Repeater Output

[PugetSounder]

I’ve been monitoring the OlyCom3 (Tiger Mt E) repeater with my RTL-SDR. I have 2 SDRs V-3 and V4. Both are reading the same.

The repeater output looks to be 1MHz off center as shown in the 1st pic. Moving the freq up to 626 (pic 2) shows the signal at center of the modulation. You can clearly see it by the station ID in CW (pic 3).

As an experiment, I tuned my SDR to a strong commercial FM station. My reasoning is these stations broadcast in Wide FM as well. So for instance 97.7 KIRO FM booms in the Puget Sound area The signal is center frequency. At 97.6 I still read the station OK but fidelity suffers. Same with 97.8.

I'm still learning so I thought I would ask more experienced people here.

Has the Repeater drifted off frequency or is this normal due to the positive offset setting? I'm unsure.

 

[wrci350]

That's not 1 MHz off.  It's 1 kHz off.  1 MHz off would be 463.625 or 465.625.

Yes, the repeater appears to be off frequency just a bit, but probably not enough that anyone would ever notice.

FM broadcast radio stations are 75 kHz (I'm pretty sure), not 25 kHz like a "wideband" GMRS channel.

[MarkInTampa]

Sounds like you have not calibrated the SDR receiver/dongle from within SDR# software. Click on the gear icon within the software and adjust the frequency correction (ppm) to a known source. I use a HT set to a known frequency (in this case 462.625) on low power, no antenna and narrow FM to keep from overloading the front end of the SDR and keep the FM width as narrow as possible to adjust the ppm. Not lab perfect but close enough.

 

 

[PugetSounder]

Thanks for the correction. And the info.

For the record, I can hear the difference on my PC w/a discone, but obviously not on my cheap HT with a small speaker and whip.

Do you think it might impact rx for people out in fringe areas? At 25 kHz, there appears to be some (perhaps minor) modulation cutoff  on the upper side.

 

 

[Sshannon]

3 hours ago, wrci350 said:

That's not 1 MHz off.  It's 1 kHz off.  1 MHz off would be 463.625 or 465.625.

Yes, the repeater appears to be off frequency just a bit, but probably not enough that anyone would ever notice.

FM broadcast radio stations are 75 kHz (I'm pretty sure), not 25 kHz like a "wideband" GMRS channel.

Broadcast radio stations are also stereo. They can’t be directly compared to a monaural FM GMRS signal. 

[PugetSounder]

2 hours ago, Sshannon said:

Broadcast radio stations are also stereo. They can’t be directly compared to a monaural FM GMRS signal. 

Thanks! And I appreciate your input.

I'm somewhat new at this. Just brainstorming and hoping to understand function and theory a bit better. -So many variables...

I spent a few years monitoring HF LSB/USB and I would just turn the VFO and tune in that way. A few Hz +- to find center becomes easy after doing it for awhile.

Now, with a couple of HTs, and considering a mobile (which are type accepted) that's not an option. So I thought I'd look into what is being transmitted and what I'm receiving. It looks to me that the output tolerances are larger than I expected.

As an example, NOAA ch3 is 162.475.00 in my area. It's loud and clear from my station but looking at the waterfall, center frequency is actually 162.475.257. Moving up to the latter frequency gives me better fidelity on my laptop. The voice becomes closer to a real human if that makes sense. Now I would think that NOAA is using the best transmitters in the world, but they still seem to be off frequency by a bit. Idn, maybe they re-calibrate on some schedule or maybe they consider that they are within tolerance and leave it be.

I don't know the answer...

[Sshannon]

1 hour ago, PugetSounder said:

Thanks! And I appreciate your input.

I'm somewhat new at this. Just brainstorming and hoping to understand function and theory a bit better. -So many variables...

I spent a few years monitoring HF LSB/USB and I would just turn the VFO and tune in that way. A few Hz +- to find center becomes easy after doing it for awhile.

Now, with a couple of HTs, and considering a mobile (which are type accepted) that's not an option. So I thought I'd look into what is being transmitted and what I'm receiving. It looks to me that the output tolerances are larger than I expected.

As an example, NOAA ch3 is 162.475.00 in my area. It's loud and clear from my station but looking at the waterfall, center frequency is actually 162.475.257. Moving up to the latter frequency gives me better fidelity on my laptop. The voice becomes closer to a real human if that makes sense. Now I would think that NOAA is using the best transmitters in the world, but they still seem to be off frequency by a bit. Idn, maybe they re-calibrate on some schedule or maybe they consider that they are within tolerance and leave it be.

I don't know the answer...

What makes you think that your VFO is more accurate than theirs?  Maybe you’re off by 257 Hertz. 
I think all upper end HF receivers have a “Clarifier” adjustment to adjust for the many radios that don’t exactly match their frequency. I tune to an integer kHz and then adjust the clarifier, with the assumption that my VFO is perfect.
Transmissions from satellites require a Doppler shift that varies based on orbital velocity and whether they’re approaching or moving away. 
Are FM transmissions modulated symmetrically about a center frequency, or do they modulate in one direction from the carrier?  I don’t know. 

[PugetSounder]

3 hours ago, markskjerve said:

Sounds like you have not calibrated the SDR receiver/dongle from within SDR# software. Click on the gear icon within the software and adjust the frequency correction (ppm) to a known source. I use a HT set to a known frequency (in this case 462.625) on low power, no antenna and narrow FM to keep from overloading the front end of the SDR and keep the FM width as narrow as possible to adjust the ppm. Not lab perfect but close enough.

 

 

Great idea. I'm going to check this out. I think I read somewhere that transmitting without an antenna would damage your radio? Is that untrue?

Also, could you tell me how you got your screenshot? I fiddled with it for a bit and finally gave up and used a camera.

[WRYZ926]

I have seen some variance with this on HF radios myself. As SShannon said, most radios have a way to deal with this. You can use the clarifier or shift function on your radio.

[PugetSounder]

9 hours ago, Sshannon said:

What makes you think that your VFO is more accurate than theirs?  Maybe you’re off by 257 Hertz. 
I think all upper end HF receivers have a “Clarifier” adjustment to adjust for the many radios that don’t exactly match their frequency. I tune to an integer kHz and then adjust the clarifier, with the assumption that my VFO is perfect.
Transmissions from satellites require a Doppler shift that varies based on orbital velocity and whether they’re approaching or moving away. 
Are FM transmissions modulated symmetrically about a center frequency, or do they modulate in one direction from the carrier?  I don’t know. 

I have two SDRs based on different chip-sets and they both center at +257Hz. Same software though, so that might be a factor.

[MarkInTampa]

22 minutes ago, PugetSounder said:

Great idea. I'm going to check this out. I think I read somewhere that transmitting without an antenna would damage your radio? Is that untrue?

Also, could you tell me how you got your screenshot? I fiddled with it for a bit and finally gave up and used a camera.

On one watt for a few seconds at a time you aren't going to hurt the radio.

Windows Clipboard is your friend

[MarkInTampa]

20 minutes ago, PugetSounder said:

I have two SDRs based on different chip-sets and they both center at +257Hz. Same software though, so that might be a factor.

I doesn't really matter what software you use, you have to calibrate the SDR to the software unless you have a really high end dongle with a crystal oscillator and clock onboard.

[Sshannon]

31 minutes ago, PugetSounder said:

Also, could you tell me how you got your screenshot? I fiddled with it for a bit and finally gave up and used a camera.

In Windows, you can either do the three key chord shift-window-s keys to bring up the snipping tool or you can use alt-prtscr or ctrl-keys.

[Sshannon]

On the Airspy groups.io, there’s a post that includes the following advice:

DO NOT use the local NOAA transmitter. While it may be on frequency, it could be off frequency by several kHz.

DO NOT use the local FM Broadcast rock station transmitter. The frequency will deviate far off frequency. A talk station might be usable if you can hit it during a "silent period", but they are overall not recommended. There are better alternatives.

https://groups.io/g/airspy/topic/69841361

 

[MarkInTampa]

42 minutes ago, PugetSounder said:

Great idea. I'm going to check this out. I think I read somewhere that transmitting without an antenna would damage your radio? Is that untrue?

Also, could you tell me how you got your screenshot? I fiddled with it for a bit and finally gave up and used a camera.

FYI, found this YouTube video explaining how to set the ppm value...

 

[PugetSounder]

7 minutes ago, markskjerve said:

FYI, found this YouTube video explaining how to set the ppm value...

 

Thank you!

This seems to be what I've been missing. -Easy enough too.

[PugetSounder]

1 hour ago, Sshannon said:

On the Airspy groups.io, there’s a post that includes the following advice:

DO NOT use the local NOAA transmitter. While it may be on frequency, it could be off frequency by several kHz.

DO NOT use the local FM Broadcast rock station transmitter. The frequency will deviate far off frequency. A talk station might be usable if you can hit it during a "silent period", but they are overall not recommended. There are better alternatives.

https://groups.io/g/airspy/topic/69841361

 

Man that is a great find! And this a helpful group. Thank you all.

[PugetSounder]

Turns out that my setting is -2. Mystery solved!

[PugetSounder]

And here's my apology to NOAA (if you are out there)

[BoxCar]

6 hours ago, PugetSounder said:

Great idea. I'm going to check this out. I think I read somewhere that transmitting without an antenna would damage your radio? Is that untrue?

Also, could you tell me how you got your screenshot? I fiddled with it for a bit and finally gave up and used a camera.

For a calibrated source to check the SDR use WWV at 10.0 MHz.

[PugetSounder]

3 minutes ago, BoxCar said:

For a calibrated source to check the SDR use WWV at 10.0 MHz.

I used 198.309.441 per the guidance in Sshannon's link. There's a solid pilot there that is easy to zero.

 Tech tip #1 - Calibrating your SDR / Finding a suitable frequency source

<snip> When calibrating a unit, use as high a frequency as you can. The higher the frequency, the more accurate the unit will be over the entire reception range. Errors will multiply as the frequency multiplies. Conversely, errors will divide as the frequency is divided. A 4 Hz error at 200 MHz will be 8 Hz at 400 and 16 Hz at 800 MHz. but if you calibrate to within 4Hz at 800 MHz, that will translate to a 2 Hz error at 400 MHz and 1 Hz error at 200 MHz.

A GPS Disiplined Oscillator is the preferred common source. If you do not have access to one, an ATSC DTV transmitter pilot carrier is a good over-the-air alternative. These can be found at 309.440559 kHz above the lower channel edge. For example, if the DTV station transmits on Channel 11 (198-204 MHz) this pilot can be found at 198.309440559 MHz. Obviously the Airspy will not resolve to MilliHertz, so use 198.309441 MHz. That will still be accurate within 1 Hz.

But I do like to use WWV @ 5, 10, 15 etc to find open bands. On a good day I can hear both the male and female voices indicating a strong signal from both Hawaii and Colorado.

[PugetSounder]

That last sentence probably didn't make much sense. So let's try again.

Male voice is WWV Ft. Collins, Colorado

Female voice is WWVH Kauai, Hawaii.

Different calls signs. But still a good propagation day if I can get them both.

Edited February 17 by PugetSounder
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