ANOTHER DIRECT CONVERSION HF SSB TRANSCEIVER
|PLL VCO||DDS VFO|
|Tayloe Mixer/Detector||Exciter & Receiver Audio|
The transceiver audio boards are: (a) the transmit audio phase-shift network, (b) the receive audio phase-shift network, (c) the receive AF filter and audio output amplifier and (d) the transmit microphone amplifier, tone generator and AF filter.
The microphone signal is amplified and processed through a soft clipper and this output together with that from an 800 Hz tone generator is passed through an active high-pass/low-pass filter to tailor the audio pass band to 300 - 3000 Hz. One half of a TL082 is used for the soft clipper while the other half is a wein-bridge AF oscillator. The other TL082 mixes the keyed 800 Hz signal with that of the microphone and offers a buffered signal to the following high pass/low pass active filters which use a TL084, quad op amp (See fig. 1).
From this board the signal then goes to the audio phase-shift board where active all-pass filters using TL082 dual op amps produce two AF signals that are 90 degrees out of phase. The first three active filters in each channel shift the signal within the audio band-pass, - 45 degs. and + 45 degs, hence the total shift of 90 degs. The two channel signals are then passed through inverting unity gain amplifiers producing signals that are 180 degs phase shifted from the source resulting in the availability of four signals in poly-phase ( 0, 90, 180 and 270 degrees). These four phases are required for driving the Tayloe Transmit Modulator (See fig.2). The original project used a passive poly-phase network but this introduced an appreciable level of insertion loss and was passed up for the active two-phase network.
Likewise, the receive audio phase-shift board follows the same general principle. Although four phases are available from the Tayloe Receive Detector post amplifiers, only the 0 and 90 degs. audio signals are used and these pass through the all-pass phase-shift network and are combined where the desired sideband comes out with the unwanted greatly attenuated. The unwanted sideband attenuation is adjusted through the potentiometer where the outputs from the two channels are combined (See fig.3). The 10 nF capacitors in the phase shift net works are 5% tolerance while the resistors are 1% tolerance.
From this board the signal goes to AF amplifier board where is passes through an active high-pass/low-pass filter provided by TL084 quad op amp. The first element is a 300 Hz 2-pole high-pass filter followed by three 2-pole 3000 Hz low-pass filters. The filtered signal then goes to an AGC amplifier where part of the output is rectified and feed back to an electronic attenuator provided by a J310 j-fet that is connected across the input of the AGC amplifier. This provided a certain level of automatic volume control. The out of this amplifier is also fed to an LM380 power IC that drives the speaker (See fig. 4).
Credit must go to Rick Campbell, KK7B who co-authored the publication "Experimental Methods in RF Design" out of which I got very useful information and whose all pass phase-shift network I used in the construction of this project.
Another article that influence the design of the receiver's AGC and audio power amplifier was the Bedford Receiver, by Rodney Green, VK6KRG (QEX Sept/Oct 1999)