[Elecraft] K3 related - non-linear audio transformer behavior

[Paul Christensen]

The usefulness of the square-wave response plots (as you and I have 
conducted) relies primarily on the complexity of the received waveform and 
system bandwidth.  Even in the case of a CW square-wave-like signal, a 
gently sloping rise/decay time (e.g., raised-cosine CW waveform), greatly 
reduces occupied transmitted bandwidth and the square-wave impulse response 
of the transformer becomes less meaningful.

OTOH, if there was to exist some form of data mode in which short rise/fall 
times are required, then a wider, flatter, overall receive bandwidth is 
required.  Presently, I know of no such common data modes for amateur use, 
but this may apply to some commercial and/or military data modulation 
methods.

Regardless of whether this is an issue or non-issue for amateur use, I have 
demonstrated (on the Elecraft mail list through Yahoo) that the problem is 
cleanly solved in even the cheapest datacom transformers through a simple 
termination of the secondary winding.  The peace of mind of knowing that a 
sound card will not be over-driven by a brief spike in transformer overshoot 
is certainly worth cost of a single resistor between the transformer and 
sound card input.

Paul, W9AC



----- Original Message ----- 
From: "Jack Smith" <jack.smith at cliftonlaboratories.com>
To: "Paul Christensen" <w9ac at arrl.net>
Cc: "'Elecraft'" <elecraft at mailman.qth.net>
Sent: Saturday, September 20, 2008 6:42 PM
Subject: Re: [Elecraft] K3 related - non-linear audio transformer behavior


> Paul:
>
> I've revised my non-linear transformer page in two respects:
> 1. Added the plot you requested. (Almost at the bottom of the page)
> 2. Added square wave ringing data for four transformers, although I am not 
> at all convinced that square wave ringing is an appropriate figure of 
> merit for a transformer used in a narrowly bandwidth limited 
> communications receiver.
>
> The page is:
> http://www.cliftonlaboratories.com/non-linear_transformer_behavior.htm
>
> Paul Christensen wrote:
>> Jack:
>>
>> With regard to the plots under "Comparisons and Conclusions," and also 
>> under "Frequency Response Compared,"  is it possible to change the source 
>> Z from 600-ohms to 50-ohms and re-test?
>>
>> It would be an interesting exercise to see if THD (at ~ 1V RMS) and the 
>> high-end frequency response of the Bourns transformer changes 
>> significantly. Tnx!
>>
>> Paul, W9AC
>>
>> ----- Original Message ----- From: "Jack Smith" 
>> <jack.smith at cliftonlaboratories.com>
>> To: "'Elecraft'" <elecraft at mailman.qth.net>
>> Sent: Saturday, September 20, 2008 8:02 AM
>> Subject: [Elecraft] K3 related - non-linear audio transformer behavior
>>
>>
>>> I've spent the last two weeks measuring and analyzing non-linear 
>>> behavior of several audio transformers, focusing on the Tamura TTC-108 
>>> used as isolation transformers in the K3.
>>>
>>> My data and analysis is now available at 
>>> http://www.cliftonlaboratories.com/non-linear_transformer_behavior.htm
>>>
>>>
>>> Jack K8ZOA
>>> www.cliftonlaboratories.com
>>>
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>>
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>