10m to 17m Transverter
by IK1ZYW 2001
This is the chronological history of this project, another crazy idea of
mine. At the present I wonder if I will ever come to a finished circuit...
3-Nov-2001
Troubles falling asleep. It comes to my mind that my modified CB for 10m
will soon be unuseful. The sun cycle is supposed to be decreasing,
therefore 10m should be closed all day, soon.
Since this CB serves at my girlfriend's place, when I'm there, I have been
thinking of a way to be on the air even with a low(er) number of
sunspots. I am still considering the purchase of an IC-706 or similar RTX,
but I just had a nice idea.
I remembered some ads on electronic/CB newspapers in 80's: they sold a
transverter from 11m to 45m, all mode (all modes supported by the CB
RTX, of course). I am sure that transverter can be modified
to cover the 40m HAM band. The very next day I was up early searching
the magazine and right after on Internet for such a device. Found it! But
the price is still too high, for gaining just a band (150 EURO or so).
Those days I had read an article by Peter Parker about his direct
conversion receiver that used a combination of analog switches as
mixer (DC-2000 with a 74HC4066 switch). That is not a new idea, but
Peter's article was very well written
:-)
There comes my idea: see if any of the XTALs in my junk box can match/mix
my CB RTX coverage (26->28.7 MHz) for any lower HAM band! Hit the
keyboard, prepared a spreadsheet to play with the values.
A low XTAL value would create troubles and many harmonics in the HF
spectrum. So a transverter for 15m is out of question, for now. Higher
would bring me easily onto 30 and 40m, but then I would need a good
amplifier to have my signal heard somewhere. Finally, a 10 MHz XTAL shows
a good combination: 28068-28168 to 18068-18168. WOW! I also like 17
meters! :-)
The decision is taken: I'll go for 17m!
8-Nov-2001
Construction started. I decided to begin from the most basic module: the
local oscillator (LO). The block schematic of the whole transverter is
only in my mind, but I am sure the 10 MHz LO is needed. :-)
Construction tecnique chosen is "dead bug", which I also have never tried
before.
Here will be the LO schematic
The LO is a reproduction of Peter's. No big changes, except for a review
(random) of capacitors' values.
16-Nov-2001
Finally assembled the mixer. Actually not all of it, just the 74HC4066, to
see if the idea actually works. My biggest doubt was whether I could actually
traslate upwards the 18 MHz using a 10 MHz sampling. This technique reminds
me of PCM modulation, thus sampling. I would certainly be undersampling
18 MHz...
DaDa! It works! Problem: I live in front of FM/TV broadcasts TX locations.
I see them all from my window. We have more than 50 88-108 MHz stations,
and some 30 TV channels (all through UHF band). Damn it! My mixer
brings them down to the HF spectrum! Noise at S7! This makes it extremely difficult to
locate a signal in HF and search for it somewhere else in HF. You need to
find a signal in the space between two FM BC images, and then see if
it's present 10 MHz above. I could locate 2-3 signals in the 16-18 MHz band and listen
to them also on 26-28 MHz. Cool, the mixer works.
Now I need to follow Peter Parker's suggestion and insert ferrite
beads everywhere around the mixer, narrow input filters and hope for good!
Note. I had already noticed that my local oscillator runs at 10004 kHz, so all signals are actually traslated of that amount.
Must be kept into account when using it for the TX transverter and calling CQ!!
8-Feb-2003
Because of a huge lack of time (and instrumentation) this project is currently suspended. If anyone is interested in it, I will honor all requests of schematics, info, suggestions.
Author
IK1ZYW, Paolo Cravero. email: ik1zyw at yahoo! dot
com