This project generates an SSB signal, on frequency, without the need for crystal filters. Sideband switching is achieved with a single 2-pole changeover switch at AF levels. The input frequency is from a VFO oscillating at 9MHz. The SSB output is the same frequency as the input frequency and may be varied, by up to 10%, without degredation of the SSB generated. Unwanted sideband and carrier are well below 40dB of the wanted sideband. At the frequency of alignment, they may be typically 50-55dB down.
The circuit described was designed to generate 9 MHz, so that by mixing a 5 - 5.5 MHz VFO with the 9 MHz I produced both 14 - 14.5 MHz, AND 4.0 - 3.5 MHz. This got me working on the two HF bands I was interested in; 80 meters and 20 meters. I will be posting the mixer and 10-watt PA as separate projects later on.
The circuit may also be modified for use with a 5 - 5.5 MHz VFO input which will make this circuit ideal for modifying old CW-only rigs for SSB operation, if the CW rig uses a 5 - 5.5 MHz VFO (Ten-Tec, Heathkit etc.).
This is a microphone amplifier which is followed by an audio phase - shift network, providing four audio signals. Each audio signal has a phase lag of 90 degrees from the previous output. These four AF signals are required for the balanced modulator which generates the SSB. The original circuit was from the RSGBs VHF/UHF manual but much work has been done to improve performance. Only the microphone pre-amp remains of the original design. The new design generates a four-phase signal that is accurate to well within 1 degree.
Other than the microphone gain control, there is no adjustment or alignment necessary. This can be done on the air. If you have access to a dual beam oscilloscope then you can test the AF Processor board:
T1 is a driver transformer, (blue) robbed out of those cheap'n nasty AM transistor radio's (you do save old radios, don't you?).
This circuit has fewer modifications as little work was necessary to better the performance. There is a two transistor buffer amplifier after the VFO, then T2, which has a very low Q and low impedance o/p to the balanced mixers. The RF phase-shift network divides the RF to two paths, each having a 90 degree phase difference. There are two trimmer capacitors for alignment. If operation is reqired on 5 - 5.5 MHz then the two fixed capacitors, marked with an asterix (*) should be changed to 100pf, and T2 + its tuning capacitor doubled in value. The original circuit worked also well on 14 MHz, but T2 was reduced to 13 turns.
I found alignment easiest with an SSB general coverage receiver that is placed close to the RF board. If you are aligning on 9 MHz and do not have a 9MHz receiver then see my article "QUICK RX".
I do have a PCB foil patern for this project but the file is too big for packet, and it is written in EASYPC. If I can get around to it I may scan the artwork and post the file.
Have fun, de HARRY, Lunda, Sweden.