30m Band - All BJT Transceiver
Iulian Rosu - YO3DAC / VA3IUL - http://www.qsl.net/va3iul/
Reason building this radio is to get a real portable QRP transceiver with low power consumption.
On receive, the current consumption is approximately
100mA, and no more than 1.8A in TX mode, at full power.
In receive mode the signal from the antenna passes the antenna switch (R1) and goes through the RF Band Pass Filter to the LNA input (Q1). This stage is used also in TX mode as an RF driver. The switching between modes of the LNA is done using two relays, R2 and R3. The gain of the stage is approximately 15dB, providing also good IP3 performance at 20mA current consumption. The 2SC5551 could be replaced with BFW16 or BFR96.
The next stage is the RF mixer (SBL-1) which translates the 10.1MHz RF frequency to the 4MHz IF frequency.
The Variable Frequency Oscillator used for mixing is a Vackar type oscillator. The main characteristic of this oscillator is a very good frequency stability and good phase noise (measured phase noise was -119dBc @ 1kHz from the carrier). The reason of this good performance is because on this type of oscillator the parametric variables of the transistor are isolated from the LC resonator, and they do not overload it.
The output of
the VFO is low, which makes necessarily to use a buffer amplifier to boost the
generated signal. The level necessarily for mixing is approximately +7dBm.
Using very high quality components, the stability of this oscillator could be close to a Crystal Oscillator. The oscillator transistor (Q9) is a medium power transistor (BFW16) supplied at low Vcc to provide the best phase noise.
Q2 is the first IF stage and is using a high IP3 transistor (2SC5551 or BFW16).
The crystal ladder filter was made from 4 identical, low cost 4 MHz crystals. The 3dB band pass frequency of the filter is less than 1 kHz.
The gain of the last two IF stages (Q3, Q4) is controlled by the AGC line. The AGC amplifiers are Q13, Q14 and Q17.
The product detector was made using two 1N914 diodes (or anything equivalent).
The headphone audio amplifier provides enough power to hear the signal into any normal low impedance headphones.
Q15 and Q16 are the CW Tone monitor.
Q11 is the BFO on 4MHz and is followed by a buffer (Q12). Q12 is keyed in TX mode by the CW key through the R5-1 relay. R5-1 is the only relay on this schematic that is located in “normal open” position in RX mode.
In TX mode the 4 MHz signal from the keyed LO, feeds the RF mixer (the same SBL-1 used by the receiver). The relays R4 and R6 should provide both together good isolation, better than 100 dB at 4 MHz, to don't get feedback from output to the input of the IF amplifier.
The up converted signal at the output of the RF mixer goes through the same BPF and RF stage (Q1) used by the receiver.
Q7 and Q8 is the driver respectively the final stage of the transceiver, providing approximately 10W at the output. Q8 is working in class C and should be mounted on a proper heat sink.
For best isolation between stages and modes all the relays should be separate one from each other. Don’t use multiple contacts relays. Cheap small 5V SPDT (Single Pole Double Throw) relays would be fine. Only R5 is a DPDT (Double Pole Double Throw) to switch the LO buffer and the audio tone generator in the same time.