Harmonics after changing electret - Page 1
Forum section: electronics / rf-microwave

author: nnixralaer
author_rank: Newbie
timestamp: August 26, 2023, 05:27:28 pm
content: Hello all,I'm just dipping my toe into RF stuff, and while I'm aware of AX.25 and such, I had this idea of replacing the electret microphone on a Motorola T42 so I could use it to transmit ASCII characters encoded simply as different frequencies with a python script. The problem is that I'm using a voltage divider connection (a 2.2K resistor and a 100uF capacitor) between the PCB's connector and a mono jack connector (KiCad's simulation told me it would keep the voltage in the normal range of an electret) and whenever I try to transmit, the T42 now produces pretty strong harmonics throughout the spectrum as I'm watching it with an SDR device and gqrx.The whole topic of harmonics could be way above my head for now but could someone show me at least some ropes on what would be the right way to do this, replacing the electret with a jack connector?Edit: Thank you in advance for any reply, my goal is really just to learn about these.

author: janoc
author_rank: Super Contributor
timestamp: August 26, 2023, 11:16:08 pm
content: No idea how you have connected your resistor and capacitor - I guess the cap and the resistor are in series, one end connected to the soundcard and the other to the radio? If yes, that is not a voltage divider and you are massively overdriving the poor microphone amplifier in the radio. That results in the splatter you see.Use a voltage divider (two resistors in series, top to your soundcard, bottom to ground and the middle point to the radio + a coupling cap of about 10uF or so) and reduce the signal to about 10mV or so. Line output of a soundcard is around 1V, so you need about 100k at the top and 1k at the bottom (to ground) resistors. Adjust as required but those values should give you a start. I would also suggest using an audio isolation transformer to protect your radio against any DC voltages that may appear at the soundcard outputs.Or, even better, don't feed the signal into the microphone amplifier, that's a bad idea. Those amplifiers are designed for microphones and voice and are not really suitable for transmitting digital data. Find the pointafterthat microphone amplifier and inject your signal there, at an appropriate level.If you want examples of how this sort of thing is done, google for "AFSK interface". The type of modulation you are attempting to implement is exactly that - AFSK. See e.g. here:https://www.ik3qar.it/rtty/1/(you don't need the FSK part, your radio likely doesn't have the FSK connector and would require modification for that).BTW, I hope you are doing these experiments into a dummy load, otherwise you are going to cause a ton of interference for others - and are violating the terms of your license (you are licensed, right?)

author: vk6zgo
author_rank: Super Contributor
timestamp: August 27, 2023, 12:22:09 am
content: Many radios have a point specifically intended for injecting CTCSS, DTMF, or other tones, which can be found by perusing the schematic.That is how I injected CTCSS tones into my old FT290R.

author: RFDx
author_rank: Regular Contributor
timestamp: August 27, 2023, 12:40:31 am
content: Quote from: nnixralaer on August 26, 2023, 05:27:28 pmHow about showing the schematic if you have it already in KiCAD?Quote from: nnixralaer on August 26, 2023, 05:27:28 pmRF harmonics (which shouldn't change/increase) or rather sidebands from the modulation process (perfectly fine with single tone FM modulation)?The whole topic of harmonics could be way above my head for now but could someone show me at least some ropes on what would be the right way to do this, replacing the electret with a jack connector?Edit: Thank you in advance for any reply, my goal is really just to learn about these.

author: nnixralaer
author_rank: Newbie
timestamp: August 27, 2023, 06:24:51 am
content: Quote from: janoc on August 26, 2023, 11:16:08 pmI think this is not in series but in parallel but correct me if I'm wrong, sounds like what you described next. I attached the kicad schematics I followed on a breadboard. (Not sure yet how to include it in a post, so it should be below I guess?)Quote from: janoc on August 26, 2023, 11:16:08 pmI have a difficult time as it's almost exclusively SMD components and I dont have the schematics of the radio but I'll keep this in mind, circumventing the amplifying would indeed help me.Quote from: janoc on August 26, 2023, 11:16:08 pmThis link looks golden thank you very much! I'll definitely read through it.Quote from: janoc on August 26, 2023, 11:16:08 pmI don't have a license yet but I do want to get an attenuator so I can properly test equipment without actually transmitting. I transmitted with my modified radio for like 2 seconds and immediately seen how the emission got worse but at this point I'm mostly interested if my idea of the electret replacement is even on the right track or not, I do not plan to test the actual emissions even if it's just a 0.5w little radio.

author: nnixralaer
author_rank: Newbie
timestamp: August 27, 2023, 06:45:44 am
content: Quote from: RFDx on August 27, 2023, 12:40:31 amI think I attached it above.Quote from: RFDx on August 27, 2023, 12:40:31 amOkay I think you are right. It was a huge spike with a +80 khz offset or so (that is not enough to be a multiple of the dominant) that I didn't really remember seeing before the modification, but I don't have the devices to properly test it yet with an attenuator. I found my other factory brand new T42 and it does the same so I think I didn't realize that it could be just FM sideband.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 27, 2023, 01:00:55 pm
content: I think your connections to the electret capsule are not correct. Here is how it should be connected:The power to the capsule goes thru the resistor, the signal goes thru the capacitor.Some electrets have three leads, on Neg one Pos and one Audio Signal.

author: RFDx
author_rank: Regular Contributor
timestamp: August 28, 2023, 03:04:43 am
content: Quote from: nnixralaer on August 27, 2023, 06:45:44 amTake a look at the attached schematic how to inject an audio signal from an external source. MIC_IN goes to where the electret mic was soldered. With R1 (5k) you can adjust the modulation level. Use your SDR receiver to measure the frequency deviation.Quote from: nnixralaer on August 27, 2023, 06:45:44 amA huge spike 80kHz away from the carrier doesn't make imo much sense and shouldn't be there at all.

author: nnixralaer
author_rank: Newbie
timestamp: August 28, 2023, 04:40:01 am
content: Quote from: RFDx on August 28, 2023, 03:04:43 amI will try the different schematics suggested when I can but maybe I should have opened this in the beginners section because I'm a bit clueless now. It's more like a 105 khz difference, but then what is this secondary emission from both the unmodified and modified T42? Then again I'm trying not to test the modified one too much until I get an attenuator/dummy load.

author: RFDx
author_rank: Regular Contributor
timestamp: August 28, 2023, 04:09:46 pm
content: Quote from: nnixralaer on August 28, 2023, 04:40:01 amCould be spurs from the synthesizer (possibly a fractional-N PLL), there are quite a lot of them. Nothing you can do about that. Do you see variations in frequency spacing and/or level when changing frequency? You would need to make a proper measurement with attenuator and cable connection between radio and receiver to make sure it's not something else. In the meantime you could do a simple relative measurement. Apply no modulation and use the S-meter to determine the level in dB(FS) of the carrier and (a few) of the strongest spurs. How many dB are the spurs down relative to the carrier?

author: janoc
author_rank: Super Contributor
timestamp: August 29, 2023, 11:53:45 am
content: Quote from: nnixralaer on August 27, 2023, 06:24:51 amThen I suggest you either get a dummy load (not an attenuator!) or get licensed. Ideally both - the license conditions explicitly stipulate that all test transmissions except the ones required for antenna tuning must be into a dummy load in order to not create interference for others. You can legally modify a radio only for amateur radio use, so you must be licensed for this type of work. That applies regardless of the fact that the T42 is a license-free PMR radio originally - once you open it and modify it you are operating outside of its type approval and the "license free" no longer applies. Even more so if you intend to transmit data over it which is not something it has been approved for.Dummy load for a 0.5W radio you can easily build yourself out of some resistors and a connector. There are plenty of guides for that online. Otherwise you will be sooner or later busted for interference and get a hefty fine, in extreme cases (e.g. during a natural disaster or because you have caused an accident or someone has died because of your actions) even prison. Even a 0.5W little radio can mess with someone's emergency communication, air traffic control or whatever else - that is actually licensed and will get you in trouble in no time.BTW, I have just realized that you mentioned you are watching the harmonics/spurs on an SDR. How exactly do you have your set up arranged? How are you coupling the signal from the radio into the SDR? And what kind of SDR in the first place?There are plenty of cheap SDR radios that have really poor strong signal behavior - front ends overload easily, little/no shielding so you get signal coupling where it shouldn't creating strong spurs/intermodulation products. etc.For example, if you have your T42 next to the SDR antenna on your desk (or have them linked directly by a cable with no attenuation), that is guaranteed to overload the receiver, you could even damage it. Then no wonder you are getting random crap on the screen. But those signals are spurious, generated by the SDR itself because of the incorrect measurement setup, not what is being actually transmitted.

author: nnixralaer
author_rank: Newbie
timestamp: August 30, 2023, 08:54:37 am
content: Not sure if I can multi-quote so I'll reply in this post, I'll try the meaurement RFDx mentioned when I can without actual emission.Quote from: janoc on August 29, 2023, 11:53:45 amI'm waiting for some SMA connectors to arrive, found this thread on the forum here with some info:https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/Even after looking at some schematics I'm not entirely sure of the difference, a dummy load is just an attenuator without the output connector? Both are resistors and ideally heatsinks that lower the signal strength, is that right? I did apply for a HAREC exam a month ago but still waiting for the reply from the given local government authority, and the exam itself will probably be in late autumn or even later, not sure yet.Quote from: janoc on August 29, 2023, 11:53:45 amI have a cheap chinese SDRPlay clone (called POP MSI SDR NANO) with a Diamond MR77S antenna, sadly these equipments are quite expensive (to me at least) so I wouldn't be surprised if I couldn't trust this SDR that much but at the moment I don't have anything else to measure RF emissions with. Do you think a tinySA would be a good investment?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 30, 2023, 12:49:32 pm
content: A load is supposed to absorb the output signal without reflectionAn Attenuator decreases the signal amplitude but does not terminate the signal and a load is required at the output of the attenuator to prevent reflections. If there is no termination the signal is supposed to continue on at a smaller amplitude. The load has to absorb the remaining energy, which is not very much after an attenuator.These things are supposed to be non reactive at the frequency used.. in other words there is no inductance or capacitance or in reality, these two reactive components cancel themselves out in the gadgetA load is frequently just a resistor of the appropriate wattage rating. At 400 MHz this takes a bit of engineering to make sure there is no "reactive" component (inductance or capacitance). Solder joints, length of leads, type of resistor and everything else physical about the "load" comes into play.Below about 30 MHz, just a big 50 ohm resistor soldered to the ends of a coax works pretty well. Lots of people have made them,. google Heathkit "Cantenna"For your purposes, a load (and/or the attenuator) would have to be rated to over 400 MHz to not produce reflections, Another way of looking at this is that a load is an antenna that does not transmit but convertsallthe RF energy into heat. You get a 1:1 Standing Wave Ratio. Or at least darn close to that.An attenuator just decreases the amplitude of the signal by convertingpart or most of the signal into heat.

author: janoc
author_rank: Super Contributor
timestamp: August 31, 2023, 10:24:04 pm
content: Quote from: nnixralaer on August 30, 2023, 08:54:37 amSo you are blasting that 0.5W output of the T42 right into the antenna of the SDR? I.e. radio next to the SDR on the desk?Well, no wonder you have spurs. If you do this with a tinySA (or any spectrum analyzer), you will destroy it.You can't do it like this, you must put an attenuator or some kind of coupler there - realize that the receiver is designed for microvolts of input signal from the antenna from across the world. The T42 at 0.5W output likely produces orders of magnitude more at the input.Whether or not tinySA would be a good investment depends on what exactly are you trying to do. TinySA series are useful instruments but don't expect the same performance as a 3000€ spectrum analyzer. SDR will also do you similar service except you don't get calibrated absolute signal levels. But you can see the spurs and other issues even with that and for many things it may be sufficient. Also oscilloscope FFT can do a lot.