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forum.funcube.org.uk • View topic - What is best "tuning frequency"?

What is best "tuning frequency"?

For the early roll-out of the Dashboard

Moderator: pa3weg

What is best "tuning frequency"?

Postby n8mh » Fri Nov 22, 2013 7:14 pm

Hi All,

What is the best frequency to tune for optimal decoding? 1800 Hz? 150 Hz? Or what? I know there is AFC, but still--there must be a sweet spot to shoot for :)

Thanks and 73! Fun collecting telemetry again.
Mark N8MH
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Re: What is best "tuning frequency"?

Postby g0mjw » Sat Nov 23, 2013 9:18 am

Mark,

It depends on what you are using, if it is a funcube dongle then 10kHz seems OK, it gives an IF in the 8-16 kHz range so you can avoid the DC spike. You may need to play about a bit to remove spurious signals. In my case I have set the LO to 145.925, the tuning Low frequency to 55 kHz and the high to 65 kHz which corresponds to an audio pass band of +7 kHz to +18 kHz. The signal itself, according to the real time graph of decode frequency, varies between 14.5 kHz and 8.5 kHz.

If it is normal SSB receiver the bandwidth is such that you need to centre on the SSB filter, so 1200 Hz. A TS2000 will allow a 4 kHz filter so open it out and tune a little higher. You will need to tune through the pass.

If it is a funcube dongle but you are using SDR# or HDSDR with USB demodulation and a sound card loop, open out the filters to about 10 kHz and then centre it in the passband.

Mike
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Re: What is best "tuning frequency"?

Postby n8mh » Sat Nov 23, 2013 2:42 pm

Thanks, Mike. I was referring to the audio passband of the IC-910. It's about 2.8kHz wide, maybe 3kHz. So I've been using 1600-1700 Hz, which is rather near the middle of course.

Only now playing with FCDP+ a bit with native support from Dashboard. Wish there was a record function!

Are you able to record IQ while running Dashboard? Seems like another program is going to be competing for dongle control, so haven't tried that yet.

SDR-Radio V2 is pretty good with recoding and all, so you can process post-pass for any packets missed.

Just tried three setups with 6 deg pass (I live in the woods...hi hi)
1-Cheap RTL dongle, no preamp, to discone--couldn't see signal at all (but not sure about user error, lashed this up quickly)
2-FCDP+ with array/preamp low to ground (6') --got one packet decoded ;)
3-IC-910 with array/preamp above roofline (25')--got 38 good ones copied, missed a good number, too.

Anyhow, having fun in the shack again, thanks to Funcube :) :)

73,
Mark N8MH
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Re: What is best "tuning frequency"?

Postby KS1G » Mon Nov 25, 2013 3:42 am

Mark's advice for setting up the FCDongle (Pro+) worked great! Using FC Desktop. Passband set to 55,000-65,000 Hz, d/l freq of 145.925 MHz. Used SDR-Radio to set IF gain to +8dB based on info in another thread. 43 packets decoded on 60 degree pass - 4 element yagi, ARR preamp and about 40 ft of LMR400 to FCDPro+. No problems from the spike signal, and momentary issues with birdies from other electronics as the antennas swung past my house or neighbors'.
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Re: What is best "tuning frequency"?

Postby Duncan » Tue Nov 26, 2013 11:01 am

I don't think the IF gain +8 is needed any more that was just a fudge to work around the blank display problem caused by DC offset removal (fixed in 809).
That said if +8 gets you better results then keep calm and carry on...

Cheers,
Duncan.
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Re: What is best "tuning frequency"?

Postby SGAARC » Fri Nov 29, 2013 9:56 am

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Re: What is best "tuning frequency"?

Postby Duncan » Fri Nov 29, 2013 10:57 am

... definitely not dim, probably a question lots of people wanted to ask, it's all down to the tuning...

The dashboard needs to get the signal down to zero hz to decode the bpsk, so by tuning the dongle to the exact telemetry frequency you'd think that would do it, however in reality doppler and other effects, mean the signal would drift around and not be close enough to zero to decode...
Hence tuning the dongle to 10kHz lower which puts the signal at 10kHz higher (or 58kHz on the dashboard display) this lets the dashboard accurately track the signal and adjust the down mix frequency.

The way the dashboard does this is in two stages, the first down mix and filter gets the signal to 1200Hz this is then decimated to a lower sample rate, which means shorter and simpler filters. Finally there is the down mix to 0Hz for bpsk decoding.
If the dongle is tuned so the signal is -100 to 3kHz then the first stage mixer is disabled, and only the decimator and final down mix is done (the final mix frequency is adjusted to match the signal).
If the signal is -100 to +100 Hz then only the decimator runs and no down mixing is done...

The result of this is that there are a only few small holes where the signal won't decode, mostly around 0hz.

NOTE: When in dongle capture mode the 809 dashboard displays frequency 0 to 96k this was just the simplest way to scale the graph, in this mode to get the actual frequency you need to add dongle freq and take away 48k...

Cheers,
Duncan.
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Re: What is best "tuning frequency"?

Postby G3WKW » Fri Nov 29, 2013 11:23 am

Maybe I am being a bit dim, but...
Not if you haven't used an SDR before like perhaps SDR# which is a good introduction.

Why set the frequency to 145.925 when the telemetry is transmitted on 145.935?
The SDR has a large spike at it's centre frequency and strong local signals tend to break through on it rendering it unuseable.

Also, adding 55kHz and 65kHz to 145.925 takes the range beyond the telemetry frequency.
But the centre frequency from the dongle is at 48kHz so you need to subtract that to arrive at 145.932-145.942. The nominal telemetry frequency would be at 58kHz. Personally I set my limits at 53kHz and 63kHz. I think 55 would limit your range a bit if you have a yagi, but I will give that a try next pass. I was still seeing the beacon over North Africa last pass.
Sorry if some of you think this is a stupid question but it's not if you don't know the answer!
Well this is an educational project, so no one should flame you. But perhaps expect answers from the non-experts.

Bob

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Re: What is best "tuning frequency"?

Postby SGAARC » Fri Nov 29, 2013 3:01 pm

Thank you it makes more sense now.

I use SDR# at home but to be honest never looked into how it works!

I have altered our settings to use 55kHz and 65kHz. It was interesting to following the signal across the tuning display and work out the transmit frequency. I can now explain it to the pupils at our next meeting.

Is any of this explained in the documentation anywhere?
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Re: What is best "tuning frequency"?

Postby mikeadams » Fri Dec 06, 2013 12:15 am

My first attempt with the dongle - which was disappointing. Set the frequency on settings to 145.925 and then set 55,000 and 65,000 to reduce the width of the receive. Large spike just below 55,000. It was a good pass but only got around 6 times locked on the signal. The auto tune seemed to jump around a lot and decoding was limited. 4 element beam here and have had good reception on this antenna from 2 m radio.

Have I set this up correctly as described above? Do I need to set anything on the FCHID software? Any comments settings would be very welcomed. 73 de Mike G0AMO
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