Re: K2: KSB2 T1/T2 Turns Ratio Question

Posted by Don Wilhelm-4 on Apr 25, 2011; 10:38pm
URL: http://elecraft.85.s1.nabble.com/K2-KSB2-T1-T2-Turns-Ratio-Question-tp6304006p6304071.html

  John,

I cannot answer your exact question (the impedance ratios), but I need
to ask a question about your "narrower than expected" filter response.  
I have never questioned it, the filter width has always been within +/-
10% for me and that is within the results due to expected component
tolerances.

Are you measuring the filter width at the -6 dB points (that is the
Elecraft "point" for the bandwidth).  If you are using the -3dB points,
your filter will appear to be about 200 Hz more narrow than advertised.

The filter bandwidth and other characteristics will be determined more
by the output impedance than the input impedance.  I would believe the
filter characteristics would be driven by the output (load) impedance,
while the input impedance would be more of an influence on the stage
driving the filter.

With the KSB2, one must consider both transmit and receive.  Looking at
the schematic, on receive the filter path is from left to right, while
on Transmit, the path is right to left on the schematic.  I am not the
designer, and I have not questioned the impedances before, so I am
presenting only a series of guesses.  For those who have the time,
energy and curiosity, some experimentation may yield better results -
but beware of degradation in the IMD products and other unexpected
anomalies if you decide to do the experiment.  Just saying "it works"
does not necessarily denote an enhancement in the operation - as with
any design, there are compromises that had to be dealt with, and without
the original designers notes, you may be treading on "thin ice".

73,
Don W3FPR

On 4/25/2011 6:07 PM, John Magliacane wrote:

> 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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