SDR receiver: Receiving 96.3 MHz on 29.2 MHz, how does it happen? - Page 1
Forum section: electronics / rf-microwave

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 23, 2025, 03:13:43 pm
content: When fumbling with a clone of this radio yesterday:https://deepelec.com/files/sdr101/DeepSDR_101_Product_Manual_V1.0_EN.pdfI tuned to 29.2 MHz and there was a very strong signal, as it gav the resemblance of musical rhythm on SSB, I switched to WFM, and indeed, it was a radio broadcast station, that I heard there in very good quality, as if meant to be transmitted there.But of course, that's 10m HAM band in Germany, too. The station identified itself, I looked it up and the transmitting antenna is only 13km from here, 15kW ERP, at 96.3 MHz.So I wonder: how do I get to receive it on 29.2 MHz?Is this some numerical effect in how the SDR works, and its filtering against this higher frequency is too weak, or something?(I didn't find what sampling frequency it uses)Side note:I received that while testing my provosorial tuned loop antenna for 11m/10m, with a coupling loop and a BalUn. I though, now that's horrible selectivity from that antenna, what's going on... disconnected the coupling loop from the BalUn, which turned out to be the antenna for this (the 50cm RG316 connected to it alone receives nothing, and if I put my hand close to the PCV box housing the ferrite ring, the reception gets bad).Is that a sign that I made an error winding the BalUn? I doubt it's supposed to be an antenna itself and mess up the selectivity of the loop, haha. Which is 2x 11 windings bifilar 0.6mm PVC insulated copper wire on FT240-43.

author: tszaboo
author_rank: Super Contributor
timestamp: July 23, 2025, 03:21:13 pm
content: FM radio stations will transmit with a lot of power. It's so the people with crappy random length antenna still can receive signals.Then this gets into your SDR, overloads the mixer or your LNA, if you happen to have one. Also the filtering in cheap SDRs for out of band signals is also not brilliant.How to solve it? Bandstop filter for FM radio.

author: Grandchuck
author_rank: Super Contributor
timestamp: July 23, 2025, 04:10:17 pm
content: https://www.amazon.com/Broadcast-FM-Block-Filter-88/dp/B01LE9LRPM/

author: iMo
author_rank: Super Contributor
timestamp: July 23, 2025, 04:35:37 pm
content: With +20dB fixed broadband LNA preamp your input mixer might be easily overloaded. Then you get plethora of various products (mirrors, spurious) off the mixer. Thus the first aid in your case would be an input attenuator (or disable the 20dB LNA if possible). A second step could a "preselector for a specific band" (aka a tunable filter), or a notch filter for the strongest signals leaking in.Those cheapo SDR radios of that kind are not designed for large gain antennas, btw.. Try to operate it with, say, 1m long piece of wire..

author: radiolistener
author_rank: Super Contributor
timestamp: July 23, 2025, 08:38:09 pm
content: Quote from: TinkeringSteve on July 23, 2025, 03:13:43 pmThere can be different reasons why you're receiving a station transmitting on 96.3 MHz while tuned to 29.2 MHz. The most likely causes are related to limitations or imperfections in the receiver design:1. Image frequency (IF mirror reception):If your receiver is a superheterodyne with an intermediate frequency (IF), it's possible that a signal at 96.3 MHz is producing the same IF as a legitimate signal at 29.2 MHz. This can happen if the local oscillator and front-end filtering aren't adequately rejecting image frequencies.2. Local oscillator spur or harmonic mixing:Another common cause is spurious mixing due to local oscillator harmonics or spur. For example, if your receiver’s local oscillator or mixer stage is generating harmonics or intermodulation products, they may unintentionally mix with strong signals like a nearby FM broadcast station and appear as false signals (spurs) at unrelated frequencies like 29.2 MHz.Also, make sure your receiver’s front-end is not being overloaded - excessive signal levels can cause clipping and intermodulation distortion, resulting in strong stations appearing at multiple unintended frequencies across the band.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 24, 2025, 04:24:16 pm
content: Ah, thanks guy, I suspected things like that, while having only vague ideas about them.That would then go for the DAB stuff that's around now to, eh? Haven't looked what ther is near me, I reckon more than the FM stations that get fewer and fewer.I wonder if it would not be more efficient if, insteadof trying to ocmbine multiple notch filters (that may introduce summed attenuation where I don't want it) I employed a low-pass filter that starts cutting off at 80 MHz or even lower.Then I need to bypass it if I want to receive e.g. amatuer 2m, 70cm bands, or switch to a high-pass filter above 108 MHzDoes that make sense?Are there cheap RF path switching boxes for such purposes readily available?

author: fourfathom
author_rank: Super Contributor
timestamp: July 24, 2025, 04:55:15 pm
content: Quote from: TinkeringSteve on July 24, 2025, 04:24:16 pmAnd it isn't necessarily overload, but instead aliasing. This is essentially difference mixing products (and this includes harmonics of the sampling clock), when the input frequency is greater than half the sampling frequency. Your receiver has a Tayloe mixer, and later an analog to digital converter and either of these can be affected by aliasing.And the proper solution is indeed filtering, and depending on the receiver architecture either a bandpass or a lowpass filter is appropriate. I do a lot of work with direct-sampling SDR receivers and unless we are trying to receive an aliased input we add a good lowpass filter at the receiver input (as tzaboo mentioned, many SDRs have a marginal at best input filter.)I make and sell filters and preamps used with some SDRs, and while I doubt that these will be appropriate for your setup my websites do have some potentially useful information on the subject:https://turnislandsystems.com/http://wb6cxc.com/

author: iMo
author_rank: Super Contributor
timestamp: July 24, 2025, 05:48:19 pm
content: It is using the Tayloe mixer, but I doubt it works fine till 150MHz (the highest RX frequency in that receiver). That mixer works nice till, say, 20MHz (ie. even the FT-DX10 is using it as a first mixer for the lower bands, afaik) because of the CMOS switches used (the OP's mixer is the TI's TS5A23157). I think in that receiver they use some DSP tricks for the higher bands, like subharmonic mixing (known from the nanovna and alike gadgets), or something like that. That may cause the issues mentioned above..Therefore they put the 20dB LNA in front of the mixer (what is basically a pretty wrong idea, indeed). You will not put a 20dB LNA in front of a mixer unless you work at highest UHF bands only - like 1GHz up (and you have a high quality narrow band-pass before the LNA)..If OP wants to use the radio for, say, 10/11m (27-28MHz) with large antenna, he has to build a good band-pass filter for that band, and switch off the LNA, (or add an attenuator in front of the LNA).For 2m band and a large antenna (like a j-pole, or a dipole, or an yagi) he has to put a narrow quality band-pass in front of the LNA.Otherwise that radio should be used with 50-100cm telescopic antenna only..PS: the subharmonic mixer - that radio is a "zero IF radio" and when you want to demodulate something at say 145MHz you set the Local Oscillator of the mixer to aprox 20.714MHz (in detail it depends what you want - LSB/USB/CW/FM..) and you pick up the diff with 7th harmonics of the LO, like 145 - (7 * 20.714) off the mixer. You feed the Tayloe mixer (the two cmos switches) with two 20.714MHz clocks, clocks' edges are with phase diff 90degree (I/Q).If you want the waterfall around 145MHz, you set the 7th harm. of the LO to the "mid" of the waterfall, and you pick up your specific frequency for demodulation in DSP via an baseband NCO mixer (with I/Q the width of the waterfall region is aprox equal the ADC sampling frequency, like with 192ksps ADC the waterfall window will be 192kHz wide with LO in middle).

author: A.Z.
author_rank: Super Contributor
timestamp: July 24, 2025, 06:54:47 pm
content: place thishttps://www.amazon.com/Flamingo-FM-Applications-Problematic-SMA-Connected/dp/B07XKY8YKBbetween the antenna and the LNA

author: 10maurycy10
author_rank: Newbie
timestamp: August 25, 2025, 04:54:18 am
content: 96.3 is suspiciously close to the third harmonic 29.2. A typical mixer works by using the LO to drive transistors or diodes as switches, but this process inevitably creates harmonics of the LO, which can cause harmonics of your intended frequency to get into the the IF: The second harmonic is usually quite well suppressed as long as the LO and mixer are symmetrical, but the third harmonic can quite strong. When combined with a strong interfering signal like a local FM radio station...One way you can tell this is happening is to slide the LO (center of waterfall at lowest zoom) around, and see if the interfering signal moves 3 times as much. If so, it's caused by harmonic mixing and you need to get a low pass filter.

author: fourfathom
author_rank: Super Contributor
timestamp: August 25, 2025, 05:57:29 am
content: Quote from: 10maurycy10 on August 25, 2025, 04:54:18 amAnd if it slides by the same amount as your tuning adjustment it's likely to be aliasing. Either way, a low-pass filter is probably needed.