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Author Topic: Transmitting FM by using a SSB transmitter ?!?!  (Read 10290 times)
DL2NEP
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Posts: 18




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« on: January 13, 2016, 04:16:21 AM »

Hi,

last week I bought an Icom IC-7200 transceiver. This transceiver does not provide the mode FM. I my opinion there is no need of FM at Shortwave. But I begin now to think about the technical possibility to transmit FM.
The IC-7200 has a build in USB-soundcard. My idea was to set the transceiver to USB/LSB and generate a "special" signal trough a PC to transmit FM. So now my question is: Does anybody have experience about that or is this idea not possible ?
I think it should be possible to do that. But I don't know the way.

Vy 73 de Bjoern, DL2NEP
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W1VT
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Posts: 3358




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« Reply #1 on: January 13, 2016, 04:24:42 AM »

Does your rig have a wide enough passband to transmit FM?  A high quality crystal filter for the rig I use costs more than an FM rig.

Zack W1VT
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DL2NEP
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Posts: 18




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« Reply #2 on: January 13, 2016, 05:01:59 AM »

I think the passband will  be very small. At the moment I am interested in the theoretical way. Is it possible to do that or not. I am not sure about that. But I am able to change amplitude and frequency with an SSB signal. So it should be possible ?!

Vy 73 de Bjoern, DL2NEP
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N8YX
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Posts: 1369




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« Reply #3 on: January 13, 2016, 06:15:18 AM »

Does your rig have an RIT/XIT function that's hardware (varicap diode) based, rather than software based?

If so...vary the varicap bias voltage on transmit with the audio signal you wish to transmit. Similar in nature to FSK but with a wider spectral range.

As was pointed out by another poster, your TX IF chain must support the wider B/W inherent with FM.
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WA2ISE
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« Reply #4 on: January 13, 2016, 10:41:07 AM »

last week I bought an Icom IC-7200 transceiver. This transceiver does not provide the mode FM. I my opinion there is no need of FM at Shortwave. But I begin now to think about the technical possibility to transmit FM.

There's some FM on 10 meters.  And a few repeaters.  Usually not too active, except for band openings when the sunspots are good. 
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DL2NEP
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Posts: 18




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« Reply #5 on: January 14, 2016, 08:34:25 AM »

Does your rig have an RIT/XIT function that's hardware (varicap diode) based, rather than software based?

If so...vary the varicap bias voltage on transmit with the audio signal you wish to transmit. Similar in nature to FSK but with a wider spectral range.

As was pointed out by another poster, your TX IF chain must support the wider B/W inherent with FM.

I don't want to modify the radio. My idea was to generate a signal that is transmitted in SSB but result in a FM transmition.
So the way should be   mic --> PC (with some strange software  Shocked ) --> SSB-TX ---> FM signal at the antenna. But Iam not sure this would be possible. There is no need to do that. But Iam interested in the idea.
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WA2ISE
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« Reply #6 on: January 14, 2016, 12:22:52 PM »

My idea was to generate a signal that is transmitted in SSB but result in a FM transmition.
So the way should be   mic --> PC (with some strange software  Shocked ) --> SSB-TX ---> FM signal at the antenna.

You could produce a constant amplitude tone about 1550Hz, and frequency modulate it, deviation about 700Hz, assuming the SSB bandwidth is like the telephone, 300-3000Hz.  Feed that into the SSB radio mic jack.  Though the modulated bandwidth would be kinda narrow.  Carson's FM rule where CBR is the bandwidth requirement, Delta f is the peak frequency deviation, and Fm is the highest frequency in the modulating signal.would give you about 700Hz max of audio.  Simulated some speech thru that, it sounds very muffled and barely understandable (and that's when I know what is being said).  Forget it, unless I got something here wrong...  Grin
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KK5DR
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« Reply #7 on: January 14, 2016, 05:42:40 PM »

The answer IS, ICOM used a 6khz roofing filter to have good rejection on SSB. To keep cost down and budgeted to a price point of $1000usd, they could not put the required 15khz roofing filter to enable passing of an FM signal, this would have pushed the price up too much.
A DSP based system can emulated any mode it's programmed to emulate. Roofing filters and budget determined the rest.
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K5TED
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Posts: 233




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« Reply #8 on: January 14, 2016, 08:55:47 PM »

Why commit to a SSB only transceiver then go through all sorts of contortions and apologies to transmit FM on the only HF band where it is allowed? There are plenty of HF rigs out there that do FM natively?

"my opinion there is no need of FM at Shortwave" Your opinion solidified your status. You do not have FM. It is not feasible with your transceiver. Why beat a dead horse?

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G3RZP
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« Reply #9 on: January 15, 2016, 04:01:15 AM »

Another little problem with the filters is the group delay variation over the passband.....
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ND6M
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« Reply #10 on: January 16, 2016, 01:22:04 PM »

...There are plenty of HF rigs out there that do FM natively.... It is not feasible with your transceiver. Why beat a dead horse?


Exactly correct,....... I don't even try to tow my bass boat with an economy car.
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W8JX
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« Reply #11 on: January 18, 2016, 09:53:26 AM »

Exactly correct,....... I don't even try to tow my bass boat with an economy car.

I remember a time when the family sedan had more towing power than many modern 1/2 ton SUV's that are the rage today for many.
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
KC2QYM
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Posts: 958




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« Reply #12 on: January 19, 2016, 09:16:02 AM »

I think the initial poster is interested in getting FM capabilities for 10 meters only on his rig; not the other HF bands.  I am not technical enough to advise in this area but if there is a technical feasibility to add FM mode to his rig, why not explore the possibility.  However, I do have a feeling that the effort may not be worth the pain.
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AE4RV
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« Reply #13 on: January 19, 2016, 10:41:13 AM »

I think the initial poster is interested in getting FM capabilities for 10 meters only on his rig; not the other HF bands.  I am not technical enough to advise in this area but if there is a technical feasibility to add FM mode to his rig, why not explore the possibility.  However, I do have a feeling that the effort may not be worth the pain.

The initial poster has stated more than once that he is not interested in using FM on HF but is very curious if it is possible with an SSB rig and a computer. It is clearly a thought experiment.
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DL2NEP
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Posts: 18




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« Reply #14 on: January 26, 2016, 04:12:58 AM »

That's right. I'm only interested in the knowing about the possibilities.
I think it will not work good enough for using it. You have to many time lacks. Transmission have to computed, and the received signal needs to be computed too. If I want to transmit FM I will use my second transceiver.

Thank you very much to help me with my idea.
Conclusion: It is possible but it make no sense in that case.
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