50MHz SSB/CW Transceiver
Iulian Rosu, YO3DAC / VA3IUL - http://www.qsl.net/va3iul/
On receive side the signal from the antenna, passed through a Low Pass Filter and TX / RX switch, is amplified by the dual-gate LNA (Q1) BF998.
Dual-FET RF mixer (Q2, Q3) converts the 50MHz signal to the first IF, which is on 45 MHz. An LC BPF filter, is placed before the RF mixer to attenuate the image frequency.
The Variable Frequency Oscillator (VFO) is a very stable Vackar FET type oscillator (Q7), followed by a buffer amplifier (Q8, Q9).
First IF use a 45MHz Monolithic Crystal filter, model that was used in old analog cellular phones (same as the 44.545 MHz crystal).
The 45MHz crystal filter provides first IF selectivity and also second image attenuation.
The IF system SA58640 from NXP is used in the second conversion of the receiver. This integrated circuit is still in production and can be requested from NXP as a sample, or get direct from Digi-Key. The internal crystal LO on 44.545 MHz is used for both conversions, on receive and transmit.
The SSB 455kHz ceramic filter is a CFJ455K from Murata (2.4 kHz bandwidth). This is a cheap filter (compared to other crystal or electromechanical filters), but with good selectivity performances. The second 455kHz filter (also from Murata) is a common 455kHz ceramic filter used in most AM/FM broadcast radios.
Q4 transistor is picking-up the RSSI output of the SA58640 and uses for the Automatic Gain Control (AGC). The S-meter placed on the emitter of Q4 is indicating the receive signal level. The last 455kHz IF stage (Q5) is a dual-gate MOSFET controlled by the AGC signal.
The dual-balanced modulator/demodulator (U2) use a circuit NJM2594 from NJR (Digi-Key). The circuit is used as a Product Detector in receive mode, and as a Double Side Band (DSB) modulator in transmit mode. The audio amplifier (U4) is a LA4425A.
The Beat Frequency Oscillator (BFO) (Q6) uses for resonator a standard 455kHz ceramic resonator. The stability of this kind of resonator is acceptable, but if is available can be replaced with a 455kHz crystal.
In SSB mode a 10k potentiometer is used to adjust the carrier suppression of the DSB modulator (U2). In CW mode a 1k resistor is used for carrier through.
The preamplifier of the electret microphone uses a low noise op-amp TL081 (U3).
The DSB signal from the modulator is amplified by a monolithic amplifier U9 (MAR-6) and routed through the SSB filter CFJ455K filter, which is the same filter used on the receive path. To switch the SSB filter between RX and TX I am using 4xBA244 diodes.
The dual-gate mixer (Q10) (BF998) up-converts the 455kHz SSB signal to 45MHz IF. A 45 MHz Monolithic Crystal filter is used at the output of the mixer to filter the desired signal, and two amplifiers (U7, U6) are used to boost the signal to feed the TX RF Mixer U8 (SBL-1).
In CW mode the biases of the monolithic amplifiers U7 and U6 are switched ON and OFF, keying the carrier signal.
An LC two-pole BPF is used at the output of the RF mixer (U8) to filter the desired 50MHz signal.
Q11 and Q12 are the last stages of amplification on transmit path. These stages could drive a higher power amplifier if is necessarily.
Click here to find an All-Mode version of the 50MHz transceiver.