HANDBUILT H.F. RADIO -
FEATURES BOTH SUPERHET
AND DIRECT CONVERSION
"SIGNAL PROCESSING" MODES


Inspired by designs; sited in Amateur Radio mag' (Australia) of May 1995;
(TCF {twin crystal filter} 40 metre transceiver by Drew Diamond VK3XU)
and direct conversion type receiver AR May 1992 (also by Drew Diamond VK3XU)
and UK radio ham David Roper M0DAD's design of DSB transceiver

I was curious to hear just what a HF direct conversion type receiver (AKA homodyne) actually sounds like?
A whole new construct was considered, but it became apparent that with a simple mod' of my previous effort, a receiver using either "superhet" or "direct conversion" modes, would result.
A SPDT changeover switch selects either mode by steering either the output of the first, or second mixer through to the audio-amp stages and loudspeaker.
In the "superhet" mode the received signal is frequency translated to an I.F. of 4 MHz, thence through a xtal lattice filter and then frequency translated (detected) in the second mixer to audio.
The advantage is good selectivity because of the steep sided xtal filter characteristic and good 'image' rejection - however fidelity is 'communications-quality' only.
In the "direct conversion" mode the received signal is translated directly to an audio resultant by frequency mixing a local oscillator signal at the same frequency as the "off-air input".
Advantage is circuit simplicity, good fidelity (i.e. no sideband cutting) but poor audio image response
The Crystal lattice filter, and 2nd L.O. use 4MHz xtals (100 units/$14 Aus! (ex China - 3Q 2010)

For frequency setting a printed paper dial calibration is taped to the surface of the dial drum
and rolls past a wire cursor in the 1" square window in front panel.
To produce the dial scale calibration; I monitored the VFO on a good receiver (ICOM R8500)
and marked up the dial in 1 MHz intervals consistent with a "swish/plop" in the receiver.
The dial calibration now has two scales; in DC mode VFO frequency is shown on dial,
and in super mode correlating dial marking is (VFO - 4MHz), because of High side local oscillator injection at ...MHz + 4 MHz
I then flatbed scan the "working" paper calibration
and artwork and print an aesthetic dial scale using the PC.