Monday, 3 May 2010

ARCI QRP 72 part challange - Update


Further to my tests ages ago with PTO... http://m1kta-qrp.blogspot.com/2010/01/pto-vfo.html

Here I am again with the same test rig but with a different construction technique. Again I post the information here incase others are interested in trying out a PTO.

I have fused a number of M5 nylon nuts together, super glued them and wound red electrical tape about them. Into this I can turn a M5 brass bolt. It is much more stable than the previous constructions. Anyway onto this I close wound about 45 turns (I didn't count them at this stage) of 28SWG enamelled wire.

Final construction of the coil and screw will involve moving this to the PCB and not being panel mounted. A second nylon bolt will hold the far end of the assembly 'true' with the brass bolt.

Then using the same setup as before I retested.

I can now get the PTO to operate between 4.673MHz and 4.3MHz which is perfect for an 8MHZ IF and 80m rig.

video

Anyway as this is a buffered PTO, needed a little lift in LO signal for the DBM but it added a couple more components to the count so will see how my rig turns out.

I'm not going to use any component that is not generally available and every one must be possible to obtain from the 'usual' suppliers.

Hope mine is going over to Dayton with a UK attendee.
My TX uses just 13 components, crystal controlled and there is about 4-5khz ability to shift frequency using two extra components. That was the easy bit.
My RX... I am still tweaking it a bit so the count is not up to date but overall I am inside 72.
The rig should have a keyer (so an AVR or PIC is my one IC!) but if I don't get the code finished, it won't.

Some images of various bits follow, it is my intention to build this manhatten/ugly style so no fancy PCB required.
The minimal component count crystal filter. "So how did I select the components to use"... no fancy calculus... I looked in the junk box and then through what 1% caps do I have > 300pf and found three 560pF, three 390pf and three 680pf. So I plugged for the 560pf and built this crystal filter.


Then just to check how it performed I used my trusty MiniVNA in transmission mode and took a look.
Nice sharp filter. Note I added the FT37-43 transformers after I measured it at c24ohms so used trial and error and made 12T:8T transformers so the input and output are both 50 ohms.
In this image the transmission appears to be down 20db (it isn't!)

The TX side is dead simple to keep the part count right down. I went for an IRF510 which adds some legs rather than a 2N3866 et al but this means /P use may be a little reduced.
The variable capacitor, series inductor and crystal are missing from this image but everything else is on the board. RF out is the white lead to left, key (basically short to ground) is the purple lead on right. The LPF is very simple just one half of a normal LPF to keep the part count down and I played about with values to get this to a reasonable level but seems to work, it is a single T50-2 and two 1000pf polystyrene caps as the oscillator is crystal controlled I did not see any spurs. I might grab an image of the spectrum plot and the LPF transmission graph and post them too.
Oh BTW don't copy the TX layout the above will not work, was an early version I took a photo of.

I went for a simple double balanced mixer. The two 0.1uF caps and 56 ohm resistor on bottom left is the AF output and they act as a RC filter. BFO in is top right, RF top left.
*** more to follow....

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