SSB TRANSCEIVER

RECEIVER SECTION


BPF AND MIXER

POST MIXER AMP.

RX I.F. AND PRODUCT DETECTOR

AUDIO AMPLIFIER

AGC CIRCUIT


BPF AND MIXER

The signal from the RX/TX relay is filtered by an 80Mtr band pass filter. This can be replaced by a filter for another band or a bank of switched filters, for a multi band rig. If you need to cover from 3.5 to 4.0 MHz, you will need to replace the 10p coupling capacitor with one of a larger value (about 18p). You can wind your own inductors, about 18 turns on a 10K type Toko former will have 5.5 microhenries of inductance.

The first version of the receiver had a passive mosfet mixer. Intermod. performance was very good, but the receiver was a little deaf. I decided to try a diode ring mixer instead. You can buy diode ring mixers in a convenient package, but high level devices are quite expensive. I used four Silicon switching diodes ( similar to 1N4148 or 1N914), and two broadband transformers. The transformers are identical to the one in the balanced modulator (6T trifilar). There is a -3dB attenuator in the I.F. port. This mixer has performed very well on 80M. The receiver sounds very clean, even when there are very strong signals just a few KHz away. Sensitivity is quite good, sub-microvolt signals can be copied easily.


POST MIXER AMPLIFIER

The post mixer amplifier, makes up for the losses in the passive mixer and I.F. port attenuator. The transistor is a 2N4427, if you don't have one, try a 2N3866, 2N5109, on any similar, medium power VHF transistor. The output transformer is 10 turns with a tap at 4 turns. The core is the same, high permeability ferrite ring, used in other parts of this project. The output supplies DC current for the RX/TX switching diodes at the SSB filter input.


RX I.F. AND PRODUCT DETECTOR

 

Two silicon switching diodes are used for receive/transmit switching of the input side of the SSB filter. The IF amp is an MC1350P. This device can give more than 60dB of gain at 7.8Mhz. I decided to use a single IF amplifier stage, followed by a relatively high gain AF amp (60dB +). The output transformer is a KANK3334R, with a 100pf capacitor and 5K6 resistor across the primary. If you are winding your own transformer, you need 18T primary, 4T secondary, on a Toko 10K former. For a 10.7 MHz IF, try a standard 10.7 MHz IF transformer. The product detector is a simple two diode balanced mixer. Try a couple of Germanium, point contact diodes, 1N34, 1N270, OA91, etc. The AGC input is connected to ground, until I find time to build the AGC circuit.


AUDIO AMPLIFIER

The AF amp has about 60 to 70 dB of gain. The transistor is a BC109C. I might try replacing it with a lower gain device. A typical 59+ signal, gives more than a volt pp at the collector. The power amplifier IC is an LM380N (14 pin version). If you intend to use the rig on CW, the LM380N must be powered up on transmit as well as receive (for the sidetone osc.).


AGC CIRCUIT

The AGC circuit uses a dual op-amp IC. The first stage amplifies the receive audio (taken from the hot side of the AF gain control). The output from this stage is converted to a DC (ish) signal by a voltage doubling rectifier. The final stage is a DC amplifier. A nice feature of the circuit, is that the meter is automatically set at zero, in the absence of any signal. I used an LF353 dual op-amp. Most fet input dual op-amps should work in this circuit, or you can use a pair of single op-amps.

Experiment with the value of the 2.2 mFd capacitor, to set the AGC decay time. I tried 1mFd (too fast), 2.2mFd(about right) and 4.7mFd(a bit too slow).


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