Sunday, 10 January 2010

New IFT

Long time no blog...

I've been doing Christmas, the Children's Birthdays (all three of which fall in the first week of January), bad weather, computer problems, work etc.. That's my excuse anyway!

The beloved got me a "Made From Junk" 969 Tuner for Christmas (which shall be the subject of a future blog). It works but - let's just say - it lives up to its name! More of that later. For the moment we return to the Valve Rx project (Blogs passim).

I ordered up an ECH81 over the holiday season to replace the ECH84 I'd been using. The new tube only cost me £4.85 from The Tube Shop so the wallet didn't take too much of a bruising!. For reference, the pin-out of the ECH81 is shown below...



The new mixer valve didn't really change much - the receiver works but only just! As I knew that all the stages worked OK, I turned my attention to the great unknown in the project - the Intermediate Frequency Transformers.

The whole project started with an idea to learn something about valve superhets, within which the IFTs are obviously an important part. I'd made some simple units (described here) , which displayed pretty disappointing Q-factor, so I modified to the ferrite-cored version described here. You can see the last two generations of (attempted) IFTs here...



I decided to look again at the whole theory behind the IFT and soon realized that I'd neglected to control the single most important variable in my prototype devices; the coupling. My IFTs (like most others) have two resonant sections (think of them as the primary and secondary of the transformer). The response of the whole system is critically controlled by the degree of coupling between these two LC resonators.

I was inspired by to make a Spice model of the IFT by Max K4ODS's work, reported here .

Here's my Spice schematic, shown with a high degree of coupling (k=0.1) between the coils of the IFT...



My Spice model showed me that - with high coupling - the "IFT" can give two quite distinct peaks (associated with the two modes of the system)...



Looking again at my second-generation IFT, I realized that this was exactly what I was seeing - I had much too high coupling (I'd managed this as an accidental by-product of the desperate bid to increase the Q of each LC section). So - I needed to build another IFT with variable coupling between the two stages - simply achieved by varying the spacing between the two coils.

I couldn't vary the spacing of the coils in the existing devices (pictured above) as there wasn't too much space available and (more importantly) I'd stuck everything fast with glue!

I quickly made a new IFT, using the same design as before (I had previously etched a batch of the "end plate" PCBs) but with...
1) more "height" (so I've got plenty of space to slide the coils relative to one another - whilst keeping clear of the end "plates")
2) coils on Ferrite formers sliding on a central dowel



Now, I could tune each section to 455 kHz (by driving the other section directly from a 50 Ohm RF generator output) and observing the response on the 'scope, THEN make the crucial adjustment of coupling to the near-critical value. Sure enough, I get the shape predicted by my Spice model (except for losses - which I shall try to understand in more detail later)...



The tuning of the IFT changes as it is mounted in its shield can (made here out of an Illy coffee tin - I've moaned before about the difficulty of getting tins without corrugations, so I've left the brand name clearly visible in thanks to Illy)...



Now we have something that actually behaves the way it should, I couldn't wait to try it in the receiver...



The good news - it really has made a difference - I can clearly copy CW in the contest currently running on 80m (albeit in headphones). I hope that, when the other "IFT" is replaced with something more worthy, the whole receiver will come to life.

That, I'm afraid, will have to wait 'til next weekend.

...-.- de m0xpd

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