radio mf tx


Posted on Feb 4, 2014

I`ve had an interest in building a simple VXO transmitter for many years, but was unsure that the limited tuning range usually associated with VXOs would be useful on the busy HF bands. However, the ability to switch a number of crystals, and cover a number of different bands in one transmitter greatly increases the chance of finding a clear frequ


radio mf tx
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ency on which to operate. The transmitter uses a FET VXO; push-pull buffer; 2N2222 pre-driver + BC212 keying transistor; 2N3553 gain-controlled amplifier; 2SC2166 driver and a pair of 2SC2166 transistors in parallel in the power amplifier (PA). With limited shack space in mind, the Multi-Four was built into a small diecast box measuring only 255 (W) It is easier to tune a crystal to the high side of its intended frequency of operation, so the crystals for the `20` and `30a` slots on the crystal switch (SW1a) were custom made to resonate at 2 kHz below the international QRP frequency, thereby providing a good tuning range above and below the usual centre of QRP activity. The VXO drives a very effective push-pull buffer, which presents a chirp-free signal to the driver stages. Note that the green LED associated with the selected frequency is only illuminated when the correct band has been selected in the Low Pass Filter circuit. The first stage of the switched-gain amplifier uses an untuned common emitter amplifier (2N2222), keyed by a BC212 PNP transistor. In my experience, many simple TX designs fail to provide adequate shaping of the keying waveform, resulting in very hard keying. To reduce the likelihood of transmitting key clicks, the keying circuit in this transmitter provides rise and fall times of around 8 ms. The waveform shaping - together with the effective VXO buffer results in a very pleasant T9 note. The keyed...




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