Wednesday, 16 December 2009

The "best laid plans"....

OK - the trimmers arrived promptly, courtesy of Peter G7JAB's excellent service (visit his website here - usual disclaimer). I quickly got them soldered atop the waiting IFTs - only to be disappointed ...

The measured Q was pathetic - I was going to be making a receiver with no more (lack of) selectivity than the "wide-open door" of the Paraset or one of the direct conversion Rx's on my QRP rigs at this rate!

I decided to investigate the problem one coil at a time, rather than try to play with the complete transformer (that way I could eliminate the central glass-fibre rod or the PCB end plates as the origin of the pathetic Q).

I took a single coil, wound on one of the sewing machine bobbins, and resonated it with a 100pF capacitor...


For a benchmark, I also took the nearest axial choke I had in stock (a 470uH device) and paralleled it with another 100pF capacitor...


I drove both networks from a signal generator, through a 470 kOhm resistor and monitored the response on a 'scope (via a conventional X10 probe). The resulting (normalized) responses are shown below...

(sorry about the frequency axis - Excel is very bad at handling logarithmic graphs)

This - it seems - is why my IFTs were as selective as thirsty man at Oktoberfest. But why is the Q so poor - what's wrong with my coils?

The first straw I clutched at was the idea that the plastic of the bobbin was causing the loss (hardly likely at such reasonably low freqs but I was clutching at straws), so I tore off one side of the "bobbin" and re-measured...


Result: no observable difference (yes - I realize that more loss may be created by the "middle" of the bobbin - but this wasn't nearly so easy to tear off).

There then followed a series of "I wonder if..." type conjectures, (during another trip to HB-land, when all I could do was speculate). I wondered if it was the coil geometry (but it looks close to the ideal "Brooks" form), if it was the wire (but the d.c. resistance was less than 2.5 Ohms and I'm too clumsy to go much finer), if it was the...

I turned in frustration to those who know more than me - in this case that's a large group! I had some interesting and inspirational discussions with Richard G8UNO and Albert G3ZHE, both of whom pointed to capacitance (self- or stray-) and so, on my return to the UK I made some similar coils on the same bobbins but with fewer turns. The results (see graph below) are an improvement - but nothing to write home about. It wasn't until I tried some ferrite cores that I started to get improved Qs, approaching (but not equaling) that I'd seen with the axial choke.

You can see my original bobbin coil (with 300 turns) and a 100 turn air-core on the same type of bobbin. Also shown are the results for the same coils with a ferrite core (a "tube" from the junk box, with O/D conveniently close to 1/4 inch and length about 3/4 inch).

I decided to run with 100 turn coils on a ferrite core for a new batch of IFTs. The graph below compares the response of my original IFT to one of the newer ferrite cored models...

This isn't "the final solution" - it is just the last one I recorded!

Now for a little more tin-bashing to make some covers and we can move on to the IF Amplifier

...-.- de m0xpd

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