm
content: Quote from: Ron Werner on May 28, 2025, 07:28:13 pmI think your question can be answered from a different angle. The fact that mains frequency radio waves may exist in the environment is not, of itself, relevant. Their existence only becomes relevant if they couple into some system or device where they become interference of some kind, that you care about. So the detector you need must replicate the device where you fear interference becoming a problem. That criterion will precisely tell you the physical dimensions and design of the detector you need, and even the characteristics of the detection circuit.That said, it is unfortunate for you (if you do not want to use modern techniques), that the most effective approach will (IMHO) be digital. You record and digitize the signal, and then apply a digital spectrum analyzer to extract and report the content over the frequency range of interest. You do not have to do the analysis on the spot. You can record a sample with time and GPS coordinates, and do the analysis later at home.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 28, 2025, 09:29:40 pm
content: Quote from: TimFox on May 28, 2025, 09:06:50 pmYou need to increase you capacitor to 100 uF or your inductor to 1 H. It's doable, but an opamp filter can be built for a few pennies with nF range film capacitors which are inexpensive and stable.

author: TimFox
author_rank: Super Contributor
timestamp: May 28, 2025, 09:38:45 pm
content: Quote from: ejeffrey on May 28, 2025, 09:29:40 pmOops: reading a LCD display in the dark. I corrected the original post.I agree that at these frequencies, an op-amp filter is more reasonable.

author: Perrold
author_rank: Contributor
timestamp: May 28, 2025, 10:08:50 pm
content: Most practical solution is a small loop or e-field antenna feeding a sound card. I'd consider focusing on the harmonics more than the fundamental for a variety of practical reasons. The higher order stuff often induces worse noise, the higher frequency is more objectionable if you're doing broadband VLF listening which it seems you are, and the fundamental can be fairly easily notched at the receiver whereas the harmonics will still bother you.Making low frequency LC filters is not an issue as already pointed out. Considering your filter will see no meaningful power and no DC bias you could also use things that innately have lots of inductance. Small audio transformers and small power transformers can be your friend here. The Q is typically adequate for such applications. Relay coils may be another option. Additional losses due to a solid core probably won't be too bad. There's a line of relatively tiny adjustable inductors from UTC or one of the older companies that give you a factor of two adjustment range centered roughly around 10H though other ranges exist as well. The Q is around 10. They're not the most common item unfortunately.

author: radiolistener
author_rank: Super Contributor
timestamp: May 28, 2025, 10:37:32 pm
content: Quote from: Ron Werner on May 28, 2025, 07:28:13 pmJust wanted to suggest sound card with and digital filter in software, but then read your notice

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 28, 2025, 10:43:59 pm
content: Your title adds more to the question. "accurately"? Are you looking for peak field strength (Volts/Meter)? Or frequency? How many decimal points? Traceability?Are you looking for a quiet place to do your VLF listening? Perhaps rather than trying to find that elusive quiet spot you should look into op-amp notch filters to bar that interference from your receiver.

author: TimFox
author_rank: Super Contributor
timestamp: May 28, 2025, 10:47:12 pm
content: By accuracy, do you mean actual field strength (in uV/m or Tesla) or just a relative meter to trace increases and decreases with position?At 50 Hz, you are in the near field region (since the wavelength is 6000 km), so if you calibrate your antenna plus amplifier, you should use the equations for induction, rather than radiation.Biot-Savart law for magnetic field:B(Tesla) = (muo/2pi) xI/r= (2 x 10-7) xI/r, whereIis the current in a long wire (amperes) andris the perpendicular distance from the wire (meters).

author: mawyatt
author_rank: Super Contributor
timestamp: May 28, 2025, 10:57:52 pm
content: A Polyphase Mixer (PPM) based receiver should perform well for this application, altho originally developed for RF/MW/MMW use, no reason it shouldn't perform well here.https://www.eevblog.com/forum/rf-microwave/polyphase-or-n-path-mixer/You can create a simple, small, low cost, battery powered, handheld mains frequency base PPM receiver and have the ability to change frequency to the mains harmonics (or other frequencies in that range) by simply changing the clocking frequency (the PPM performs over decades of frequency range). Consider an 8 Phase PPM Version (performance sweet spot) with Hz level bandwidth (or even lower) which directly produces I and Q baseband signals that are easily digitized with low speed ADCs (for DSP processing), or simply Analog displayed for level detections (Log Amp). BTW no inductors required (just Rs and Cs and CMOS (NMOS) switches), and input Narrow Band High Effective "Q" Pre-Select Filter and Antenna Impedance Matching tracks LO clock without tunable elements!There's a long story (see link for snips of such) behind this PPM way back that caused quite a stir in the academic and research community because it (apparently) breaks the laws of Physics and Fourier Mathematics with sub 2~3dB basic Noise Figure (theory teaches the NF limit for Bi-Phase Mixing is 3.92dB)!!Best

author: coppercone2
author_rank: Super Contributor
timestamp: May 29, 2025, 12:52:02 am
content: Well just keep in mind you can BUY inductors that are resistor sized up to 100mH of inductance. I have a 4H "choke" that is approx 1cm², it looks like a transformer and uses very thin wire (but the package is terminated with some 30AWG pvc wire for installation.So I would take that suggestion about the weight with a grain of salt. However, I don't know how it will perform for what you want.If you get a nanocrystaline core with high inductance per turn, its easy to get a rather large inductor. With my 20mm diameter ring I got 40mH from I believe 16 turns and it REALLY started going up after that.If you can get the coils to work there is massive simplicity. I like simple for portable. Also easier to design. Ring cores are rather well isolated.Other thing to keep in mind is that in the last 10 years it has gotten easy to buy high quality shielding online for low prices. The most available is "pure iron sheet" and also permalloy sheet (i believe mistakenly they call it mu metal but you can get mu metal too, just most stuff is supermalloy afaict). These two materials are 'standard' sheet material.You can also get a very thin metal like material, called metglas... this is a special magnetic material that is made by pouring a special alloy on a water cooled copper drum with a skiv on it so as it rotates, it peels the solidifed metal into a thin foil. This foil has the properties of metal and glass. The key is that the steel cooled 1000000 degrees C per second, on the copper, so it does not really form a grain structure. It is very very thin (registers as less then 0.0005 inches on micrometer) and it is very springy, if you get a larger piece it can wrap around stuff nicely.Anyway the point is you can get some very high inductance values with the nanocrystaline core ferrite cores, or buy old high value inductors, and also create rather advanced magnetic shields.I don't have experiance trying to make something with these magnetic materials yet, and I can attest to the quality of active filters with op amps (my preferred method) but I am just saying there is an appeal to a passive filter and it might be possible to make a very nice one if you figure this stuff out a little more. The fact that you are so far away from the emitters makes one of the biggest downside of passive filters go away (which is being effected by stray near fields of hustle and bustle of real life)also read the websitewww.vlf.itwhich is specialized in this low frequency stuffalso for antennas, stormwise sells stuff on ebay like a giant low frequency ferrite that is 1 inch by 1 foot for $100, which seems to be used to make giant ferrite rod antennas (which people seem to like to use for lighting)and the metglass shield material you can get is called 'amorphous shield'. I think its similar to what is in those nanocrystaline cores, but its a thin sheet you can wrap around something. Just no sharp bends. Cuts with scissors OK or you cna fold it and it cleaves. sharp though

author: moffy
author_rank: Super Contributor
timestamp: May 29, 2025, 01:25:23 am
content: You can improve sensitivity by using a PLL to lock onto the nearest 50Hz signal, they are basically a self tuning BPF, and then demodulate the output. They are great at extracting low level signals from the noise.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 29, 2025, 02:53:08 am
content: Quote from: Ron Werner on May 28, 2025, 07:28:13 pmHow do you want to detect or display your measurement ?A Sync detector and low pass filter is one way to get precise frequency detection, and good rejection of unwanted frequencies.That can be a CMOS divider and analog switch - eg if you start with a 32.768kHz clock source, you get numbers like32768/4/164 = 49.95121951 A .05Hz beat frequency will take a while to decide is there.32768/4/163 = 50.2576687132768/4/162 = 50.5679012332768/4/161 = 50.8819875832768/4/160 = 51.2 A 1.2Hz beat can be filtered and detected.32768/4/149 = 54.97986577 This will detect 50Hz and 60Hz equally, and gives a 5Hz beat frequency you can further bandpass filter and detect/measure.Standard RTC crystals take a long time to start, but there are 32.768Khz active oscillators that internally run a higher crystal, that have faster startup, much less temperature drift, and still draw < 100uAThese low flutter frequencies could drive a VCO, to make the difference audible.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 29, 2025, 12:36:35 pm
content: I'd wind a bunch of turns of fine wire on a ferrite rod scavenged from an old AM radio for the antenna. There might be a better material for down that low, maybe a DIY laminate made of sheet iron. Resonate it against a low loss capacitor and feed it to an IC headphone amp. Get some cheap over-the-ear headphones and just listen. About $13 at a dollar store or Amazon might do it, just make sure they have a wired input, not just Bluetooth. For more selectivity, add a single op-amp 4-pole filter and a gain stage. Should be able to make the whole thing run on a 9V battery.

author: Datman
author_rank: Regular Contributor
timestamp: May 29, 2025, 12:59:55 pm
content: I've done it with an ATmega328P (Arduino): measuring 25 or 50 or 100 or more cycles I obtain 1mHz resolution, but it is wired:Making it wireless should not be so difficult, but signal must be filtered well.EDIT:...Or you want to measure the strength of a 50(60)Hz field?..."Accurately" let me think to a frequency measure...

author: CaptDon
author_rank: Super Contributor
timestamp: May 29, 2025, 01:21:58 pm
content: Look at a Potomac Instruments AM field strength meter for ideas. They contain a built in directional loop antenna in the lid/cover and they make measurements in Volts Per Meter which is a standard measurement for field strength. I am not sure what you mean by 'accurate' measurements? Chances are anything home built can only make 'relative' measurements. You will find an H-Field magnetic gradient away from the power lines and an E-Field electrical gradient also when very close to the power lines. I used to yard jockey 53' trailers that were under 500KV power lines in a factory parking lot. If you lightly rubbed the back of your hand along the trailer you could feel the 60Hz sensation on your skin. You could hear the corona crackle on high humidity days. We refused to move trailers during thunderstorms as the whole operation was on top of an open hillside.

author: tggzzz
author_rank: Super Contributor
timestamp: May 29, 2025, 01:29:16 pm
content: Quote from: CatalinaWOW on May 28, 2025, 10:43:59 pmIt would help if the OP would answer those questions, and also what "accurately" might mean in a system with a moveable antenna.I note many of the responses ignore the harmonics of 50Hz, which can be surprisingly (to me) high. They also don't directly address the issue that the mains frequency is not exactly 50Hz, so it may be necessary to change the centre frequency to track the current mains frequency.One type of filter can automaticallyincludethe harmonics: the N-path a.k.a. commutating capacitor filter. That's a variant of the PPM filter noted by Mike Wyatt above; the thread he noted is excellent, and is well worth reading.To add to the points made by Mike, I'll note that many decades ago I made an N-path filter with a Q of ~4000, using 10% components.

author: mawyatt
author_rank: Super Contributor
timestamp: May 29, 2025, 02:14:43 pm
content: If OP wants to display mains frequency, then consider what we did in ~1970 to measure such. We had a very precise and stable OCXO (vaguely recall was ~3.84MHz), this was divided down to 60KHz and used as a reference for a custom discrete PLL that was locked to the mains line (60Hz). The PLL had a divide by 1000 in the feedback path, so when mains locked the VCO output was ~60KHz which was then phase compared to the 60KHz reference. The phase detector output drove an Up/Down counter/accumulator which displayed the "counts" as Mains Frequency Deviation in millihertz since 1 count represented 1/1000 of the mains frequency, and the resolution could be set by the gate count interval.For the analog frequency deviation output the VCO control voltage (very linear VCO) was Low Pass Filtered and scaled to use with an Analog Chart Recorder. Remember during a Seminar watching the North America East mains frequency drop ~30 milihertz and later found out that the Turkey Point Nuclear Power Plant had tripped off-line which was a common occurrence for this and the Crystal River Nuclear plants. The loss of this power source causes the overall grid to drop in frequency which we could monitor and correlate with various events and one could "see" by observing the Chart Recorder paper trail how the grid would speed up at night to compensate for the days load and keep in sync for a precise day of exactly 24hr!Best

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 29, 2025, 04:19:10 pm
content: If all that is wanted is detection of mains (and its harmonics) the synchronous filter architecture (Multiply input by reference, LPF and measure resulting in phase and quadrature DC values) works when the reference is a square wave. Acts as a comb filter. With appropriate attention to detail a quantitative result (total power in the mains and harmonics) could be developed, at least to one or two significant digit accuracy.

author: IanB
author_rank: Super Contributor
timestamp: May 29, 2025, 06:07:41 pm
content: This whole thread is a pointless waste of time. The OP does not respond, does not discuss. They just opened a thread and then disappeared.Right now this thread is just a group of posters speculating in a vacuum.

author: Ron Werner
author_rank: Contributor
timestamp: May 29, 2025, 08:28:28 pm
content: Dear IanB,I am sorry to read that you think you are wasting your time. But, again sorry, I only spend an hour or so a day on my PC for the timebeing. I am located in Norway, that means for me an hour in between 20:00 and 24:00 local time. (14:00-18:00 in New York or 11:00-15:00 in LA.)And I also apologize for not being able to hurry and stress; I have enough of that in my life and job...But anyway, all those who replied, many sincere thanks. A lot of useful thoughts!First, about "accurately". I got this idea when letting out a dog, and saw some newly built houses directly underneath high power lines. I believed this to be illegal in Norway, but it started me wondering about the field strengths. Then I thought back on the times I went out to some remote marsh for VLF listening to tweeks and whistlers, and remembering how much hum I still received, even though I was about 3-4 km away from the nearest powerline.My idea was to start a small pseudo/halfway serious-scientific project to measure the field strength at different locations and get a more or less sensible impression of how much stronger/weaker the fields are. I have no hope to get a scientific correct figure in nT, but if I can say that the field is 1000x or 100.000x weaker at the location where I did my VLF listening, I would call that an accurate enough result.What also struck me, is that I didn't manage to find a ready-to-use circuit diagram for such a device. I googled and used AI, but apart from some serious studies, I did not find anything I could use for the actual construction.AI recommended:https://patents.google.com/patent/CN2503494Y/enhttps://www.homemade-circuits.com/field-strength-meter-circuit/(adviced me to use the 433MHz type and changing the LC values)It is also possible to buy some detector from Ebay/Banggood/Amazon etc. But these are not frequency specific.I have so far been experimenting with a few LC-circuits and a first opamp circuit. But I am not an expert, and wasn't convinced that my results would lead me anywhere at all.About the display. I could use a simple panel meter (1 mA or so) or a digital voltage meter module. Again, a result in uV/m or T is not necessary in this stage.About 50 Hz and harmonics. As you by now understand, my project has as such nothing to do with VLF listening, but was partly inspired by it. I wouldn't rule out that a next stage could focus on the harmonics, but first, I intend to focus on the 50Hz signal.I understand that the use of LC's is in itself not problematic. I found my tests not easy to tune and not selective at all.I am now in the middle of a first opamp circuit I found in an old Dutch Elektuur magazine. It is a simple circuit and the calculation for the capacitors is easy. It is called a critically coupled band-pass filter, and I calculated it for 45 and 55 Hz, assuming that even if the tolerance of my components is not optimal, I should be able to get a result that is close enough.I found several band-pass filter calculators, but the standard ones result in somewhat impractical resistor values:https://robots.wa4dsy.net/bandpass-filter-calc.I found another version here:https://onechip.com/2nd-order-bandpass-biquad-filter-calculator.htmlI also attached a pic of my opamp BPF and my loop antenna with a simple opamp amplifier. I try to get resonation at 50Hz with an elco. The loop antenna was originally for another project and only has an inductance of 193mH, requiring 52.4uF.OK, this is as far as I manage to come this evening.

author: tggzzz
author_rank: Super Contributor
timestamp: May 29, 2025, 09:32:19 pm
content: So, you already have a device that detects what interests you.Put a 50Hz bandpass filter on its output, and use a voltmeter set to AC to measure the strength.If there is a way in which that would be inadequate, tell us.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 29, 2025, 10:46:21 pm
content: Quote from: Ron Werner on May 29, 2025, 08:28:28 pmMaybe using a gyrator approach is better at 50Hz? - (gyrator uses a capacitor to emulate a large inductor)Some examples and formula here :Very High-Q Gyrator Filterhttps://sound-au.com/project218.htm

author: Someone
author_rank: Super Contributor
timestamp: May 30, 2025, 12:46:46 am
content: Quote from: Ron Werner on May 29, 2025, 08:28:28 pmStill no accuracy defined here. But trying to measure multiple orders of magnitude difference with analog needs some additional steps: log amp? multiple ranges? how to calibrate/verify those?

author: mawyatt
author_rank: Super Contributor
timestamp: May 30, 2025, 03:20:19 am
content: Quote from: Ron Werner on May 29, 2025, 08:28:28 pmSpend some time with the link we posted in #11. The PPM (N-Path Mixer/Filter) does everything you want, a low noise, narrow band, mains frequency direct down conversion receiver that's small, cheap, uses no inductors, low component sensitivity, tunable by clock, with high dynamic range and good out-of-band signal rejection, without any conventional type bandpass filtering!!Best

author: gf
author_rank: Super Contributor
timestamp: May 30, 2025, 06:50:52 am
content: Quote from: mawyatt on May 30, 2025, 03:20:19 amSince the result is quadrature, I'm wondering how to easily measure sqrt(VI²+VQ²)? as scalar value? [ w/o ADC and MCU ]If I understand correctly, the measurement is supposed to be done kilometers apart from civilization. So I'm also wondering what voltage levels are to be expected at the output of the antenna? Millivolts? Microvolts? If the signal level is very low, how to amplify the zero-IF w/o suffering from DC offsets and 1/f noise? Zero drift / chopper opamps?Independent of PPM I also wonder:How narrow does the bandwidth need to be to dig the signal out of the noise floor? With a Q of ~100, the bandwidth still covers the full frequency range of the primary reserve, so fixed tuning to 50Hz (or 60Hz) is likely OK. But if we need a higher Q of say 1000 or 10000 that's no longer possible and exact tuning to the current mains frequency is required. But how to find the correct frequency? Relatively easy if we do the narrow-band filtering and spectrum analysis with a DFT, but MCU is prohibited by the OP. I'm unsure if a PLL is capable to auto-tune and lock to a weak carrier at a negative SNR? Furthermore, as Q increases, the filter settling time and measurement time increase proportionally to Q. This makes a frequency sweep (e.g. 100 steps from 49.8...50.2Hz) increasingly impractical, too. And within such long measurement times, the mains frequency may even change. In the end, it all depends on the a priori signal level we can get from the antenna and the SNR...

author: tggzzz
author_rank: Super Contributor
timestamp: May 30, 2025, 08:28:29 am
content: Quote from: gf on May 30, 2025, 06:50:52 amThe filter I made in '79 wasn't a "4 section" IQ filter, it was 8 section, so the output was a sampled sine wave with 8 steps/cycle. Put that through a crude bandpass filter and a pure sinewave emerges.

author: tggzzz
author_rank: Super Contributor
timestamp: May 30, 2025, 08:31:12 am
content: Quote from: Someone on May 30, 2025, 12:46:46 amYeahHence the text under my moniker <----But the OPhasprovided some useful information, and the accuracy requirements are minimal.I expect he could make his measurements with the equipment he already posesses.

author: mawyatt
author_rank: Super Contributor
timestamp: May 30, 2025, 11:46:56 am
content: Quote from: gf on May 30, 2025, 06:50:52 amFor the scaler value of the I and Q PPM output (or N-path filter output which is scaler), one could employ the old analog squaring and square-rooting functions like those employed in old DMMs.Gain staging shouldn't be difficult (altho we don't know the range as OP hasn't provided such) and could employ a Log Amp as we mentioned to allow an Analog Meter or an DVM chip set for displaying a large DR. Of course range switching could also be considered.Anyway, seems like the PPM or N-Path is an ideal candidate from what we know about the OP application/requirements.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: May 30, 2025, 12:19:55 pm
content: Quote from: gf on May 30, 2025, 06:50:52 amThe OP should provide some info wrt what the signal environment is around the mains and harmonics, this would help establish the signal levels and relative SNR.As for PPLs locking to and tracking low SNR signals, they are quite good at this and often employed for "yanking signals out of the noise". In the distant past we developed a number of PLL based sensitive receivers including a unique phase detector and control (patents 4904958, 4912434) for use with such. Phaselock Techniques by Floyd Gardner is an excellent reference which has a discussion of the NASA Voyger PLL based tracking receiver which is the ultimate in low SNR PLL receiver performance.Since the OP is interested in amplitude, not sure if a PLL based receiver is the best approach, as they are preferred for Signal Frequency/Phase Acquisition not Amplitude.Best

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2025, 03:19:19 pm
content: When the fundamental frequency is slowly varying about its nominal value, a PLL is ideal for monitoring the signal.

author: tggzzz
author_rank: Super Contributor
timestamp: May 30, 2025, 03:26:47 pm
content: Quote from: TimFox on May 30, 2025, 03:19:19 pmEnsuring a PLL locks on to a signal with a very wide dynamic range in the presence of noise is not trivial.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 30, 2025, 03:42:10 pm
content: For LF and VLF, it has been suggested that the ground its self can be used as an antenna.

author: tggzzz
author_rank: Super Contributor
timestamp: May 30, 2025, 04:31:15 pm
content: Quote from: Terry Bites on May 30, 2025, 03:42:10 pm"Ground" is a term that has little meaning when considering electromagnetic fields.Simple example: it is not difficult to calculate the capacitance of the earth-moon pair of spheres. Is one side of that capacitor "grounded"?Another simple example: look at any basic texts about antennas, and see that the earth is effectively a mirror.

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2025, 04:58:54 pm
content: The "image" or "mirror" construction is useful for such geometries.The non-physical voltage and current below the "ground" plane would result in an equipotential surface at that location, so calculations ignoring the ground but including the "image" will give the same result as the physical situation with the equipotential plane.

author: Ron Werner
author_rank: Contributor
timestamp: May 30, 2025, 09:54:50 pm
content: @TGGZZZI have probably not enough sophisticated equipment to do the kind of testing that will assure me the results that I think would be sufficient for my project.I did simple testing: I fed a signal varying in frequency from 0 to 100 Hz into these circuits, and checked the output signal on my scope. Regardless of what I did, I wasn't very successful getting a clear peak at 50 Hz. The selectivity wasn't nearly what I would have expected.About "N-path a.k.a. commutating capacitor filter": this is well beyond my current skills.@PCB.WizI checked the gyrator and it might actually be an option. I definitely will try to construct such a circuit, and see what it does.IN GENERAL:Several replies assume more skills than I currently have, and however grateful I am, like moffy/Mawyatt's PLL solution, that is just way too complicated.I will definitely try a ferrite rod, as I have several in stock.I will look for better core materials, though that would only be an extra plus after first getting the electronics working properly.As soon as I have some concrete results to show, I will come back and present these here!Thanks to you all!Ron WernerNorway

author: coppercone2
author_rank: Super Contributor
timestamp: May 31, 2025, 02:42:25 am
content: just keep in mind with core materials it might not be making it better it might be making it achievable with a low mu core it won't be practical to build high value inductors. loopstick from old radio is the right thing., but if you try to use random cores you might be disapointed, i.e. switching power supply chokes that we have alot of are not meant to do anything for low frequenciesits like a resistor, you can improve size (optimize) but it also needs to be the right value in the first placethat frequency is so low that it needs special ferrite, metal transformer material or possibly the nanocrystaline stuffif you take some RFI filter inductor from a fast power supply it's just probobly gonna be a dead end / waste of time

author: moffy
author_rank: Super Contributor
timestamp: May 31, 2025, 04:52:17 am
content: Quote from: tggzzz on May 30, 2025, 03:26:47 pmIf you use an analogue rather than digital mixer the loop filter becomes more effective, but yeah, effective locking in does require a certain minimum signal level.

author: tggzzz
author_rank: Super Contributor
timestamp: May 31, 2025, 07:28:50 am
content: Quote from: Ron Werner on May 30, 2025, 09:54:50 pmYou don't have a handheld DVM with an AC range to measure the output of a bandpass filter?Don't doubt your ability to build an N-path filter. Here's a picture of one I recently recreated based on one I made in ~1979. The yellow components are capacitors, the IC is a DG408 analogue switch, and there is one resistor dangling above the -ve PSU connection (and two tant bead PSU decouplers). The external connections are the PSU and a 3-bit binary counter, plus the BNC connectors for input and output. And that's all.Its properties are shown inhttps://www.eevblog.com/forum/testgear/ad3-or-budget-bench-setup/msg5546397/#msg5546397The only changes to make a 50Hz filter would be the clock frequency and increasing one resistorHere's a complete circuit of how to build a "Mains Rejection Tracking Filter - using a tracking 'n-path' filter with wide dynamic range" from p374 inhttps://www.worldradiohistory.com/UK/Wireless-World/70s/Wireless-World-1974-10.pdfMaking a bandpass filter is marginallysimplerthan a band stop filter.Fig 1 shows the basic principle, and Fig 2 is an illustration of Fig 1's response. I used N=8 capacitors, it looks like article has N=16 and uses JFET switches (I used a DG408)