Posted by
Alexey Kats on
Apr 25, 2011; 10:43pm
URL: http://elecraft.85.s1.nabble.com/K2-KSB2-T1-T2-Turns-Ratio-Question-tp6304006p6304080.html
Here is my guess.
If you take a look at K2 schematics you'll notice that the crystal ladder is
loaded with a STEP-UP transformer T7 with ratio 5:20. This is needed because
it is ultimately loaded with the input of MC1350, which has impedance in the
2.5-3k range. So, two transformers 22:4 and 5:20 give you a total ratio of
1.375 which is approximately 1:1.9 impedance transformation. If your ladder
output impedance is 1.5k it'll give you about 2.85k output impedance. So, my
guess is that the ratio was chosen to match input of MC1350.
On Mon, Apr 25, 2011 at 3:07 PM, John Magliacane <
[hidden email]> 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
>
--
Alexey Kats (neko)
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