That is way cool Mark. We made some boards and I am building some with
friends right after fishing season.
On Wednesday, March 29, 2017, Mark Pilant <mark@xxxxxxxxx> wrote:
Well, it seems to be quiet around here.. :-)
I've finally managed to make some time to build my own Minima. My plan
is to make is as a series of modules so I can try out different some of
the various suggestions posted here. Plus, I got a good deal on a bunch
of RG-174 and SMA connectors :-)
One thing which delayed things a bit was wanting (and to a lesser extent
needing) to build a scalar network analyzer to check out the various
modules. I built my own (hardware and software) before I found out about
the PHSNA. Oh well, timing is everything :-) One thing I added early on
was a color TFT display to be able to see all the "pretty" graphs :-P
So with the SNA built...
I was able to sort through my pile of crystals and build the crystal
filter. It worked out pretty well. It has a 2.555KHz bandwidth, centered
on 19.994MHz.
Next was the front-end filters. After a bit of a learning curve winding
toroids, I was able to get the filters constructed. I used my SNA to
check out the actual filters before wiring up the relays; to make sure
they actually worked as expected. I need to adjust the 15MHz LP filter
as it is falling off a bit too soon.
After I completed the circuit, I encountered one "big" problem. When the
30MHz LP/20MHz notch filter was used, the shape looked OK, but the signal
was about 40dB lower than it should have been. After some careful poking
around with my scope, I found the problem. The relay I mounted on a
18-pin DIP "MePAD" (cut down to 10 pins) had a very marginal trace between
the pin contact and the solder pad (to connect to the pin). A little
solder "bridge" cured this problem and the signal was right at the expected
level.
The other "problem" I need to fix is what appears to be a shorted relay
driver transistor. When I hook up +12VDC, the relays engage; regardless
of the voltage on the base of Q23 (float, ground, or +3-5VDC). I don't
expect this to be too hard to fix.
It turns out I was right. There was a shorted bypass cap, from Q23's
collector to ground. Replacing it with a good bypass cap fixed the
problem.
Next up is the mixer. I'm going to start with the dual FET design, but
because everything is modular, I expect I'll try some other mixer designs
as well.
That's it for now.
73
- Mark N1VQW
