[ARC5] Easy selectivity improvement....isn't...

[Kenneth G. Gordon]

On 6 May 2013 at 18:06, Richard Knoppow wrote:

>      There are several old text books though I can't cite 
> one off the top of my head, with "universal selectivity 
> curves" in them that show the shape of the curves of coupled 
> transformers with various degrees of mutual inductance 
> coupling.

Yes. I have Terman's here, among others. But how do you easily measure 
that mutual coupling?

The only way I have here to do that is to connect a100 K resistor in series 
between my sweep generator and the input of the IF transformer, then my 
scope to the output, and manually adjust the spacing to see what I get.

Given the way the transformers are built and the "tight quarters" this is not 
going to be a short-term job.

>      The optimum spacing of the ARC transformers could be 
> found by using a sweeper and indicator or a spectrum 
> analyser with coupled tracking generator.

I don't have that fancy equipment. I have a B&K sweep/function generator 
with a sawtooth or ramp output which will drive the horizontal input of my 
scope, and which covers the necessary frequency range, and I have a 
couple of good scopes. I also have a good HP-8640B. Those will have to do.

>      Its sometimes possible to broaden out a normal IF 
> transformer by adding an auxilliary winding to increase 
> mutual inductance. Theoretically, such a winding with 
> opposing couping would serve to decrease coupling but I've 
> not seen anything practical on this.

That would be too much work for me. I want to use what is there.

Thanks for the info.

Ken