Wednesday, April 13, 2016

How to make an IF transformer for that odd frequency.

Someone ask where to get IF cans another asked what the IF in an antique radio was. They are after parts to build that radio of their dreams. Sometimes NOS is available or a parts only relic which has already seen its last days of service other times it is best to just build your own. One of them was after a 1600Khz IF transformer so that will be the goal. If I were building one to use I would put a trimmer cap on it but this is a demo only so I'll use a fixed cap and get 'close'.
The first steps to inventory the parts bin and find suitable material.
Look at the chart I can use a core from 1/2" to 12". I was involved in a plumbing project not very long ago and saved some PEX. The 1/2" pipe is 5/8" OD.
First piece of scrap I picked up is about 3-1/2" long, that could do nicely. Let's try a design around it. I'm using #24 wire which is 47TPI and I'm going with a 3" length. Drawing a line between 47TPI on the second column and 3" on the third column establishes the index. I'm using a 5/8" core so I draw a line from 5/8" on the first column through the index point. This gives an inductance of 65 micro henry.  All that is left is to determine the wavelength and draw a line from the inductance to through the wavelength to determine the capacitance. If everthing stays on the chart the design is good. Now to build the coil.
The PEX uses a locking ring connector so I use one of the rings to mark the end mark. Measure the other end and mark it the same. This is not necessary but it helps keep everything in line.

I threaded a #14 wire on both ends and drilled and tapped the PEX to accept it. Screw the wire in and trim it to length.
I had to decide if the coils will terminate on one end or both ends. I went with both ends so i put two terminals on each end.

The meter shows 70 micro henry. I wound one coil 3" and the other 1". giving a 3 to 1 turns ratio.

I put a cap across it and feed a signal until the output peaked.  Just the first cap I pulled out of the bin. The input was 25 mv with 1.33v out. It could use a slug or a trimmer capacitor to tune it but for now it is a proof of concept. 
I started designing for 1600KHz. The frequency meter is displaying 1882KHz. With more capacitance I could achieve the 1600KHz goal.

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