Change a 153-279kHz LW AM radio to receive 15-150kHz ultrasounds, is it doable? - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: July 27, 2024, 10:05:47 am
content: A couple of years ago I've spotted a bat flying near the balcony, and was curious to hear it. Meanwhile the bat has left, but this summer I've seen it again.Found already some electret microphones that are good enough to receive ultrasounds, too, not only audio. The range of interest for bat ultrasounds is somewhere between 15-150kHz at most, but mostly between 50-100kHz or so. The exact frequency and bandwidth depend with the bat species. 15-150kHz is not a must, only a nice to have.So far I have tried a direct conversion receiver (a digital approach, after sampling at 192kHz with a soundchip), and also a simple frequency divider (a comparator followed by a digital counter and a speaker). They kind of work, but not very well. And they work only when the bat flies very close, at less than 3m (10feet) or so, which doesn't happen very often.Another type of receiver would be one using an intermediary frequency, which I never tried. I've remember today that I already have some scrapped boards from former boomboxes, no idea the exact model, so no schematic. They do have AM radio for MW-LW, and LW is close to the ultrasounds range. The boards are in very bad shape, but most probably their AM radio is still OK.I thought maybe it would be possible to shift the receiving band from 150-300kHz to 15-150kHz or so, and use an electret microphone instead of the ferrite antenna. Another option might be to make an AM modulator modulated by the ultrasounds, and then to feed that signal to the input of the MW or LW radio. Not sure which one is more feasible.The hope is that a superheterodyne would have a much better sensitivity and dynamic range than direct conversion, or than a comparator followed by a counter. Maybe that would make possible to receive the bat from a farther distance.I know how a superheterodyne receiver works in theory, but I don't know the practical details, I'm not a ham, and never designed, or even built a superheterodyne receiver in practice. For example I know there used to be typical RF cans for IF, and osc, but I don't know the details of their tuned transformers, or their color codes, or the typical boombox topology of the AM radio from back then, things like that.The project is just for the fun of tinkering something this weekend, a one off, doesn't have to be reproducible. My questions are:1. - is this a good idea, to try to change the receiving frequency (I have a DSO and an AWG, but no SA or VNA), or should it be more practical to just use an AM modulator, and leave the radio as it is?2. - by looking at the filter cans in the pics, which of the 3 boards should be easier to reuse/transform?3. - I'm very bad at estimating time, how long it might take? Is this doable in a weekend or two, or is it expected to take much longer?

author: wofritz
author_rank: Contributor
timestamp: July 27, 2024, 03:16:19 pm
content: I read your other thread and wondered if one of the relatively cheap SDRs like the SDRPlay RSP1 could be used. The RSP1A goes down to 5 kHz.Since these are software defined radios, you need software running on some computing device which could make portable operation a bit cumbersome. I don't know if there is any SDR software running on mobile devices like phones or tablets.These RSP1A can receive a wide frequency range simultaneously, so 5 kHz to 150 kHz is no problem. You can monitor the whole spectrum on the screen.I have such an SDR running but I have no frontend (microphone and maybe preamplifier) to receive sound but I would really like to test it because I think there are bats flying around my house.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 27, 2024, 04:14:03 pm
content: That should work, too. There are many types of receivers for ultrasounds (they have some commercial applications too, for example finding gas leaks, etc). Some are dedicated to receive bat ultrasounds, some are DIY, other commercial, and so on.This time I am curious in particular about an analog solution. In particular, how practical would be to try to modify an existing analog AM radio, so to repurpose it as an ultrasound receiver.The hope is to get a better dynamic and better sensitivity by using an intermediary frequency (i.e. the typical 455kHz used by AM superheterodyne), but I'm not very sure if it would make any difference in practice (when compared with just down mixing the 50-100kHz ultrasounds to audio without any IF). Or if the repurposing of an AM receiver is doable at all.Maybe would be better to just build a new one?

author: Andy Chee
author_rank: Super Contributor
timestamp: July 27, 2024, 04:24:13 pm
content: Although the full article is behind a paywall, perhaps the introduction description and block diagram are sufficient to get you pointed in the right direction?https://www.siliconchip.com.au/Issue/SC/2006/August/An+Ultrasonic+EavesdropperAlternatively, I can chase down a physical copy at our library and scan it for you if it helps.

author: coppercone2
author_rank: Super Contributor
timestamp: July 27, 2024, 04:35:08 pm
content: I think what you need to do is make a roost and put the microphone by the roost when it flies in there it should chirp for landing, it would be alot easier then trying to catch him mid air. They probobly are pretty quiet because they have competition from other bats for food.... they probobly avoid advertising their location unless they have some need for doing it, they are trawlers. I think its kind of like how fishing boats don't like being near each other, and how fishing captains naturally want to be secretive about their lucrative fishing areas.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 27, 2024, 05:13:28 pm
content: Quote from: Andy Chee on July 27, 2024, 04:24:13 pmYou are too kind, thank you. Found the article. Did try already a mixer (not this one). Though, the idea of using a dish-like reflector might help. Didn't try that yet. Well, aiming at a flying bat won't be easy, the bat I've seen was flying in a very "acrobatic" manner, but it might worth the try of adding an ultrasounds reflector.I guess my title for this topic was misleading (changed it). Already have a lot of schematics of ultrasound receivers from another older thread:https://www.eevblog.com/forum/projects/want-to-hear-the-bat-flying-in-my-front-yard/The two DIY ultrasounds receivers I've already tried (one was a digital down-mixer, the other was just a frequency counter/divider) only work when the bat flies very close, but the ultrasounds become very weak with distance.This time I am curious how practical would be to try to repurpose an AM radio.Does modifying an AM radio looks like a doable approach, or does this rather look like a destined to fail idea? If doable, will a superheterodyne receiver (and in particular, heaving an intermediary frequency) bring any advantage for ultrasounds receiving? I know heaving 1 or even 2 IF is advantageous for a radio, but will an IF be helpful in this case, too?

author: nali
author_rank: Frequent Contributor
timestamp: July 27, 2024, 05:30:41 pm
content: Just wondering, have you seen this project?https://www.omenie.com/pip.htmlUses a rPi Pico for decoding. (I've not tried it, just bookmarked it as I have a simple hetrodyne detector which I was thinking of upgrading some day.)

author: iMo
author_rank: Super Contributor
timestamp: July 27, 2024, 05:59:18 pm
content: The IF BW with those AM radios is like 4-6kHz, afaik. The span of bat's chirps is XXkHz as you wrote, and they do frequency hopping, perhapsHow that span and IF BW would fit together?What would you hear? I do not know..

author: wofritz
author_rank: Contributor
timestamp: July 28, 2024, 07:07:40 am
content: Quote from: iMo on July 27, 2024, 05:59:18 pmThis is my concern too. With the SDR, you see the spectrum on the screen, so maybe it would even be possible to identify the kind of bat by their sound signature (assuming the reception is working in the first place). The longer I think about it, the more a like to test it. I'll go back to the other thread and look for information about microphones etc. and see if I can set something up.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 28, 2024, 08:01:18 am
content: Indeed, I've read the radio channels in LW are spaced at 9kHz, but LW broadcast is not SSB. Both sidebands has to fit in those 9kHz, which means only 4.5kHz of audio bandwidth.Quote from: wofritz on July 28, 2024, 07:07:40 amIt's not much info about microphones in that thread. To summarize it, usually any electret mic will tend to work for ultrasounds, too, unless specifically limited. Some have a capacitor in parallel, to cut away their unwanted response to higher then audio frequencies. As a thumb rule, the smaller the diameter of an electret mic, the better its response to ultrasounds.I've harvested most of my electret mics from former 2G/3G mobile phones. The best I have found so far was an analog MEMS microphone (not electret), and has an audio preamp inside. The MEMS mic was also a found from a former mobile phone. Beware that most MEMS mics are digital now (and usually limited internally to audio only).I've browsed the docs for the SDR you mentioned. Looks much better than an average SDR dongle. For example, the AGC is adjusted in 3 places, in the LNA, in the mixer, and in the IF.https://www.sdrplay.com/docs/SDRplay_AGC_technote_r2p2.pdfThe AGC was one of the main reasons that made me want to use a radio, the second reason was sensitivity.

author: iMo
author_rank: Super Contributor
timestamp: July 28, 2024, 08:31:18 am
content: Afaik the electret mic capsules end up at ~20kHz. You would need something special..https://micbooster.com/ultrasonic-microphones/146-2319-ultrasonic-mic-board.html#/209-connector-with_fitted_header_pinshttps://micbooster.com/home/360-2385-Clippy-Ultra-XLR.html#/220-single_or_stereo_pair-single

author: radiolistener
author_rank: Super Contributor
timestamp: July 28, 2024, 01:22:47 pm
content: For me the best option is to use high speed ADC with ADC driver instead of RF transformer for impedance matching. But such hardware may be complicated to obtain, especially for newbies.Another option is to use sound card with 384 kHz sample rate, it has Nyquist bandwidth 192 kHz which covers almost any frequency which is used by the bat. But sound cards have 20 kHz low pass filter on the input which needs to be modified for 190 kHz. Also some cards may have DSP filter in addition to input analog LPF, such cards are not suitable for this purpose.Regarding to SDR receivers, cheap dongles like RTLSDRv3 or RSP1 have very bad sensitivity below 200-500 kHz, so this is not a good choice. It still can be used, but it will have very bad sensitivity.Regarding to the software, for windowsHDSDRis the best choice, it has best usability and DSP. For Linux there is no so good analog, but you can useSDR++, it has a lot of issues with DSP, but looks like the best choice at the time. I made my modSDRPPwith fix for FFT processing and visualization, but it still has some issues from SDR++.

author: iMo
author_rank: Super Contributor
timestamp: July 28, 2024, 04:10:44 pm
content: You may try with classic "softrock" sdr radio design, you would need one audio transformer from old radios, a 4066 switch (the mixer) and an I/Q VFO set to 60kHz (240kHz oscillator with a 7474 creating the I/Q clock for the 4066). Two opamps with A=100 (or more) and their I/Q output fed to a sound card or to 2 simultaneously sampling ADCs.With that setup you will detect the bats "over the horizon"Here below is a standard schematics - all you need is to replace the HF trafo with an audio transformer (or better make it yourself as you would need 100kHz BW)and adjust perhaps the decoupling capacitor values for your band.I would set C1=1..10u, and the C4/5/6/7 to 3n3 or 3n9.And of course use a mic instead of the antenna (with a biasing, so decoupled via a capacitor). The sensitivity of this stuff will be uV levels..On the SDR screen you will see the spectra from aprox(60-soundcard_sampling_rate/2).. to .. (60+soundcard_sampling_rate/2) in kHz..For example with 96kHz sampling rate your visible spectra on the SDR screen will be from 12kHz to 108kHz..https://www.pianetaradio.it/blog/pz-sdr/Note: you have to use I/Q mixer with the BAT detector, as a simple direct conversion mixer will not eliminate the mirror frequencies, which are close or within your spectra of interest..Note1: on the picture the "LO" is the input for your 240kHz ttl level local oscillator. It is a fixed frequency, so you may generate it from a small mcu for example, or simply by an RC oscillator, like NE555 or other (the duty cycle is not important here as you divide by 2 at the clocks' edge).Note2: the opamp's outputs I and Q are fed into the stereo soundcard (left/right). The actual L/R could be switched to R/L in the SDR applications usually, as well as many other params..Note3: I would start with a cheaper opamps there, the opamps shall be ideally low noise, bipolar, and working at 5V. Also 7474 or 74HC74 or LS version may work fine, as well as any 5V variant of the analog switch 4066.Note4: the transformer - you may use a ferrite pot core. The two secondary windings must be perfectly symmetrical, therefore you have to wind them with twisted wires. With for example a ferrite pot core with AL=1000nH you would need aprox 160 turns in order to get aprox 3kOhm reactance at 20kHz..Also mind the dots at the windings in the schematics.

author: iMo
author_rank: Super Contributor
timestamp: July 29, 2024, 09:42:29 am
content: Quote from: radiolistener on July 28, 2024, 01:22:47 pmI cannot confirm that. I built many sdr radios while running the DSP on several PCs/Notebooks with at least 5-6 various soundcards inside them and when I set 192kHz sampling rate I got nice aprox 180-190kHz wide flat spectra.In opposite, there could be a small annoyance around Zero audio frequency (in the middle of the HF spectra on the DSP screen) because of the soundard's finite input capacitive coupling creating a high pass at very low audio frequencies (like first 10-20Hz).PS: FYI - this is perhaps one of the first SDR backends we were using ~two decades back.. Still well working and pretty useful..https://www.dxatlas.com/Rocky/https://www.dxatlas.com/Rocky/Advanced.asp

author: radiolistener
author_rank: Super Contributor
timestamp: July 29, 2024, 05:38:51 pm
content: Quote from: iMo on July 29, 2024, 09:42:29 amit depends on the card. I also have sound card with almost flat response, but other card has pretty limited bandidth.

author: mark03
author_rank: Frequent Contributor
timestamp: July 29, 2024, 07:05:34 pm
content: I've had good success with surface-mount MEMS microphones for my bat experiments. I was told by an application engineer that pretty much all of the analog-output models have good ultrasonic response. (This may depend on front or back port---I use the more common back-ported devices.) You don't need one advertised as having ultrasonic capability. Digital output would be OK too except I'm not sure what DSP is already done on chip, possibly limiting the upper corner.I suspect your issues have more to do with the front end, sufficient gain, etc. than the downconversion topology. It's not complicated, and any Cortex-M microcontroller can handle it with some care. The Pi-Pico project linked above looks nice.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 30, 2024, 09:39:36 am
content: Quote from: mark03 on July 29, 2024, 07:05:34 pmSo, you didn't use any AGC in the front end? Only manual gain adjust?Maybe my receiver is too close, and that's why the need of an AGC.The bat that sometimes visits the front yard (seems to be a solitary one, never recall to see flying more than one at once) flies above a green area that is roughly 10 by 20 meters (30x60 feet), and this green space is sited right in front of a tall block. A street encircles the other sides, so the bat flies inside that green area, and right in front of the building's balconies, doing flight rounds, coming as close as 1 meter, and going as far as 20-30 meters away. There are some trees, too.In less than one second or so, the bat flies between 1m and 30m away, which is a lot of distance variation, and the variation in sound direction. Overall, when the bat flies on the far side I receive nothing, and when it flies right in front of the balcony the ultrasounds saturates the input.

author: radiolistener
author_rank: Super Contributor
timestamp: July 30, 2024, 12:08:13 pm
content: Quote from: RoGeorge on July 30, 2024, 09:39:36 amI always using manual gain control, it allows to keep signal with low distortion.

author: mark03
author_rank: Frequent Contributor
timestamp: July 30, 2024, 02:16:18 pm
content: AGC isn't a bad idea, especially if you aren't using headphones, just to compensate for the limited dynamic range of yours ears. You should be able to get close to 16 bits of dynamic range in the front end, though. (Possibly, electrets don't do as well as MEMS at the upper end?) I probably did have some clipping on mine... I didn't pay that close attention to it.