The receiver had to match a lot of requirements that should be described first:
- Particularly on the lower bands and with effective long wire antennas the receiver front end will see high signal levels that it has to cope with. IMD always is a serious topic in this case.
- Sensitivity particularly on the higher bands, where noise level is ow and signals are weak, is also an issue.
- Dynamic range and extensive AGC gain compensation should be as high as possible.
This lead to a circuit that has proven its stability in lots of my radios:
- Band filtering for each band with a double and loosely coupled LC circuits
- Dual-Gate MOSFET (part of the AGC chain) as the first amplifier
- Diode ring mixer (with Schottky diodes)
- Post mixer amplifier with Dual-Gate MOSFET (part of the AGC chain)
- SSB Filter (now 10.7 MHz) also used for transmitter (relay switched)
- Main IF amplifier with MC1350 (part of the AGC chain)
- Audio preamp with bipolar transistor
- Audio final amp: (once again! 😉 ) LM386
Before describing the receiver itself we will have look at the band pass filter unit, that is shared between receiver and transmitter:
To minimize stray energy traveling from the input to the output of the filter, two SMD relays have been used on each side of the filter per band. And to reduce feedback fromt the transmitter (when the BPF is used to filter the TX signal after the TX mixer) the filter has been placed far away from the TX amplifier section.With an overwhelming result: The transmitter is nearly unconditionally stable now (compared to the TX section used in the “Give me 5”-Transceiver that had severe shortcoming in this aspect.
Control leads for the relays follow a designated coding scheme:
- 160m: green
- 80m: blue
- 40m: brown
- 20m: yellow
- 15m: grey
- 10m: violet
The receiver’s circuit
VFO signal is coupled into the DBM via a 10nF capacitor. The same is valid for the amplified RF signal from the output of the first amplifier stage using a Dual-Gate MOSFET (40676, BF900 or equ.).
Another Dual-Gate MOSFET is used as the post-mixer amplifier. All Dual-Gate MOSFETs so far are part of the AGC-Chain. This maximizes the possible gain swing to about 40 to 50 db. and enhances the receiver’s capability to handle even the strongest signal levels without distorting the output signal and the end of the audio chain.
Next is the SSB-Filter. Due to this is an “experimental” transceiver, the filter has not been soldered to the circuit board. Instead it is fixed with an aluminum clamp into two parts of header strips. Thus I can compare numerous SSB-Filters (9-, 10.695-, 10.7-MHz off-the-shelf ones, various home made ladder filters etc.). Here the different performance is very interesting to be explored.
The filter is accompanied by a special rf relay (manufacturer “Teledyne” with excellent performance concerning separation for the two channels) so that it can be used as the SSB filter for the transmitter section.
After the filter section the IF amplifier follows. This one uses an MC1350 video amp (old but good and still available, even in SMD!) and this IC also is controlled by AGC. The input is unbalanced (PIN6 to GND) the output is balanced and terminated with a tuned circuit.
Demodulator is an SA602 mixer IC.
After that the signal is handed over to the audio chain. But before the signal is processed in the next stage the frequency range is limited by a low-pass filter to reduce hiss. This filter also has two switched capacitors (controlled by MCU via NPN-driver stages) to adapt the sound to the preferred settings of the user. The software contains a respective function.
The audio amplifier consists of two sections: A preamp with a bipolar transistor and the inevitable and well-know LM386.
The full circuit on a 6×8 cm veroboard:
Starting from left top corner there is a 1:4 input transformer (not in the schematic), the preamp, the DBM, post mixer amp, SSB filter, relay, MC1350 as IF amp, demodulator and 2 stages of audio amp.
Performance is excellent. The circuit has no problem with high signal levels (in-band and out-of-band) especially on 40 meters. No IMD problems are noticeable even when used with high gain antennas like a 2×25 meter doublet with a tuner. On the higher bands noise figure is pretty OK what I think is based on the usage of Dual-Gate MOSFETs in 2 of the 3 amplifier stages. The MC1350 deteriorates this to a certain degree but is still very much acceptable for a shortwave radio.
Vy 73 de Peter