facelift for the 7MHz TRX

Posted on Feb 6, 2014

Had developed the 7 MHz CW TRX for my first trip to the US. The receiver worked very well. (Actually a lot better than here in Europe since they do not have high power broadcast stations on 7. 1 MHz. ) The transmitter is also quite ok. 1. 5 Watts is not a lot, but that was the compromise I had chosen to save current. The thing that was not really good was the temperature stability in transmit mode.

facelift for the 7MHz TRX
Click here to download the full size of the above Circuit.

While it was no problem at all to listen to a certain station for a couple of minutes without retuning, the LO really started to drift in TX mode. If I made a longer transmit, giving my name, my QTH and the report I really had a hard time to recognize the other station when I was back in RX mode. And it was really hard to find friends on a certain frequency since the analog frequency readout was quite coarse. I decided I needed a PLL (phase-locked-loop) synthesizer with a crystal as frequency standard. I did not know about a PLL IC for 7 MHz that would give a small enough step width. Having a RX filter that is only a few hundred Hertz wide, I needed a maximum of 100Hz as tuning step width. So I choose a quite unusual solution. I used a 900 MHz VCO (readily available from a couple of suppliers, usually used in mobile telephones) and a MB15E07 PLL synthesizer from Fujitsu. It can give a step width of 8kHz at 1GHz. The 900 MHz signal is divided by 128 by a cheap U893 prescaler from Temic Semiconductors. This reduces the frequency to 7MHz and the step width to 62. 5 Hz. For programming the PLL I need a micro controller anyway, so I decided to remove the analog instrument and replace it by a LCD-Display. This way I can realize a digital frequency readout. To be frank I liked the analog instrument but the digital frequency readout makes it a lot easier to find your QSO partner if you have an appointment (a sked) with a friend or...

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