Unlike CW, where a couple of watts will make plenty of contacts, its hard to get anyone to talk to you on phone if your signal is weak. With this in mind, the original Simple SSB rig has been redesigned to add a 25 watt linear amp to it. Rather than making everything fit on one board, the linear is a separate module with a separate receiver/exciter board. One advantage of doing this is the receiver/exciter board could be used on bands other than 75 meters with just changing the receiver input and transmitter output tuned circuits. And of course, an appropriate LO frequency.
75 meters was chosen as the default operating band for this project, as the linear works well at this frequency, a suitable 5 MHz VFO is easy to make and finally, this rig would make a good emergency rig for local communications, should one live in an area prone to natural disasters. The rig draws little current on receive, making for use with a gell-cell power source very practical.
A few minor changes have been made to the original Simple SSB rig circuit, a simple R/C high pass filter added to the mic input to help reduce hum pick up when using a speaker mike and changing to 2N7000's for switching in a couple of places. A new board layout has also been produced. This layout is single sided and requires a number of jumpers, but no long floating point to point jumpers across large areas of the board. Even though the layout is for through hole parts, the board is only 3" X 3".
The linear uses IRF-510 MOSFETs in push-pull and a push-pull driver stage using 2N4401 transistors.
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T1 and T2 are 10.7 MHz IF transformers. The 470p cap shown across the secondary on T2 retunes it for 75M. A 330p cap is used on T1, as the 2N7000 used to shunt the input to ground during transmit adds some capacitance. The typical inductance of a 10.7 MHz IF can is about 4.5 uHy. If one were to want to build the exciter for 160, this inductance is getting a bit low to be practical. With care, it would be possible to open up the IF can and rewind the core with more turns, but very small wire is required. The upper frequency limit of 10.7 MHz IF cans (with the internal cap broken out) is 17 meters. At 15 meters, stray capacitance makes the coil self resonate.
The crystal filter is switched between the inputs and outputs of the two SA612 mixers for T/R using two sections of a 74HC4053 analog switch. The third section of the switch moves a by-pass cap to shunt the input which isn't used depending on the direction the mixer is being used for at the time. Q1, a 2N7000 mosfet is used to shunt the input of the LO mixer to ground when the mixer is being used in the transmit mode. Q8, another 2N7000 was added to shunt the audio input at the BFO mixer to ground during receive, as it was found noise was being picked up here.
Q2 buffers the transmit signal output of the mixer so that it can drive the low impedance side of the IF transformer and not load down the mixer output. Sufficient voltage is developed on the secondary of the transformer that it only needs to be buffered by the j-fet buffer and then buffered again by a 2N3904 so that it can drive the low input impedance of the linear amp.
T/R switching is sensed when the mic switch is closed and the voltage to the inverting input of U2a becomes lower than than the voltage at the non-inverting input. R16 and R19 form a voltage divider so the non-inverting input is slightly less than 6 volts, so that the op amp output is ensured to be low if the PTT is open. When the PTT is closed, the output of the op amp goes high, turning on Q5, which in turn turns on Q4. The turning on of Q5 is delayed slightly by C37, so that the filter and mute switch can switch before power is applied to the Tx output buffers and linear amp T/R relay.
C16, R2, C1 and V1 form a highpass audio filter so keep hum pickup from a hand held speaker mike from modulating the output. V1 can be used to adjust the power output by lowering the audio level. It might be worth adding a active low pass and high pass filter to the speech path, for better shaping of the audio. The simple high pass filter shown rolls off at a fairly high frequency in order to attenuate the 60 Hz hum pick up sufficiently.
The BFO trimmer cap C14, needs to be adjusted to move the carrier above the cut off frequency of the crystal filter for proper sideband rejection.
A bit mapped file of the circuit board layout for the SSB exciter board and component location overlay can be down loaded here: board files
The layout is "through board" view. This allows printing the layout directly to toner transfer film. The image is mirrored when you transfer it to the board. To print to scale, open the file with MS Paint and print from there. Other bit map file viewing programs might not print actual size. The component designations on the component location overlay maynot match the designations used on the schematic.
25 Watt Linear amp:
The linear was built on top of a 3" x 3" heatsink with 3/4" long fins. A single sided piece of copper clad board is placed on the flat side of the heatsink. As can be seen in the photo, rectangular holes were cut into the board around the PA MOSFETs. I used a carbide router bit to cut the holes. Messy, but quick. Ugly, dead bug construction was used, with a few isolated "Islands" cut into the copper at the supply end of the PA transformer, hot side of the bias adjust trimmers and RF input. The RF input is DC isolated from the PA ground to prevent DC ground loops. Full output is achieved with a few volts p-p RF input.
The PA output transformer is made with two BN-43-302 cores. Two cores are used to ensure enough inductance and prevent flux saturation. The primary to the Drains of the MOSFETs is made with a single hairpin turn of #14 solid magnet wire. Ideally, some brass tubing would be used instead, but I didn't have any which would fit into the holes. The output secondary is two turns of #26 stranded wire. The driver stage input and outputs are made with BN-43-2402 cores ( I think this number is correct). The cores are available from Diz, at www.kitsandparts.com
A miniature DPDT relay is used for antenna switching between the PA and receiver input. This was a little easier than making a diode QSK switch which can handle 25 watts. The relay also turns power onto the driver stage and gate bias circuit. Relay T/R switching is a lot less annoying on a phone rig than it is in a CW rig, where the relay is constantly chattering.
The bias controls are adjusted to produce about 10 ma of idle current in each of the PA MOSFETs. Short the RF input to insure there is no RF output. With an amp meter in series with the supply, put the rig into transmit mode and note the current. Adjust one of the bias trimmers to increase the current by about 10 ma, then the other one for an additional increase of 10 ma.
Low pass filter caps should be rated for 200 volts.