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.
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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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