HF transceiver

The choice of IF is usually determined from availability of filters with desired characteristics. For communication receiver or transceiver the requirements for main selective element are very high, and usually two types of filters are used: electromechanical (f~200-500kHz) and quartz crystal (f~5-9 MHz). But if low IF is used, the relative difference in frequency between fundamental
HF transceiver - schematic

and mirror channels becomes to small for input signals with F>4-5 MHz, and necessary suppression of mirror channel becomes unachievable by using common resonant LC-band-filters. two fixed intermediate frequencies, the first one in range about 3-8 MHz, and the second one - about 465-500 kHz. In this case, main signal filtering occurs on the second IF. variable first IF and fixed second IF. This is very common variant for homebrewed transceiver, because it has following very important advantage: variable frequency oscillator (VFO) works on rather low frequencies and is not switched by bandswitch. However, in this case all bands have the same width, which doesn`t correspond with amateur radio frequencies first IF higher, then highest received frequency, for example 35 MHz. This principle is used in many modern communication receivers. The advantage is, that no band-pass filters (BPF) are required for each band, because mirror frequencies are outside HF and can be suppressed by appropriate non-switchable low-pass filter at the input. But it requires high-frequency VFO (usually digital frequency synthesizer), which is difficult to make at home My SSB/CW transceiver, that i describe below, has 2 fixed intermediate frequencies, the first one is 5. 5 MHz, and the second (main) one is 500 kHz. This choice is not mine, i just followed the described UA1FA transceiver schematics. The main selective element is electromechanical filter...

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