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Author Topic: icom 910h and fsk 1200bps  (Read 7350 times)
MERSAULT
Member

Posts: 21




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« on: November 19, 2012, 10:03:27 AM »

Hello,

I'm having problems using 1200 bps (A)FSK modulation with an ICOM 910H.
The hardware setup that I have is an ICOM 910H and a TNC which outputs are connected to the MAIN data socket in the rear panel of the rig.
I'm using two symbols for modulation (so, it is 2-FSK or AFSK with two levels) and I used a signal of 1's and 0's (A square pulse with 600 Hz) to test the RF output.

With an spectrum analyzer I saw the RF signal from the ICOM. What I expected to see was two peaks in +/- freq_sep/2, something like this
http://i48.tinypic.com/14brf5u.jpg (This is an image taken using the max hold feature of the analyzer)

but what I saw was this
https://vimeo.com/53870405

As you can see, the output is kind of "noisy"
I checked all the cables and everything was fine. I used a signal generator (to discard problems with TNC and a noisy output) to simulate the FSK signal and the RF output was the same and I also checked ICOM's configuration (9600 mode off).

Well, my first question is: Is this behavior normal?
I think that is NOT normal because with the same analyzer I've checked the FSK signal transmitted by a small transceiver (which I want to communicate with the ICOM as my final objective) and the signal received was "clean" as in the picture, so, what can be wrong? the TNC, the ICOM? I've read about frequency stability problems of some ICOM 910H, Can be this the reason fo the problem?

Hope that you can help with the possible solution or ideas to test my equipment.
Many thanks.

Kind regards.




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MERSAULT
Member

Posts: 21




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« Reply #1 on: November 23, 2012, 04:37:21 AM »

any clue?
I've trying every option in the Icom to discard it, but all the test have failed.
Thanks.

Regards.
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NM3G
Member

Posts: 47




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« Reply #2 on: November 23, 2012, 06:37:58 AM »

It appears you are in FSK mode, and not USB-D (upper side band - data input).

The signal you are generating has all of the appearances of a (poor) Gaussian Minimal Shift Keyed (GSMK) emission at about a 600 Hz data rate, not an AFSK signal.

Please verify your audio signal is routed to the rear panel audio input, and you are in USB (LSB will work as well, just with inverted mark/space signals).

Good luck and hope this helps.

73,
Rick
NM3G

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MERSAULT
Member

Posts: 21




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« Reply #3 on: November 29, 2012, 10:41:35 AM »

Thanks NM3G,

I checked and all the connections are OK.

I'm using data input, data output, ptt and ground on the Data socket in the rear panel for the main band.

I'm using an Icom-910H that doesn't have true FSK mode, only AFSK. What I guess that should happen if my input is a signal with only two voltage levels is a frequency spectrum with two peaks (mark and space)  [Something like this http://www.n8mdp.com/images/mmtty_tuning.jpg] but what I'm seeing is the two peaks with many others as my first post shows.

Can be a problem with a ground loop?
Could be a problem with crystal oscillator? (I've seen this problem in ICOM 910H around the internet)

Anyone with an ICOM 910H could share the experience, configuration, leassons learned with this kind of data input and modulation?

Thank you.

Kind regards.
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NM3G
Member

Posts: 47




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« Reply #4 on: November 29, 2012, 02:34:23 PM »

I missed the point where you are injecting a square wave. Square wave is loaded with harmonics!

Try injecting a single tone sine wave, and make sure the microphone is disconnected from the front panel (see IC-910H manual, page 53). You should, in SSB mode, see one tone on the spectrum analyzer (R&S ... nice!). Change the sine wave tone frequency and you should see the tone on the spectrum analyzer change.

In 1200 BPS mode, the microphone is active ... and will also modulate your signal.

Make sure you don't overdrive the audio input into the data jack ... see page 54.

73,

Rick
NM3G



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MERSAULT
Member

Posts: 21




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« Reply #5 on: December 03, 2012, 01:57:40 PM »

Yes, square signal has a lot of harmonics. I thought the same and I also put a sine wave as input but I got almost the same waveform in the spectrum analyzer. In my opinion, that's because is AFSK modulation so, it modules from 0 to Vpp. (~0.4 Vpp to not overdrive the input)

Also, I'm using FM mode, because that's the mode to communicate with the small transceiver that will complete all the comm system.

In all the tests, I had disconnected the mic.

Thanks.
Kind regards.
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