Re: Follow-up Antuino question
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Barry, I too built a direct conversion transceiver a few months ago. I chose a fully analog design without any ICs. A carefully designed free running vfo is used. It is so stable that I can continuously monitor the FT8 spot! On the other hand, when I substitute it with an Si5351, the difference is immediately obvious. The CW is noiser and the SSB audio has less fidelity. I can no longer copy the signals that were barely above the noise floor. Given a choice, I would use a free running VFO any day. Except that it is very expensive to build them with a good slow motion and implementing multiband design is always a?challenge. - f
On Sat 14 Nov, 2020, 9:55 PM barry halterman, < kthreebo@...> wrote: Hi fellow DC rx fans. Back in 1992, Rick Campbell had a design in QST for a direct conversion receiver he called the R1. Later he upgraded this to the R2. I have the R1 that I have used the si5351 to drive the SBL-1 mixer with excellent results. I use a 6 db pad on the LO port, per a recommendation from Ashar. Later I changed the LO to a PLL oscillator with a sine wave output. I did not notice any difference at all between the two oscillators. The recovered audio is fantastic from this R1 board. Rick had a few different LC designs for filters, SSB narrow and wide and a cw filter.? For those who want a serious DC receiver, I highly recommend looking at his design. It is a little more complicated then just a mixer and LM386, but very well worth the effort. K3BO
Would be interesting to try sine wave vs square wave.
But I doubt you will notice much difference,
Assuming you have a low pass (or band pass) filter on the RF input,
the harmonics in a square wave should not cause any first order products in the resultant audio.
Also, your mixer is likely to have a non-linear voltage vs current relationship
at the local oscillator port anyway.
However, the clean audio one can get from a DC receiver make it?
an excellent testbed for trying this sort of thing.
And I have heard that a sine wave is somewhat preferred for driving
even a diode ring mixer, which has an extremely non-linear LO port.
Note that the Antuino has a very nice ADE-1 mixer.
Might be possible to include that in your DC receiver.
Jerry, KE7ER
On Fri, Nov 13, 2020 at 08:35 PM, Bob Lunsford wrote:
There was some comment in the webpage about the signal being a square wave. If this shows up in the signal, causing harmonics, then included in the amp should be a "smoothing" circuit or some way to make it show up as a sine wave. This is theoretically simple since an inductor's flywheel effect would actually convert it to a sine wave. How well it does this is another question and it would/could cause some refinement of the signal. A simple resistor-capacitor circuit followed by an amp may be better.
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