H.F. SUPERHET Mk 2|
with "Digital" frequency readout
In the Mk 2 version I have replaced the IF strip/2nd L.O. 4.43MHz (recycled) "colorburst" crystals with new 4 MHz units.
These are so cheaply available now, I obtained 100 units for $14 Aus! (ex China - 3Q 2010)
As a consequence the analogue dial can now be replaced with a "digital" readout, based on:
A simple frequency counter
As the freq counter cct uses a 4MHz reference oscillator; it was then an easy matter to measure the receivers L.O. frequency and using some logic gates,
deduct 4MHz from the count, which would then indicate the received frequency!
Local Oscillator is 'high-side' injection to mixer at + 4MHz above receiver frequency. (see schematic)
By constructing a truth-table based on what would otherwise be displayed and what we need, the combination of the 7432 (or gate) and the 7886 (xor gate), effectivley removes a 4Mhz component from the displayed 'count'
In the "KHz" switch position this subtraction is not needed and bypass switching is acheived with a 74157 quad 1 of 2 data selector.
I could have used a simple 3PDT mechanical switch here, but small examples aren't easy to source so I used a TTL chip instead ( with a SPDT switch)
Other considerations ?
Another excellent circuit (designer unknown),
published in Lo-key magazine (Australia) of Dec 2007
RF Amp: BC547
Mixer: 3N140 (FET)
Local osc: 74HC04
Xtal filter: 4.434 MHz (3 xtals)
IF amp: BC547
Det: 1N414 x 2
BFO: BC547 x 2
AF preamp & amp: TDA2822
A short video of a good working example of this receiver (courtesy of VK3YE) can be viewed at :
Homebrew Superhet Amateur Bands Receiver
Additionally, here is an exhaustive listing of all kinds of QRP inspired receivers/transceivers/ transmitters.... etc etc Handbuilt QRP rig designs
Also another idea (as yet untried) from Silicon Chip Magazine (Australia) Dec 1989: