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K2: KSB2 T1/T2 Turns Ratio Question

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John Magliacane

K2: KSB2 T1/T2 Turns Ratio Question

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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wayne burdick

Re: K2: KSB2 T1/T2 Turns Ratio Question

Administrator

I started with textbook impedance ratios, then adjust the windings 1
turn at a time to find the optimal combination of power transfer and
passband flatness. This can vary with crystal characteristics, circuit
strays, etc.

73,
Wayne
N6KR

On Apr 25, 2011, at 3:07 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
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

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Don Wilhelm-4

Re: K2: KSB2 T1/T2 Turns Ratio Question

In reply to this post by John Magliacane

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
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

Alexey Kats

Re: K2: KSB2 T1/T2 Turns Ratio Question

In reply to this post by John Magliacane

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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John Magliacane

Re: K2: KSB2 T1/T2 Turns Ratio Question

In reply to this post by wayne burdick

--- On Mon, 4/25/11, Wayne Burdick <[hidden email]> wrote:

> I started with textbook impedance ratios, then adjust the windings 1 turn at a time to find
> the optimal combination of power transfer and passband flatness. This can vary with crystal
> characteristics, circuit strays, etc.

Thanks, Wayne.

When I built my K2/100 back in 2003, I changed the coupling capacitors in the crystal ladder network according to KI6WX's 2.5 kHz bandwidth mod. I ran a TX sweep, and found my 6 dB bandwidth was only around 2.1 kHz. I never measured the bandwidth prior to this change, but I know the change made the rig "sound wider", with less ripple in the passband.

So, I'm looking for reasons that might explain why the bandwidth is so narrow. According to what I've read, the crystals I received with the kit in mid-2003 should have been matched well enough for the bandwidth mod to work. This is what directed my attention elsewhere, such as the transformers surrounding the ladder.

However, I'm beginning to think the crystals I received were not matched well enough to provide much beyond a 2 kHz bandwidth.

If crystal matching is indeed the problem, are new crystal sets available to match the calibration number (3.7 in my case) from earlier KSB2s, or will all 14 crystals need to be changed so all filters have well matched center frequencies?

73, de John, KD2BD
K2/100 #3563

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Don Wilhelm-4

Re: K2: KSB2 T1/T2 Turns Ratio Question

John,

Do you have the older or the newer crystals installed? The newer ones
have a "-S" suffix. If yours have a "-20" suffix, they are older and
should be replaced with the newer ones.

It is not a question of crystal matching - the crystals in a set are
matched, but one of the crystal characteristics - mainly the motional
inductance (Lm) parameter.

73,
Don W3FPR

On 4/25/2011 7:00 PM, John Magliacane wrote:

> --- On Mon, 4/25/11, Wayne Burdick<[hidden email]> wrote:
>
>> I started with textbook impedance ratios, then adjust the windings 1 turn at a time to find
>> the optimal combination of power transfer and passband flatness. This can vary with crystal
>> characteristics, circuit strays, etc.
> Thanks, Wayne.
>
> When I built my K2/100 back in 2003, I changed the coupling capacitors in the crystal ladder network according to KI6WX's 2.5 kHz bandwidth mod. I ran a TX sweep, and found my 6 dB bandwidth was only around 2.1 kHz. I never measured the bandwidth prior to this change, but I know the change made the rig "sound wider", with less ripple in the passband.
>
> So, I'm looking for reasons that might explain why the bandwidth is so narrow. According to what I've read, the crystals I received with the kit in mid-2003 should have been matched well enough for the bandwidth mod to work. This is what directed my attention elsewhere, such as the transformers surrounding the ladder.
>
> However, I'm beginning to think the crystals I received were not matched well enough to provide much beyond a 2 kHz bandwidth.
>
> If crystal matching is indeed the problem, are new crystal sets available to match the calibration number (3.7 in my case) from earlier KSB2s, or will all 14 crystals need to be changed so all filters have well matched center frequencies?
>
>
> 73, de John, KD2BD
> K2/100 #3563
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

John Magliacane

Re: K2: KSB2 T1/T2 Turns Ratio Question

In reply to this post by Don Wilhelm-4

Hi Don.

Thanks for the reply.

> 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 plotted a sweep of my TX chain a number of years ago, the results of which can be seen here:

http://www.qsl.net/kd2bd/sweep2.png

It's a linear plot, so the ripple and other imperfections really stand out. It was taken by sweeping audio through the KSB2 well below the level of ALC action while measuring the corresponding audio level picked up by a nearby crystal controlled, direct conversion receiver having no filtering or AGC.

Looking at the red LSB curve, if we estimate the center of the passband to average around "70" in amplitude, -3dB would be around "50", and -6dB would be around "35".

>From this, I estimate the -6 dB bandwidth to be around a 2.2 kHz. And this is with the 2.5 kHz -3dB bandwidth (2.6 kHz -6 dB bandwidth) coupling capacitors in the ladder.

And, lastly (although less scientific), if I optimize my CAL FIL settings so that my transmitted audio has the identical quality on LSB that it has on USB (as observed on a wideband receiver), my BFO settings are only 2.3 kHz apart. This is 400 Hz less than the spacing suggested in the KSB2 instruction manual for a 2.2 kHz (-6 dB) bandwidth.

73, de John, KD2BD
K2/100 #3563

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Don Wilhelm-5

Re: K2: KSB2 T1/T2 Turns Ratio Question

John,

Given that information, I conclude that you have an unusual situation
with your KSB2 filter.
The easy way to attempt a fix is to replace both the crystals and the
capacitors (SSBCAPKT). Total cost is only $30 plus shipping, and your
aggravation is likely more costly than that amount.

The design works and produced the proper results - if the components are
the correct values.

73,
Don W3FPR

On 4/25/2011 7:35 PM, John Magliacane wrote:

> Hi Don.
>
> Thanks for the reply.
>
>> 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 plotted a sweep of my TX chain a number of years ago, the results of which can be seen here:
>
> http://www.qsl.net/kd2bd/sweep2.png
>
> It's a linear plot, so the ripple and other imperfections really stand out. It was taken by sweeping audio through the KSB2 well below the level of ALC action while measuring the corresponding audio level picked up by a nearby crystal controlled, direct conversion receiver having no filtering or AGC.
>
> Looking at the red LSB curve, if we estimate the center of the passband to average around "70" in amplitude, -3dB would be around "50", and -6dB would be around "35".
>
> > From this, I estimate the -6 dB bandwidth to be around a 2.2 kHz. And this is with the 2.5 kHz -3dB bandwidth (2.6 kHz -6 dB bandwidth) coupling capacitors in the ladder.
>
> And, lastly (although less scientific), if I optimize my CAL FIL settings so that my transmitted audio has the identical quality on LSB that it has on USB (as observed on a wideband receiver), my BFO settings are only 2.3 kHz apart. This is 400 Hz less than the spacing suggested in the KSB2 instruction manual for a 2.2 kHz (-6 dB) bandwidth.
>
>
> 73, de John, KD2BD
> K2/100 #3563
>
>

AC7AC

Re: K2: KSB2 T1/T2 Turns Ratio Question

In reply to this post by John Magliacane

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