(Fairly) Simple 75M SSB rig with PTT

NOTE: The original simple SSB presented on this page has been replaced by an improved design. These changes increases the total parts count some, but are well worth it. The improvements include active MOSFET QSK switching instead of a passive, series L/C with clamping diodes. This improves receiver sensitivity and eliminates possible IMD caused by strong signals clipping in the diodes. The Audio muting scheme has been changed. T/R switching has been improved to eliminate spurious outputs during T/R switching. Filter termination has been added to flatten out response of the crystal filter. A second tuned circuit has been added to the transmitter mixer output. The Mic input has a high pass filter added to eliminate possible low frequency hum pick up when using a speaker mic. The transmitter circuit has been improved, allowing for more drive and power output. The circuit board layout has been improved to maximize the amount of ground plane and to incorporate the circuit changes.


Nearly all the simple SSB rig designs you will find use relay or mechanical switches to go between receive and transmit. Relays use extra power and take up a fair amount of space. They can also be expensive. Mechanical switches don't use power, but also take up a fair amount of space and often have to be located in inconvenient mechanical locations to minimize lead lengths. They also make the rig harder to use. The solution is to use electronic switching. Often this is done with diodes or bi-directional amplifiers. Both of these approaches can get complicated. The approach used here is to use a 74HC5053 analog switch and it works well. Two sections of the analog SPDT switch are used to toggle the crystal filter between the inputs and outputs of SA612 mixers. The third section switches a by pass cap between the inputs of the mixers, depending in which direction their being used for.

Transmitter chain:

The output of the Tx mixer is buffered by a 2N4401 emitter follower to drive the low impedance input side of a 10.7 MHz IF transformer. The transformer is returned to 75 meters by an additional, external 330 p cap. A second IF transformer is used to make a double tuned band pass filter, although for 75 meters, a single tuned circuit would be sufficient, as the IF and LO are far removed from the transmit signal. The output of the band pass filter is amplified by Q9 and further amplified by Q10 to provide sufficient voltage swing to drive the gate of the IRF-510 PA. C47, a 100p cap has been added from the collector of Q9 to ground, which eliminated some spurious outputs at some drive levels. Power output of 5 to 6 watts is produced by Q10, the IRF-510. Power output is greatly influenced by the match presented by the low pass filter. Minor adjustment of the spacing of the wire used to wind the low pass output toroid will make significant changes to the power output. Therefore, the spacing of the turns on these cores should be done to produce the best power output. Power output will vary with frequency, with about a 1 watt variation in a 200 kHz tuning range. Therefore, the tuning range and LPF should be optimized for the segment of the band you wish to primarily operate in.

Matching Crystals:

Closely matching the crystals for a SSB rig like this isn't as important as it would be for a CW rig. You can probably get away with out matching them at all. However, if you do have a frequency counter and a test oscillator to use, matching the crystals to with in 100 Hz or so isn't a bad idea. The crystals can be the inexpensive microprocessor type, either tall or short can, available from Mouser and Digi-Key by various manufactures.


The receiver input signal first passes through the transmitters low pass filter and then into the QSK switching FETs. During receive, Q2 is turned on, allowing the signal to pass through it to the receiver input. During transmit, Q2 is turned off, disconnecting the receiver input from the LPF. In addition, Q4 is turned on, shunting any leakage through Q2 to ground. Q5 is used as an inverter so the proper logic levels are applied to the gates of Q2 and Q4.

The input signal is then mixed with the LO in U6 to produce the IF of 9 MHz. The mixer output is routed through the crystal filter with U5 and into the product detector U4. The resulting audio signal is amplified by U3b and passes into the volume control. U2 provides additional amplification and drives the speaker. During transmit, audio is muted simply by shunting the audio input to U3 to ground using Q11.

MIC/PTT input:

If you use a speaker Mic, the mic can simply be connected to the MIC pad. Inside a speaker mic, the Electret mic is connected through a NO PB switch, which grounds the mic element. This causes a voltage drop across the mic, which is sensed by the U3a op amp to switch the rig into transmit. Some (if not all) speaker mics have a resistor across the mic element. If this is the case with your mic, the value of R4 will likely have to be reduced so that there is 3-4 volts across the mic element. The resistor across the mic element also reduces the output level, so it is actually better to remove the resistor from the mic and use the 22 K value shown. This will alllow you to get full power output with out the mic gain trimmer turned all the way up.

Separate Mic/PTT inputs:

Using the voltage change across the mic to switch to transmit can cause a spurious output from the transmitter as it switches on and off. This is because as the voltage across the mic changes, a voltage transient propagates through the coupling caps to the balanced mixer, causing it become unbalanced for a few milliseconds. To avoid this or to use a separate PTT input, the track connecting the MIC and PTT pads on the bottom of the board can be cut. In this case, a 22K resistor is used in R3 location. Now the mic is always on, so there is no voltage change across it. The rig is put into transmit mode by grounding the PTT pad.


There are a number of options you can choose from for a VFO. The simplest being traditional analog techniques. A frequency range of 5.00 to 5.50 MHz is required for full 80 M band coverage.

There are two standard crystal frequencies available which will put the rig in the phone segment of the band, 5.068 and 5.185 MHz., producing an operating frequency of 3.932 and 3.815 respectively. 3.815 would probably be the better choice. By using a VXO circuit, the frequency could be pulled somewhat, though with 5 MHz crystals, it wouldn't be too much.

A simple and surprisingly stable VFO (PTO) can be made from a brass screw and a Nylon threaded spacer. I used a 1" long, #6 nylon threaded spacer. A longer one might be better. Find the longest #6 brass screw you can find, at least 1 1/2" long. Cut the head of the screw off and put a short 1/4" diameter #6 round brass spacer on the end of the screw and solder it on. Now you have something to attach a knob too, with out it being all wobbly. Construction details can be seen in the photo at the bottom of this page.

The PTO is built into a box made of copper clad board. The PTO coil is supported on the back side with another #6 screw. A #6 brass nut is soldered to the copper on the front side, where the tuning slug (screw) will go into.

To wind the coil on the Nylon spacer, if its a hex spacer, put a little notch near the front and rear of the spacer in one of the sharp edges of the hex. Put the wire in the notch at the beginning and end of the winding. A short screw in the spacer can be used to hold the wire ends tight. Once the coil is wound, cover it with a layer or two of epoxy cement to hold the wires in place.

I built the oscillator and buffer circuit on to a separate piece of singe sided copper clad board. "Island" pads were cut from the copper to mount the parts on. The board is mounted into the box with short stand offs. This reduces any capacitive coupling from the circuit to the rest of the box which might affect drift.

The oscillator frequency is set by the series value of L1+L2 and C2//C3. Since I couldn't wind enough inductance on the nylon spacer to use with the 560p caps, L2 was added. This also has the advantage of providing a means of setting the VFO frequency, with out having to add trimmer caps. Simply change the spacing of the turns on the core. It might be a good idea to start with an extra turn or two on the core, as the actual frequency can be influenced by how the circuit is built and parts mounted.

I used polypropylene caps for C2 and C3. NPO or C0G could be used instead.

C4, L4 and C5 form an output filter to remove harmonics and adjust the output level to something suitable to drive the SA612 mixer. The 1K resistor terminates the filter, ensuring proper operation.


A printed circuit pattern, component location overlay, hole drill size diagram and parts list with supplier part numbers (sorry, haven't had time to make up the parts list yet) are provided in separate pdf files in a zip folder which can be down loaded here: ssb80_files.zip

To print to scale from the Adobe pdf reader, in the print set up menu, unselect "fit to page" and select "print image".

Note that the board tracks are through board view. If you use toner transfer sheets to make boards, you can print onto the sheet as is and the layout is reversed when you do the transfer. If you use photo sensitized boards, you need to reverse the image by putting the printed side against the board.

When soldering the components, be careful not to short to the ground plane surrounding most of the pads. Be sure to use sufficient heat when soldering components to the ground plane, as thermal relief tracks are not used. Before soldering in U5, note that a point to point jumer is required under the chip, though this jumper could go on the bottom of the board. In all, there are seven point to point jumpers required on the board. The reason for using most of these is to ensure that the ground plane was not broken up or became isolated in places. Also note that two long jumpers are required to connect the holes labeled "+DC" together.

Tune up and adjustment:

The most critical adjustment is the BFO trimmer, C57. If the BFO frequency is too low, excessive carrier will leak through the SSB filter and audio high frequency response will be attenuated. If set too high, low frequency audio response will suffer. A trimmer from the parts supplier is set to maximum capacitance. Give the trimmer about a 1/4 turn to set the capacitance to mid value. Connect your VFO to the board, a speaker, antenna and finally DC supply and turn on the board. Assuming no other problems to track down, you should be able to hear signals when you tune around the band. Peak T1 for best signal strength. Find a SSB signal to listen too and tweak the BFO trimmer to give the best signal clarity.


Disconnect power to the board and insert a DC amp meter in series with the supply. Turn the PA bias control trimmer to full counter-clockwise. Connect a RF power meter and 50 ohm dummy load(min10 watt) to the antenna. Re-apply power. The board can be put into transmit mode by using a clip lead to ground the MIC input. Note the current being drawn by the board. Now turn the PA bias trimmer clockwise until the board current increases by about 10 to 20 ma. Be careful not to exceed this. It is possible to fully turn on the PA, at which point it will suck as much current as the supply will deliver. If the supply is not current limited in some way (electronically or with a fuse), either the PA or the power supply will fry.

The transmitter band pass filter, T3 and T4 can be peaked by re-adjusting the BFO trimmer to maximum capacitance. This will allow some carrier to pass through the crystal filter. With the MIC input grounded, adjust T3 and T4 for maximum power output. Once this is done, re-adjust the BFO trimmer so power output just goes to zero. This will likely be the best BFO setting.

An alternative way of peaking T3 and T4 is to insert an audio tone into the MIC input. A 10K resistor will have to be placed between the MIC input and ground to switch into transmit mode. A 1000 Hz test tone with adjustable level would be ideal. Leave the MIC gain set to mid setting. Adjust the tone level to produce some power output and peak T3 and T4. Now increase the tone level until power output no longer increases. At this point, you can tweak the spacing of the turns on L3 and L4 in the output low pass filter for best power output. Doing this can make a remarkable improvement in power output by optimizing the matching between the PA and load.

MIC gain adjustment:

Setting this control can be a little trickier too. Too much gain and the signal will distort, while too little will result in low power output. The power you see on an average reading watt meter when using normal speech should be about 1/3d that which is seen when you whistle loudly into the mic. The best way to set the gain is to look at the output with a Scope and make sure it looks like the pictures you see in the Handbook.

73, Steve KD1JV

Main board schematic

5.00 to 5.5 MHz, brass screw tuned PTO

Close up of PTO assembly. Back and Right side removed for access. Wax may be melted over parts for additional mechanical stability