Audio Distortion on 2 Meter FM

[ShawnInPaso]

I've had an SDS200 for a couple of months or so and after a little learning curve have really enjoyed it so far. One issue that kind of nags at me is some distortion.
As a ham radio operator, I listen a lot to 2 meter FM (144 to 148 mHz) and typically narrow band FM. In the busier part of the band, 146.500 to 147.99, audio from the SDS200 is fine. But there is a frequency in the lower part of the band I listen to a few guys chat which is in the 144 mHz range. It is only in these frequencies that the audio is distorted. The distortion occurs for anyone I hear and at any distance close or far. These same people transmitting on the upper part of the 2 meter band sound just fine. Likewise, if I transmit with my handheld radio on 144.500 my signal is distorted but on 146.520 it is not? These are all simplex signals, no repeaters involved and certainly not simulcast and I've ruled out phase distortion. Just point to point transmissions.

When I listen to the 144.xxx transmissions on my 2 meter transceiver or my BCT15X they sound fine, no distortion.

So far I have tried every available filter setting, tried setting to WFM and tried adjusting slightly off frequency (e.g. 144.500 vs. 144.505). None of these correct the issue.
Frankly it's not a big deal but its kind of gotten under my skin why it is occurring. I did search the forum but everything I found with the keyword distortion had to do with simulcast.

Just wondering if others may have some ideas or experienced this?

Thanks

 

[AB5ID]

Are there any strong carriers nearby could be desense with the SDS series using analog.

 

[tvengr]

Typically, 2 meters modulation is FM, not NFM or WFM. Try using IFX on the frequencies that are distorted.

 

[ShawnInPaso]

Are there any strong carriers nearby could be desense with the SDS series using analog.

No strong signals nearby, I'm in somewhat of a rural area. Thanks for the suggestion though, I wouldn't have thought of that.

 

[ShawnInPaso]

Typically, 2 meters modulation is FM, not NFM or WFM. Try using IFX on the frequencies that are distorted.

Thanks' for the info, I'll check it out as I didn't even now IFX was a thing. When I say narrow band FM, I should have said the standard of 12.5khz.

 

[tvengr]

No strong signals nearby, I'm in somewhat of a rural area. Thanks for the suggestion though, I wouldn't have thought of that.

You may have a local source of interference. USB switching power supplies are notorious for wideband emissions. IFX may take care of that. You can try unplugging all of the electronic devices in your residence. If you can power the scanner with a 12v battery, kill the master circuit breaker to see if that clears the problem. That will quickly tell you if you have local interference. Your 2 meter transceiver and BCT15X most likely use different IF frequencies and will not be affected by interference on the same frequencies as your SDS200.

 

[popnokick]

Thanks' for the info, I'll check it out as I didn't even now IFX was a thing. When I say narrow band FM, I should have said the standard of 12.5khz.

Nearly all ham / Amateur Radio FM on 2M and other bands is 25kHz rather than 12.5kHz… and thus is not NFM (Narrow FM) in the industry-standard Part 90 sense. Check to see if your radio is set for 25kHz.
Also … much of the lower end of 144.xxx is SSB. Which I doubt you’re mistaking for FM but wanted to check. When I pickup SSB on any of my FM receivers it’s pretty garbled and would be hard … but not impossible… to mistake it for some sort of garbled FM.

 

[ShawnInPaso]

You may have a local source of interference. USB switching power supplies are notorious for wideband emissions. IFX may take care of that. You can try unplugging all of the electronic devices in your residence. If you can power the scanner with a 12v battery, kill the master circuit breaker to see if that clears the problem. That will quickly tell you if you have local interference. Your 2 meter transceiver and BCT15X most likely use different IF frequencies and will not be affected by interference on the same frequencies as your SDS200.

Those are good thoughts. While I receive the same stations just fine on 146.520, I suppose there could be something local that just happens to cause the distortion lower in the band. I'll do some testing, thanks.

 

[ShawnInPaso]

Nearly all ham / Amateur Radio FM on 2M and other bands is 25kHz rather than 12.5kHz… and thus is not NFM (Narrow FM) in the industry-standard Part 90 sense. Check to see if your radio is set for 25kHz.
Also … much of the lower end of 144.xxx is SSB. Which I doubt you’re mistaking for FM but wanted to check. When I pickup SSB on any of my FM receivers it’s pretty garbled and would be hard … but not impossible… to mistake it for some sort of garbled FM.

Oh yes, this is all FM. These guys have used the 144.xxx for years as their frequency of choice for some quirky reasons. I understand the definition of wide/narrow can be a bit perplexing, it's too bad the SDS manual doesn't provide more technical info about the receiver. At least in California, most 2 meter FM transmitters (i.e. repeater transmitters) are designed for narrow band FM (+/- 5 khz deviation).
When I last tried switching through wide/narrow or auto on the SDS200, using wide the audio was barely perceptible, the narrow and auto were the same (the distortion was the same) but the best.

I'll have to wait for them to be on the air again to do more testing but I am grateful for the help and ideas.

 

[popnokick]

Don't confuse total FM deviation (e.g. 5 kHz) with the highest modulation frequency (e.g. 25 kHz with most ham FM) that is being applied to modulate the FM signal. More info and explanations here -
FM Bandwidth
- and here -
FM Frequency Modulation Index & Deviation Ratio » Electronics Notes

 

[ShawnInPaso]

Don't confuse total FM deviation (e.g. 5 kHz) with the highest modulation frequency (e.g. 25 kHz with most ham FM) that is being applied to modulate the FM signal. More info and explanations here -
FM Bandwidth
- and here -
FM Frequency Modulation Index & Deviation Ratio » Electronics Notes

Well the discussion about deviation and WFM or NFM is pointless as it applies to the SDS200 and 2 meters because selecting WFM on 2 meters results in a barely audible signal on all frequencies because there isn't sufficient deviation. If you have a SDS200, or any other 2 meter receiver, try selecting WFM and you'll find the audio is down in the noise and maybe 20% of what it should be. More to the point though, it does not correct the distortion I am hearing.

 

[popnokick]

When you enter the frequency for one of your local NOAA Weather transmitters (162.4xx or 162.5xx... several freqs used around US).... does it sound better if you select NFM or FM as the modulation type for the channel? NOAA uses 25 kHz bandwidth, not 12.5. The setting that sounds best for NOAA should also cover most ham radio analog FM transmissions.

 

[ShawnInPaso]

When you enter the frequency for one of your local NOAA Weather transmitters (162.4xx or 162.5xx... several freqs used around US).... does it sound better if you select NFM or FM as the modulation type for the channel? NOAA uses 25 kHz bandwidth, not 12.5. The setting that sounds best for NOAA should also cover most ham radio analog FM transmissions.

That's a good question. It sounds better on NFM. On WFM I have to crank the audio way up and it sounds okay but not quite like NFM. For example, I usually have the SDS200 at volume 4 but have to turn it up to 12 or 13 for similar audio on WFM.

 

[tvengr]

That's a good question. It sounds better on NFM. On WFM I have to crank the audio way up and it sounds okay but not quite like NFM.

You should not use WFM. NFM (Narrowband) is for 2.5 KHz deviation. FM (Wideband) is for 5 KHz deviation. Public safety and business radios in the VHF low, VHF high, and UHF bands all used 5 KHz deviation until the narrowbanding mandate was passed. Some frequencies in the VHF high band such as the VHF marine band and NOAA weather and all of the VHF low band frequencies were exempted from the requirement. VHF marine band, NOAA weather, and VHF low band frequencies should still be programmed as FM (Wideband). WFM is far wider. It works well with 15 KHz deviation radio microphones and the old 25 KHz deviation analog TV aural transmitters. Some of our old news radios were licensed as remote pickup and used 15 KHz deviation. FMB is for FM Broadcast radio stations which use 75 KHz deviation.

 

[ShawnInPaso]

Well @tvengr you nailed it. All is good now with ham radio frequencies set to FM, no distortion on 144.xxx or elsewhere.
I realized too late that the common understanding for "wideband" changed when commercial and public service agencies were forced to 2.5KHz which shifted the interpretation of 5 KHz to wideband. Of course the SDS200 manual doesn't help as it shows NFM for the amateur vhf/uhf bands.

Anyway, I appreciate your help and also the help of @popnokick .