The 10.7 Mhz Xtal filter
The input and output of this filter must be loaded with 450 Ohm,
The IF amps is re-used in both directions. And has 50 Ohm in/out.
So 2 transformers will be used for this job, at the X-tal filter.
The filter connected with the two transformers for test.
The left transformer has been left open for the photo.
If you just add another vinding on top of the other and make
this one 1/3 turns, it will have the right impedances, BUT !!
It will have about 2-3 db loss, so make a trifilar vinding like this
Be sure to connect it right, then the conversion loss is now only 0.1 db !
If you write down what you are doing then it will be easy to get it right :-)
Just solder the three a1-a2-a3 connections, when you are done adding all the vindings
it is easy to measure what b1-b2-b3 that goes where.
I used 0.2mm cu and there was room for 14.5 turns
First connect the two transformers and test the loss at disired frequency, I had only 0.2db @ 10.7 Mhz
They are broadband enough for most ham applications, a torid core could also be used, USE low AL values.
The the Xtal filter can be tested..
At 10.7001 at max output, the loss was found to be -2.5db including the two transformers, sounds good to me..
Now I recalibrated the db scale to make 10.7001 my zero db.
Then I have measured:
-60 db 10.6958
-30 db 10.6980
-6 db 10.6988
0 db 10.7001
-6 db 10.7013
-30 db 10.7023
-60 db 10.7033
BW @ -6 db 2.5KHz
BW @ -60 db 7.5KHz
Factor = 2.0 and 3.0