LSB Conversion of RT321 and RT320 HF Sets

Posted on Feb 5, 2014

The RT320 and RT321 Clansman HF sets are designed for Upper Side Band (USB) operation only. This conflicts with the Amateur radio convention which is to use LSB on bands below 10MHz. In both radios the IF used for SSB generation is 1. 7500 MHz and there is a sideband inversion between the IF and the antenna. So LSB at the IF comes out as USB and US

LSB Conversion of RT321 and RT320 HF Sets
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B at the IF comes out as LSB. The IF filter passband for SSB is roughly 0. 2 to 3. 0KHz below the IF frequency so the 1. 7500MHz carrier is blocked leaving pure LSB. To get LSB at the output it would be necessary either to change the filter to a USB one or to move the carrier below the filter passband. The latter approach is what was first documented by Dr. Andrew Smith G4OEP in his excellent article now available on the VMARS Manuals web site: A crystal at 3. 2KHz below the original 1. 7500MHz = 1. 7468MHz is needed for the G4OEP LSB modification. Unfortunately the size and cost of a 1. 7468MHz crystal is now somewhat of a problem and other cheaper solutions have been sought. One is to use a programmable oscillator comprising a VHF crystal and a PLL to generate programmed arbitary frequencies. Such an oscillator can replace the transistor CIO in the G4OEP circuit provided a suitable (5 V TTL) power supply is available and the output is adequately filtered and at a level compatible with the radio. A suitable oscillator is the SG8002-JAPTB which covers 1 to 125MHz and has a 0 to 70 degrees C tolerance of 50ppm (so would be up to 87. 5Hz off - not a problem for SSB but would affect narrow data modes). At room temperature samples have measured within 5ppm of the programmed frequency. Physically it is a J-Leaded plastic package about the size of an 8 pin DIP op-amp. The data sheet is at Important Note: The Output Enable is...

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