Dennis WC8C is the event coordinator for the radio club in Michigan that I recently spoke with. He mentioned to me that he was working on a homebrew 6 meter rig. FB Dennis. I see lots of tribal wisdom in your approach, especially in your decision to do this in a stage-by-stage modular form.
Dennis's rig is obviously a work in progress, so if anyone out there has any helpful hints (especially on the carrier suppression and on the testing for spurs and splatter) please share them with him via e-mail or blog post.
Thanks Dennis!
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This is my 6 Meter homebrew transceiver, currently work in
progress. It is a single conversion
super –heterodyne design. I constructed
each stage independently with SMA connectors.
This is so I can re-make sections as needed, and will allow me in the
future to swap sections to experiment with alternate designs. The VFO and BFO are controlled using a SI5351
with an Arduino micro controller. I
currently have separate SI5351 modules for VFO and BFO because I suspected
issues with cross-talk. These issues may
not actually be real, so once I am happy with the performance, I will test
again with just one module to see if it is OK.
The Power Amp is still on the to-do list, so output is well under 0 DBm
The Blue boards were designed by me and ordered
on-line. The other boards I etched
myself. Construction is mostly surface
mount because I find it easier than drilling all the holes. SMD components are mostly 805 and 1206 size.
Transistors are SOT23.
The Band Pass filter is a 5 coil design made with air-core
inductors.
3 bi-directional termination insensitive (TIA) amps are used
(blue boards). Total RX gain is about
44db. Total TX gain is about 16db. Each board has its own independent RX/TX
switching circuitry (mosfet based) and is fed with +12.5, GND, and RX/TX logic
signal from the Arduino (3V logic and up will work)
The Mixer and modulator are both Diode Ring mixers.
The 12 MHz SSB filter is a crystal ladder filter similar to
the one used in the uBitx.
The Mic and audio pre-amp (also a blue board) is made on a
modified TIA amp board. I had 10 of
these boards made, and the needed circuitry was largely the same, so I modified
the board with a rotary tool and jumpers.
The Audio amp is a PAM8403 module and drives a headset. I did make some modifications to the module so
it runs in-spec and to eliminate the power on audio pop.
The challenges I have been having are mostly related to
spurs, splatter, carrier suppression and TX audio quality. I have been gradually tweaking these things
to improve operation before I start on a power amp. My IF is 12 MHZ, and I was using the LSB side
of the crystal filter because it is sharper (VFO 62 – 66 MHz) but have recently
changed over to the USB side of the filter (VFO 38 – 42 MHz). This eliminated the spurs I was seeing near the
pass band. I still need to make some
adjustments to the crystal filter as it is too broad.
I still have some splatter and audio quality seems low, but
I am starting to doubt my test setup. I
see the splatter on the RTL SDR, but I don’t see it on the Tiny SA. The spatter happens at ~160 KHz
intervals. I am hoping to find someone
local with a better spectrum analyzer to help me verify if it is the rig or my
SDR dongle/test setup.
The modules to the side of the picture are my
rejects/experiments. The one covered in
copper shows how I eventually will shield all the modules. I 3D printed a cover for the board, when
wrapped it with copper tape, soldered to the bottom ground plane. The one shown is a diode ring modulator. For some unknown reason the carrier
suppression is rather poor. I had
previously made a junk-box modulator that had much better carrier
suppression. I don’t know why it is
better than the one I more carefully made for the radio, but until I figure it
out, I am using the junk box version.
The junk box modulator uses unmatched schottkey diodes, whereas the
“final” one uses a 4 diode SMD package because I wanted them matched – I
thought this would be better, but maybe not.
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