For a radio club rig-building project we decided to assemble a crystal controlled 10-meter transceiver with a direct-conversion receiver. Such a rig is relatively simple and to make our members familiar with several amateur construction techniques, some parts were built in dead-bug stile, other parts on experimental PCB boards and even a small part in amateur-SMD construction style. 10 HAM's took part of the project 7 finished it with success and three dropped off due to lack of time or other reasons. This article gives a technical description of the rig, the schematic diagram and a number of pictures.
Let's start with the block diagram. We used 14 MHz crystals for the VXO (Variable X-tal Oscillator). These crystals were at hand for free. But it is also possible to use 28 MHz crystals instead. In case of 14 MHz crystals, the tuned circuit at the drain of the oscillator FET is tuned to 28 MHz to obtain the desired transmit frequency. A driver stage amplifies the oscillator signal and the PA (power amplifier) delivers about 500 mW to the antenna filter. The PA is keyed. So far for the transmitter RF circuitry. The receiver is of the DC (Direct Conversion) type. It starts with a low noise RF amplifier which gain can be controlled in a 60 dB range. A mixer converts the RF amplifier's output signal to an audio frequency signal which is filtered to obtain the desired receiver selectivity. An audio amplifier delivers enough signal for driving a headphone or small speaker.
Let's start with the schematic diagram of the VXO. The Clapp oscillator is built around T6. This is a SMD dual gate MOSfet which is capable of delivering sufficient output to a tuned circuit formed by L9 and C31, resonating at 28 MHz. The oscillator circuit is straightforward. A 14.17 MHz crystal in series with a small inductor and a couple of variable capacitance diodes defines the transceiver's tuning frequency and can be varied + and - 3 kHz by potmeter R55, resulting in a 12 kHz tuning range at 28 MHz. The value of L12 depends how low the oscillator can be tuned. But a too large value will cause instability. The supply voltage of the oscillator circuit is stabilized (10 V). The choice of the variable capacitance diodes depends on what is available. Choose a type with a capacitance of 30 to 50 pF at 1 V bias voltage. And make sure it is a type which will have a large capacitance swing between 1 and 10 V bias voltage. All small capacitors around the crystal must be of the C0G quality to ensure frequency stability. A 28 MHz band pass filter is formed by the L9/C31 tuned circuit and the L10/C37 tuned circuit. This will sufficiently remove the 14 MHz sub-harmonic and the multiple harmonic signals. The 28 MHz signal is further amplified by T8. The drive signal for the PA-stage is taken from the emittor and the drive for the receiver mixer is fed to the mixer via the primary winding of TR1, which is part of the double balanced mixer.
SMD CONSTRUCTION TECHNIQUES
For this part of the transceiver a small part of prototyping board is used on which SMD (Surface Mounted Devices) components are soldered. It is amazing how easy it is to build with 0805 sized resistors and capacitors as is shown in the pictures of the VXO part. No glue is needed to fix the prototyping board to the solid copper clad substrate. Just solder all ground points and the small board is fixed! I use standard 0.5 mm resin core solder and a soldering iron with a very fine tip. when soldering an SMD component, first apply a little soldering tin to one pad on the board. Pick up the component with a fine tweezers and place it with one end on the tinned pad and solder it. then solder the other end of the component to the adjacent pad. Wiring between the components is done with AWG30 wirewrap wire. Note that only the circuitry around T6 has been built in SMD style. T8 with its surrounding resistors is built in well known dead-bug style which takes a much more space than building with SMD components.
PA AND KEYING CIRCUIT
Now it is time to build the transmitter's power amplifier (PA). the PA gets its excitation from the emitter of T8 (VXO-driver part) via C46 and R35. T7, (2N3866 or aequivalent), delivers 500 mW of output power. a low pass filter (C40, L7 and C41) lowers the amplitude of the unwanted harmonics. The path to the receiver input is formed by C44/C45 and L11. Two anti-parallel diodes D9 and D10 protect the receiver input against high RF voltages. T10 acts as an electronic switch to key the power amplifier. During the first experiments, it became clear that the leakthrough of the oscillator signal to the antenna with key-up was only 40 dB, so nearby stations reported they could still hear the oscillator when the transmitter was off. This problem is solved with T11 and T9. T11 acts as an inverter and T9 short circuits the oscillator signal at the base to T7, thus improving the leaktrough damping.
receiver RF stage and mixer
Now it's time to build the receiver circuit. The antenna signal comes from L11 in the PA and keying circuit. L1, C13 and C19 form a LC resonant circuit tuned to 128 MHz. T3 is a dual gate MOSFet which provides a 20 dB of RF gain. This gain can be varied from + 20 dB to -40dB (60 dB range) by RF gain control potmeter R19, which controls the G2 voltage of T3. In order to silence the receiver while transmitting, T4 clamps the G2 voltage to ground when the transmitter is on. The receiver double balanced mixer is formed by TR1, TR2 and D1 ... D4. The local oscillator signal comes from the VXO/doubler and driver circuit we already have discussed. A low-pass filter (C12, L3 and C11) defines the AF bandwidth of the receiver. The photograph shows the physical construction. Also this part has been built in amateur-style SMD technology. The lower part of the picture is the RF amplifier and in the upper part the ring mixer is visible, which contains two BAT54 double diode pairs.
audio amplifiers and sidetone switch
After the post mixer audio low pass filter, two low level amplifier stages (T1 and T2) take care of sufficient amplification of the mixer output signal. Both amplifier stages are heavily decoupled (R12/C10 and R6/C14) to avoid hum and parasitic oscillations. The bandwith is restricted by C27 and C26. An audio switch (U2) is used to couple the receiver signal to the final audio amplifier (Q1) during reception, during transmission the output of the sidetone generator (to be discussed later) is coupled to the final amplifier. One section of U2 is used as a digital inverter for the logic levels controlling both the switches. T5 acts as a squelch switch during startup. The LM386 audio amplifier gives enough output to drive a speaker or a pair of headphones. Both the audio switch and the LM386 amplifier are mounted on a piece of prototyping board (middle and top section of the photograph). At the bottom side of the photograph the sidetone generator is visible. The pre-amplifier stages with T1 and T2 are built in dead-bug style.
Sidetone generator and voltage regulator
It is nice to hear your own CW transmission. Therefore, a sidetone generator is included in the design. One part of the dual opamp (U4a) is used as a mid-point power supply, providing a 5 V DC level which is used for the audio switches in the previous chapter and as a midpoint voltage level for the Wien bridge oscillator (U4b). this oscillator generates an almost sinusoidal output. The voltage regulator is a low-drop type, providing a stabilized 10V supply for the VFO and other voltage sensitive circuits.
Although the design is simple, it is amazing what distances you can bridge with such a device. Sensitivity is good, -133 dBm (0,05 uV) and 500 mW of RF power, in combination with a reasonable good antenna makes it possible to perform CW QSO's with local HAM's. A simple case for the rig has been made from double sided PCB, available on Dutch HAM fests, with outside dimensions of 10 x 10 x 5 cm. It was real fun for all participants to build their own rig and to make test QSO's with it. The pictures below show the finished transceiver and the bottom side of the internal chassis with the RF amplifier (center), the RF low pass filter (left top), the voltage regulator (left bottom), the audio low pass filter (right bottom) and the AF preamplifier (right center and top). interconnections between upper and lower side of the chassis are made by feed through capacitors for the supply lines and simple holes for the RF and other signal lines. The wire for the trifilar wound toroids for the double balanced mixer is AWG 30 colored wirewrap wire. The colors make it easy to distinguish between the different wires of the transformers.