There are many ways to continue with the Minima:
First:
If one crosses over to the fulll H-mode mixer, it might solve the leakage
problem. I have been trying to keep the design simple with the KISS mixer.
Our (Joe and Mine) experiments wiht using MOS switches were encouraging.
Second:
The other option is to just go with a diode mixer. I already have a
transceiver like that in the works with Si5351. It is a low-cost design
that works upto to 21 MHz and uses two oscillators for the BFO and the VFO.
A a post-mix amp had to be added between the diode mixer and the crystal
filter to provide decent termination to both sides as well as overcome the
filter loss. This is a pretty simple design. The BFO and VFO are entirely
software controlled. A single LPF that cuts off at 21 Mhz is all that it
takes. I will soon post the details.
Third:
The JFETs as mixers could be trouble as their gate-source and gate-drain
can go into conduction easily. Instead, if we use BS170 or 2N7000 kind of
switching MOSFETS it might just work.
The interesting thing is, though Minima itself was not a very replicable
design, it did teach us quite a lot. It is a robust receiver and it has had
a large number of spin-offs.
- farhan
On Tue, Jul 26, 2016 at 11:04 AM, Mark Smith <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
Thank you, all three of you who gave a great answer to my question. The
first IF being high allows continuous tuning but crystal filters are
rubbish up there. So convert to the second IF where we can filter nice an
narrow as needed. Then, down to base band audio. That makes a lot of sense.
I wasn't aware of that limitation of crystal filters.
So, the KISS mixer from the minima was really a problem? The write-up made
it sound like it was better at rejecting the carrier than you all make it
out to be. Not as good as matched diodes like the HF1, then?
Next question: any reason that I couldn't use an Si5351 for at least two
of the LOs (since I can do two completely independent PLLs, but the third
is dependent on one of the other two), probably the VFO on the front end,
and the second IF mixer, so that I can just control the USB vs LSB in
software at the LO, and remove the whole complex oscillator? Why would that
be a bad idea?
Add a third oscillator, and you could implement an IF Shift by moving both
LOs around the crystal filter up or down a bit.
-Mark
On Jul 25, 2016, at 9:44 AM, allison <ajp166@xxxxxxxxxxx> wrote:
On 7/25/16 11:36 AM, Mark Smith (Redacted sender smittyhalibut for DMARC)
wrote:
Thank you Ben for that. I looked at the schematics and understand them.
I've been a ham since 1991, but am just getting into RF electronics and
circuit design. Schematics like that one make it so much easier to
understand what's going on. Thank you!
One question, that's not specific to your design. I've never understood
the benefit of more than two conversions, one IF. Why does the HF1 convert
to 45MHz, loosely filter with LC, then convert again to 10MHz and filter
with crystals? How is that better than converting to 45MHz, filtering with
crystals, then converting directly to base band?
Continuous tuning from some low frequency like 1mhz to 30mhz without
crossing the IF, or having image problems.
That's the advantage of having the IF higher than 30mhz. The pain is that
you need to down covert from that IF to
something lower where selectivity using crystals is easier.
The down side is more gain stages but they are simple and fairly cheap.
The other is more mixer and oscillators
to cause birdies.
The transmitter low pass filters are still required for harmonics.
In general a multiband rig is harder to do well than a monobander.
There are also a couple more amp stages in there, but those are probably
just to make up for losses in the filters and conversions, and could be
added anyway without the extra conversion.
There were two issues with Minima, RF from the LO making it to the
antenna and IF getting to the antenna.
The filter requirements were hard to meet.
Allison/KB1GMX
To be clear: I'm not saying you're wrong, I'm totally ignorant and asking
for education. :-)
Thanks!
-Mark
On Jul 25, 2016, at 7:23 AM, Ben Aupperlee <beninturkye@xxxxxxxxx> wrote:
