Visualizing the operation of SSB filters

 Visualizing the operation of SSB filters


Many pre 2000 HF radios are double conversion on TX and triple conversion on RX. a popular crystal filter IF freqeuncy is 9MHz. The BFO (beat frequency oscillator works at the same frequency . HOwever some method (usually involving capacitors) is used to slightly change the exact frequency to either 9.013 Mhz or 9.010 MHz. Here is an illustration on how, in the TX mode, the desired sideband is chosen.

For example we want to transmit USB on 14MHz. To achieve this, we have the produce a double sideband signal using a 9.013 carrier frequency. This double sideband signal is made to passthrough the crystal filter. Only the LSB will appear on the output of the filter. The USB signal will not allowed to pass through. This is because of the carrier frequency we used. (If we had chosen a different carrier frequency , for example 9.010, the double sideband output will be entirely shifted down up. And if we pass this through the crystal filter, the LSB will be chopped off and only the USB will go through)
Since the radio is double conversion a 9Mhz LSB signal is subsequently converted to a 70MHz LSB signal. This simply done by mixing it with a 61MHz signal. Since this is simple addition no sideband transformation occurs.
At the last transmit mixer the signal from a local oscillator operating at VHF (typically from 70 Mhz to 100 MHz) is mixed with the previously mentioned 70MHz LSB signal from the 70MHz ceramic filter output. As with all mixers, we expect the sum and the difference signals to be produced. It is the job of a low pass filter at the output of the mixer to remove the sum signal so that we can retrieve the difference signal. If our transceiver is set 20 m band our local oscillator will be at about 84MHz. 84 MHz minus 70 MHz will give 14MHz and since we subtracted, the LSB of the original 70MHz signal will be transformed to USB signal now at 14 MHz. This is now amplified and output to the antenna terminals.
A similar process happens in the RX side.
May be an image of text that says 'By selecting either 9.013 MHz or 9.010 MHz as carrier, we can get either LSB or USB !!! 9.013 9.010 9 MHz Filter'





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