K2: KSB2 T1/T2 Turns Ratio Question

Posted by John Magliacane on Apr 25, 2011; 10:07pm
URL: http://elecraft.85.s1.nabble.com/K2-KSB2-T1-T2-Turns-Ratio-Question-tp6304006.html

Hi Folks.

I've been perplexed for quite some time as to why RF transformers T1 and T2 that sit at opposite ends of the crystal ladder filter on the KSB2 board have different turns ratios.

T1's turns ratio is 22:7. while T2's is 22:4.

The impedance of the crystal ladder network appears to be in the neighborhood of 1200 - 1500 ohms.  Since the KSB2 sits inside an IF having a 150 ohm impedance, T1's 22:7 turns ratio, which performs a 9.87:1 impedance transformation, makes perfect sense.

The Balanced Modulator feeds T2's primary with an impedance close to 1200 ohms, which also makes perfect sense.

However, T2's secondary, having only 4 turns, presents an output impedance significantly lower than the 150 ohm IF system it is feeding.

What's the deal?   :-)

And I ask this question as I troubleshoot a "narrower than expected" crystal filter response.  I'm curious whether the T2 impedance mismatch is intentional, or something else is going on that I'm not aware of.

Any relevant enlightenment on the subject would be appreciated.

Thanks.


73, de John, KD2BD
K2/100 #3563

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