6 METER SSB Transceiver. HOME

This project was going to be a 20 to 6 meter transverter, but I decided it wouldn't take too many more parts to make it a stand alone SSB rig and it would be more practical in that configuration. The transmitter power output is only about 1 watt pep, so eventually I will need to come up with a linear to get a more reasonable power output of say 10 to 15 watts. That will take at least a $25.00 order to RF Parts for the transistors. More likely, I'll see what I can get out of an old 6M FM "brick" amplifier I picked up at a hamfest some time ago, with the PA biased for linear operation of course.

The transceiver uses a 14.318 MHz IF and a 64.5 MHz LO. The LO is generated by a 9.126 MHz VXO oscillator which is passively multiplied by 7. This is done by converting the oscillator output into square waves using a 74AC00 NAND gate. This creates a harmonic rich output and the 7th harmonic is picked off by a double tuned circuit. Fixed capacitors and air wound inductors are used for the tuned circuits, which are inductively coupled. The circuit is tuned by spreading the turns on the coils. A following amplifier produces about a 1 v p-p LO signal to drive the SA612 Tx and Rx mixers. I have not checked the tuning range of the VXO yet, as I only have one 9.126 MHz crystal on hand at the moment. However, I expect the tuning range to be from about 50.15 to at least 50.20 MHz. I would use at leat a 100 pfd variable cap for VXO tuning.

A SA612 is used for the Tx mixer and is tripple tuned at the output frequency. I had originally tried double tuned output, but there were a couple of fairly close in spurs which weren't being attenuated enough. Going to triple tuning did the trick. The 50 MHz output is amplified by several stages of MPS5179 transistors, which have a hft of 900 MHz. Small binocular balun cores are used for coupling transformers between stages. The PA is a 2SC1970 VHF transistor, rated at 1 watt output at 175 MHz. Although I can get 1 watt peak output, 750 mw is more likely once the PA warms up.

Q7 and D4/D5 provide linear bias for the PA and isolated by L4. D5 is thermally coupled to the PA to prevent thermal run away. Q6 is used to switch the DC supply to the transmitter circuits when transmitting.

The receiver section uses two more SA612 mixers. An inexpensive reed relay is used for T/R switching of the input signal from the transmitter LPF. The IF output of the 1st mixer is amplified by Q3, which improves receiver sensitivity at the expense of increasing the noise floor. However, with out this extra stage of amplification, the receiver is fairly "deaf", taking about a 5 uV signal to be heard in a speaker. With the amp, the sensitivity is a more reasonable 0.5 uV. The IF signal is then routed to the crystal filter through a 74HC4053 analog switch. During transmit, this switch will connect the crystal filter to an buffer/amplifier stage to drive the Tx mixer. Also during transmit, Q4 shorts Q3 out. Without this, there isn't quite enough isolation to keep the Rx input mixer from back feeding into the filter, causing a feedback situation resulting in power output when none should be present. Adding Q4 was easier than turning power to the Rx mixer off during transmit.

The other end of the crystal filter is connected to another section of the HC4053 switch and routes the signal to the input of the BFO mixer. During transmit, the filter is connected to the output of this mixer. The audio output of the BFO mixer is routed to the 1st audio amp stage through the third section of the HC4053 switch. During transmit, the input to the audio amp is grounded through the analog switch to mute the audio. Finally, the audio goes into a volume control and then a LM386 audio power amp to drive a small speaker.

The BFO mixer is also used as the transmit balanced modulator. Audio is injected into the input of the mixer at the same pin as the IF signal, so is isolated with an RF choke. Q2 is used to short the microphone input to ground during receive, which reduces noise pick up. C1/C2 and R3 form a high pass filter to reduce 60 cycle hum pickup. A Electret mic is used for the microphone. This doesn't have quite enough output to fully modulate the mixer by its self, so a single stage transistor amp is used so one doesn't have to shout into the mic.

U1 b is used for T/R switching. The + input is biased slightly lower in voltage then the 5V supply. A PTT switch in series with the mic element makes the voltage at the junction of R2 and the mic sit at 5V when the PTT switch is open. This makes the output of U1b high. When the PTT is closed, the voltage across the mic drops, causing the output of U1b to go low and turn on the transmitter circuits. This allows using a speaker mic with the transceiver.

Some parts were added to this board during development. The mic pre-amp can be seen at the lower right hand corner of the board. I only had two 14.318 MHz crystals on hand, so the IF filter only has one crystal in it. Even so, it is enough to get a USB signal out of it, with reasonably good LSB suppression. The board is 5" wide by 4" long. .01 and 0.1 ufd chip capacitors were used on the bottom of the board for by-passing and where ever a 0.1 ufd cap was needed. This saved some board space and made layout easier. Peaking the LO tuned circuits and Tx tuned circuits could be done with a diode probe, but a 100 MHz BW scope would be better. Peaking the Rx input circuit could be a little tricky if you do not have a signal generator or other source of a 50 MHz signal in the tuning range of the receiver.

I'll eventually add the board layout files here, but first I need to make the final layout and make sure all the schematic part designations match those on the board layout.