SSB TRANSCEIVER, EXCITER BOARD


MICROPHONE AMPLIFIER AND LPF

USB/LSB CARRIER OSCILLATORS

BALANCED MODULATOR

LOCAL OSCILLATOR


I decided to begin at the microphone end, and work my way gradually towards the aerial (antenna).The first circuit is of the Microphone amplifier and audio low pass filter. RF is kept out of the input by a PI filter. The microphone amp. has about 40db of gain. The gain of this stage can easily be changed to suit the microphone you are using. The amp. is followed by an active LPF. This is a standard 18db per octave filter, which rolls off at about 2.7 kHz.

 

MIC AMPLIFIER AND LOW PASS FILTER

I used an LF353 dual op-amp. Most 8 pin DIL dual op-amps are pin compatible with the LF353, so try whatever you have in the junk box. The value of the input inductor is not critical, anything between 47 and 470 microHenries, should be ok.


USB / LSB CARRIER OSCILLATORS

The USB/LSB carrier oscillators are fairly conventional, common collector Colpitts oscillators. I used BC548 transistors, not because they are particularly well suited to the job, but because I had a few of them in my junk box. The crystal frequencies are chosen to match the SSB filter in your rig (7.7985/7.8015 MHz in my rig.)

As crystal SSB filters are very expensive these days, the circuit has been designed to work at any IF between 7.8 MHz and 11 MHz. This gives you a better chance of finding a surplus filter at a reasonable price. Some CB rigs have very good filters (I got mine from a Panasonic CB), but watch out! a lot of single conversion CB's use the same filter for SSB and AM. These filters are too wide for use on the amateur bands. Most double conversion CB's have a good six or eight pole filter, with a bandwidth of 2.7KHz or less. I have built several SSB rig's, and have never spent a penny on crystal filters.

OOPS! I forgot to include the value of the output coupling capacitor (100pf).


BALANCED MODULATOR

 

DETAILS OF TRANSFORMER PHASING

 

The simple, two diode balanced modulator is still one of the best ways of generating a DSB/SSB signal. You can use Silicon switching diodes, 1N914, 1N4148, Germanium point contact diodes, 1N34, or Schottky (Hot Carrier) diodes. I used a pair of 1N34's. You should spend a few minutes with a fistful of diodes and a multi-meter to find a pair that are well matched for forward and reverse resistance. Both transistors are BC548's, for the reasons mentioned above. The transformer is wound on a high permeability (ui = 850) ferrite core. I used three lengths of insulated miniature 'hook up' wire, twisted together (trifilar winding). Try about 6 turns. If you use three different colours, it's much easier to get the phasing right. The 2K2 resistor at the output, supplies a few mA to a diode switch in my receiver. This allows the same crystal filter to be used for transmit and receive. If you are only building a transmitter, this resistor should be omitted.


The modules constructed so far will work on any band. Decide what band you want to use, work out the local oscillator frequency, allowing for the IF offset, then build a suitable oscillator / PLL / DDS. I have decided to build an SSB transceiver, for the 80 Metre band. The IF frequency in my rig is 7.8MHz, the local oscillator frequency range required is, 11.3 MHz to 11.6 MHz to cover from 3.5 to 3.8 MHz. This local oscillator can easily be modified to work on other frequencies, if you are using a different IF.


LOCAL OSCILLATOR

 

 

The local oscillator is based on the Franklin oscillator, that was so popular in the first half of this century. The Franklin circuit used two vacuum tubes, this circuit uses two direct coupled FET's. The Inductor L1 is wound on a ceramic former, about 1/2 inch (12.5mm) Dia. You can make a ceramic coil former from an old 1KW electric fire (heater) element. Nick the element with a hacksaw, and break it on a hard surface. Don't try drilling holes in it, I destroyed a couple of HSS drill bits, before I gave up. You will need to experiment with the number of turns on L1, I needed about 1.5 microHenries, I started with 15 turns (about .25mm Dia. wire) but had to remove 3 turns to get it right. The main tuning capacitor is a double bearing type, value about 20pf. The trimmer cap is a good quality air spaced type. The 50pf fixed cap (47pf actually) is polystyrene. The trimmer marked, 5p should be adjusted to the minimum value that gives reliable operation, I used a 5mm ceramic trimmer, 3/20pf.

The VFO should be built in a well screened box, I built mine in a homebrew box, made from copper clad PCB board. The VFO output is about 700mV pp, further amplification will be needed to drive a high level diode, or mosfet mixer. I am using a digital VFO stabilizer (ELEKTOR Mag. Feb. 98), so good long term frequency stability will not be too difficult to achieve.


LOCAL OSCILLATOR AMPLIFIER

The L.O. AMP. increases the VFO signal level to about +17 dBm (50 mW). I used a 2N2219A transistor. The transformer is wound on a small ferrite ring (the same type as used for the balanced modulator), 6 turns, bifilar winding. Twist two lengths of pvc insulated wire together, use two different colours. The low pass filter is designed to work at 11.5 MHz. The inductor is 12 Turns on a T50-2 core. If your rig has a different L.O. frequency, you will need to re-calculate the LPF component values. The 5R resistor is two 10R resistors in parallel.


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