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Wednesday, 22 June 2011

A most Diplexing Mystery

Well, finally a day when I can spend some time at play here in the shack; the next part of the project has to be the beginnings of the receiver front end. So, in true form, and trying to stay faithful to the articles in RadCom Homebrew, I've moved on to the 1st RF mixer and the diplexer that follows it. The basic concept of a diplexer is really a band pass filter. Post the RF mixer there will be a mix of the RF input (through a bandpass filter from an antenna) and the LO. My LO isn't yet built but I have a DDS VFO here which will do just nicely. My IF is already set at 9MHZ, but the RadCom articles are assuming an IF of 10.7 MHz so I have some maths to do.
There are loads of on-line references to diplexer design, but I found this to be a particularly good starting point:

and more specifically, this:

I'm wanting to build a Bridge-Tee RF Diplexer, a la:

and I now understand that I'm looking for a reactance for C1 and L1 of about 250R and for C2 and L2 a reactance of 10R at my chosen frequency of 9MHz.

Using my basic maths, and also this great on-line reactance calculator:

I've come up with values of C1 = 70.5pF, L1 = 4.42uH, C2 = 1760pF and L2 = 177nH.

When I wind coils I always tend to measure them out using my AADE LC meter:

So here I have wound 4.5uH when the theory suggested that 33 turns would give me 4.4uH, but it's close enough. I've built the diplexer here, and included some trimmers hopefully to peak the resonance:

And as per design, it peaks nicely at 9MHz. So post 1st RF mixer we should be able to bin all the other junk thats generated and have a clean (ish) signal. This is my homebrew signal generator:

Apart from suffering from the worlds brightest power light, it works very well. I stole the design from here:

I'd like to add a second output at higher amplitude, but that's another project for another day.

As per normal, chief technical assistant has been close to useless today:

Fun, egh?

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