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Old 19-11-2006, 10:31   #1
pd1acf
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HAM Project : Build our own DRM transmitter

Hello All,

Via this forum i would like to see if it is possible to build a DRM transmitter .

We are licenced radio amateurs, and are currently in the process of making a VHF FM repeater system. This would 'copy' received signals on 1 frequency to another frequency. This all within the allowed HAM frequencies.

Next to the above, i am thinking about 'copying' the same audio, adding a second transmitter, but now with the audio 'encoded' in DRM.

Until now i have only found 1 software for generating DRM signals, but i am seeking something more into hardware.

Although this project would require quite some time and experiments, i think it could be usefull to see how DRM signals behave on 145 Mhz and up.

Best regards,

ron
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Old 24-11-2006, 09:42   #2
CT4RK
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Hi Ron
Of course is possible you building a TX on DRM in VHF band. I was maked a local experiments with upconverters, but only for "play". I and CT1ADT are planning to build a 100 watts 26 MHz TX but is not for now yet.
You can from the 12KHz base band make a upconverter, with a DBM (is important to use a DBM for supress the LO and BB signal) to 455KHz. The LO should be low noise, and you can use a color TV cristal 4.433 Mhz and divide by 10 with a decade counter, ( I use the 74LS90 ) followed a low pass filter, to obtain a pure and clean 443 KHz signal. After, you need supress the image QRG, and you can use a good FM filter for 455KHz with 12,5 KHz band wide. Now you have a god DRM signal on 455 KHz. The next step is up this QRG to 10,7 MHz and use again FM filters. When you have a good DRM signal on 10.7MHz, You can easily mix this baseband on 10,7 MHz with a LO on VHF band for the QRG do you want. I think (never tested) we cannot double the DRM complex signal like you can in FM or AM mode. Also in SSB you only can convert the signal, not double or divide.
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Old 29-11-2006, 16:08   #3
Digger
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Hi,

The simplest DRM transmitter without any mixers and filters, try this:

Just feed the DreaM 12 kHz output from the sound card of your PC to the External Modulation Input of an FM Signal Generator.

Set the Signal Generator's frequency to a frequency in the FM Broadcast band, connect some form of an antenna onto the RF output jack and increase the generator output and receive the signal with a "normal" VHF-FM Receiver. Now take the 12 kHz AF signal directly from the Earphone Jack and feed it to the Line Input of a second PC, and enjoy DRM reception. Works perfectly.


I haven't tried this in the 2 m Ham band, but it would be worth a try doing the same with a pair of normal 2 m Transceivers. I believe you have to inject the 12 kHz DRM after the Mike Amplifier's Low Pass Filter, and on the receiver side you may have to retrieve the 12 kHz signal at the detector output.
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Old 30-11-2006, 15:47   #4
fibber
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2M FM ham rigs for DRM

I have fed the 4.5k bandwidth DRM on 2M and had success. In order to get 5k or wider it seems you would need to do some injection on the board rather than the mic input. I have never gotten that far, but the 4.5k will even pass on a local ham repater. WinDRM also works fine on FM or SSB ham rigs as it is narrower bandwidth.
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Old 07-12-2006, 18:09   #5
pd1acf
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Hi All,

Thanks for the feedback so far.
Happy to hear there is a posibility with Dream to also en-code.
I have today received the sat-schneider mixer to modify my FT817 rig (receive only offcourse).

When i have convert that succesfully i will have at least a good receiver, and the ability to compare the signal i will be creating, compared to regular DRM stations.

Will keep updates posted,

ron
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Old 28-12-2006, 19:22   #6
tedex
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any way to remove the irritating "trial" audio tag in while transmitting?
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Old 29-12-2006, 20:52   #7
tedex
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aaackk.... the "trial" message was coming from VAC

Anyway.. Here's what I played with today:

Dream transmitter 12khz output into a Ramsey FM-30 FM transmitter, mono, then received that signal at other end of building on Yaesu Vx-7r in FM wideband, decoded OK with Dream.

There is the problem of FM pre-emphasis built in to the transmitter that skews the response so that the left side fo the received signal is about 20db higher than the right, but careful input setting to the Dream receiver fixed it.

I might try this with an FM-25 that is easier to hack on to bypass the pre-emphasis.

Of course, the ideal thing would be to put together an AM transmitter...
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Old 22-02-2007, 07:38   #8
DRM-OM
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Quote:
Originally Posted by Digger
The simplest DRM transmitter without any mixers and filters, try this:

Just feed the DreaM 12 kHz output from the sound card of your PC to the External Modulation Input of an FM Signal Generator.

Although this off course will work, this has nothing to do with DRM as it is just a conventional FM-Signal with data modulation (and, by the way, much more broadband than the DRM-Signal itself).

DRM claims some of its advantages from using OFDM on the RF side - not conventional FM.

These claimed advantages are
  • some resistance to multipath reception (also called selective fading)
  • Resistance to narrowband interferers (which I cannot prove)
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Old 06-03-2007, 17:13   #9
yo7hli
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you should have a look here
http://www.yu1lm.qrpradio.com/homebrew.htm
there are several versions of a SDR receiver and also a SDR transmitter.
While the SDR receivers work ok with DRM, i don't know how the transmitter performs with DRM modulation applied.
Once I get the time to finish it I'll post the results.
Also if anyone built the yu1lm SDR transceiver it would be nice to share his/her thoughts with us
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