[minima] Re: for minima project
- From: allison <ajp166@xxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Wed, 27 Jul 2016 08:57:13 -0400
On 7/26/16 1:54 AM, Ashhar Farhan wrote:
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.
MOs switches or a fully balanged mixer works.
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.
DBM is a easy way to get a balanced mixer. RF/IF gain is easy and cheap.
Filters are both easy and hard, easy to suggest and without instruments
or a procedure that
uses easy to acquire gear hard to assure they work as expected. This is
hampered by fear of
making coils.
Also the IF is too magic as crystal filters are scary for most and high
HF filter being more so.
As it works the 20mhz is not much better for images or wide tuning than
9, 10 or 11mhz
because there will be some frequency or range that will be inaccessible
as its the IF. Also
the lower frequencies its easier to obtain quality crystals.
One solution is a preselection system. The tube radios did this and at
least one vendor
(Tentec) did that on their early transceivers. Ganged L or ganged C are
a mechanical problem
but with Varicaps I have prototyped this and considered it a possible
approach. Its another
knob but it is less a problem if its a simple pot and can provide a
higher selectivity front end
which helpful in Eu and many places with high power SW broadcast stations.
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.
capacitance from gate to drain is first order issue. Then the drain
source parasitic diode is the next problem.
I did some work trying them using some TI parts (lower gate to dran
capacitance) and it was not as good as
Jfets properly biased. The trick with Jfets is matched for Idss and
gate cutoff (dual parts are good for this
but scarce) and then raise to source to the gate cutoff value. The
problem is its still a single balanced mixer.
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.
Its trouble spots were the 20mhz filter, and the first mixer. Both are
easily solved.
I know from prior work I've done that a 16mhz filter is easier to
realize and crystals
had better temperature stability vs band width. Also 16mhz is far
enough from
14 and 18mhz to avoid spurs in ham bands and images.
At HF a wide band RX or TRX is easier with a VHF if (45mhz or higher).
But for monoband or
a limited number of bands single conversion and a if at 6-12mhz makes
for a simple design
with fewer issues with retaining good performance.
Having built many monoband transceivers (160 through 2M SSB) and a few
multiband plus
a large number of receivers for various uses and tuning ranges there is
experience. There are
few magic circuits (thought the DDS and SI570 may qualify) the basic
radio is more a matter
of trade offs and choices. This has not changed since the 40s though
the devices to attain
the needed gain have.
Allison/KB1GMX
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