Abstract
An SSB transceiver for the five “classical” amateur radio bands (3.5, 7, 14, 21 and 28 MHz) will be introduced briefly. The output power is 50W PEP on all bands.The design involves an ATMega128 microcontroller, a large colored LCD, a DDS controlled VFO, a single conversion superhet receiver with 9MHz interfrequency ( IF) and a 5 stage power transmitter equipped with bipolar transistors. The design is compact, the cabinet expands to the size of 19 x 12 x 7 centimeters. Basically it is a development based on the “6 band experimental transceiver“.
Microcontroller (MCU) board
The MCU is an ATmega128 8-bit RISC controller made by AVR/Microchip. It is clocked by an external crystal at 16MHz. The display is an 8-bit 320×240 pixel color LCD. The 5 band relay bus is driven by an ULN2003 driver IC. This IC uses open collector circuit and also has preinstaled clamp diodes to cope with high inductance voltage generated in the relay’s coil when switched.
Receiver, SSB signal generator and TX mixer
Description
On the left we start with an rf amplifier stage equipped with a dual gate MOSFET (all types can be applied, from 40673 to BF991 or its equivalents). The main purpose of this stage is to improve S/N ratio on the higher but also increasing gain for the lower bands when band conditions are weak. The stage is AGC controlled to avoid overdrive of the first mixer stage.
The mixer stage uses the SL6440 integrated circuit which has become a problem to purchase on the market. Therefore a resign using another mixer IC has been projected but not realized so far. The main advantage of this IC is its high IMD3, depending on the current that is applied via the 820Ω resistor at PIN 11. 820Ω in this project has proven to ensure a good compromise between IMD3 behavior and current consumption. The mixer is balanced at the input and output which also helps when coming to maximum performance. The rf transformers data can be deduced from the schematic.
We next see two interfrequency amplifiers using cascode circuit. As typical for this sort of amplifier we find the emitter of the second transistor connected to the collector of the first. in “cascade” amplifiers this connection goes from collector of transistor #1 to the bass of #2. AGC voltage controls the bases of the two transistors wich has relatively low resistance. Thus the AGC voltage is a little bit lower for the dual gate MOSFET in the first stage which reduces gain to a certain degree. Spoken in practical terms this has not shown to have a significant influence on the receiver’s overall performance.
The two interfrequency amplifiers before an after the SSB filter are identical. The overall gain of this part is around 60 to 70 dB.
The product detector applied is an NE602 or one of its equivalents. This stage provides another some dB of gain and is terminated by an audio frequency low pass filter.
The audio signal is subsequently amplified by a simple audio stage with an bipolar transistor in common emitter mode. The final stage is made up of the TBA820M integrated audio power amplifier which has been used because the distortion rate is significantly lower than with the LM386 IC.
The SSB signal circuit starts with an electret microphone and the necessary DC supply which provides about 1.5V DC to the microphone. The microphone gain can be set properly with a 10kΩ variable resistor in the front panel. The connections must be made using shielded audio cable to avoid coupling of radio frequency energy from the antenna.
The double sideband modulator (DSB) uses an IC that has been used in SSB CB radios and is still available on the web, the AN612. Carrier suppression provided by this IC can be experienced in the range of 45 to 50dB.
The filtered DSB signal turn into an SSB signal after having passed the SSB filter and is amplified in a simple stage where a BC547 bipolar transistor enhaces the gain to around 12 to 15 dB.
This SSB signal fed into the transmit mixer which in this radio is also made of differential amplifier with an added current source. The operation principal is identical to the DSB modulator.
Band Pass Filter for RX and TX
The Bandpass filter for the 5 bands in this radio (3.5, 7, 14, 21 and 28 MHz) is switched with small signal relays:
Each set of filters consists of two LC circuits coupled by an “upper” capacitor. The coils are wound on 5mm TOKO coil formers. The relevant data is given in the table visible in the schematic.
50W Power Transmitter
Following the filter we see the 50W power transmitter circuit.
As broadband power transmitters tend to have different gain on different frequencies in addition the negative feedback loops in each stage a power control stage has been inserted. For each band there is a resistor in the emitter degeneration circuit switched by a PNP transistor in accordance to the respective band.
Between the 4th and the 5th amplifier stage an impedance matching network (Pi circuit) has been added. By selecting the capacitors C1 and C2 output and input impedance can be chosen in a wide range to ensure proper matching between the driver and the final stage.
The final Low Pass Filter
This filter unit uses 5 filters for the 5 bands in the radio thus each band can be optimized concerning output and harmonic suppression issues.
T/R switching unit
To switch between transmit and receive mode my standard circuit has been used. Two power MOSFETs (p-channel) make the switching device:
Practical
The radio is mounted into the cabinet using two different layers. The first layer makes up the MCU board, the oscillators, the complete receiver, the band pass filter and the T/R switching unit.
Layer 2 consists of the power transmitter and the final low pass filter:
On the air
Checked with a Kiwi Wed SDR in Finland and a two tone signal, the transceiver can be examined for spectral purity giving the spectrum shown underneath. Output power is 50 Watts PEP on 14MHz
Thank you very much for reading!