SSB Exciter with IF-Clipper for QRP.
DL6ZB, Rolf Heine



This is a description of a simple but effective single sideband exciter for 10.7 MHz with a built-in IF clipper. By changing the crystals, this concept can also be used for other IF frequencies (e.g. 8/9 MHz).

I have developed a special 50 Ohm SSB (USB/LSB) crystal filter for this exciter. It is a so-called bridge design. These filters are more suitable for SSB generation than ladder filters.

This is because bridged filters can be tuned very easily without changing the impedance significantly. Just change the 33pF capacitors to change the bandwidth of the filters. For IF stages of receivers, these filters can also be used. For higher filter orders just cascade them. The bandwidth is only dependent on the value of the capacitors (see C1/C2).

The higher the value, the narrower the bandwidth will be. Within a 2.5:1 VSWR bandwidth (50 ohms) these filters can be tuned from 6000 down to 300 Hz bandwidth. The sideband is changed by tuning the carrier oscillator crystal. The crystals don't have to be selected or tuned. However, I recommend to use crystals of the same type (manufacturer).

The switchable built-in IF clipper is a well-known simple but effective diode clipper. It consists of germanium diodes and a step-up transformer. It was first described by Wes Hayward, W7ZOI. The IF clipper is followed by a second crystal filter. This provides a clean SSB output signal.

In theory, a clipper offers a +6 dB improvement in readability. This could be a big help for QRP. I use this exciter in my 160/80/40m QRP SSB transceiver with good results.

The "SSB EXCITER 10,7.brd" is an Eagle CAD file. A free (limited) version of Eagle PCB layout software is available from CADSOFT. Search for the free version of Eagle.

The bottom layer is the routed layer, the top layer is just ground.

Try this link for the CAD software: https://www.autodesk.com/products/eagle/free-download
Click here for the 10.7 MHz SSB Exciter PCB layout file

UPDATE:
I would like to pass on some of the experiences of people who have rebuilt this SSB exciter.
The exciter is apparently safe to rebuild and works well. However, the attenuators at the inputs and outputs of the crystal filters seem to attenuate too much, so that the clipper does not work at full intensity.

I recommend not using the attenuators at the input and output of the first crystal filter which is just ahead of the balanced modulator. However, you should check the frequency response of the first crystal filter after you removed the attenuators.

Removing the attenuators will allow the clipper to be much more efficient. In addition, I would recommend to only use either Germanium diodes or small Schottky diodes for the clipper.


Circuit Diagram:
Click on picture for a large scale view





Place plan:
Click on picture for a large scale view




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