[AMRadio] Transformer phase shift

[Donald Chester]

A quick test for phase  shift in  a transformer with a balanced winding 
(such as a driver transformer: or the primary of a modulation transformer):

Ground the CT.  Connect one side of the balanced winding to  the horizontal 
plate of the scope (better to couple directly to the plates than to use the 
amplifiers in the scope).  Connect the other  end of  the winding to the 
vertical plate.  Load the transformer with a resistor of the rated winding 
impedance.  Feed the output of a signal generator into the other winding of 
the transformer.  Start at about 1000 cps.  Note the scope pattern.  If 
there is no phase shift between the halves of the balanced winding, you will 
get a straight diagonal line.  Sweep up and down the audio spectrum, until 
the straight line begins to separate into an oval shape.  That indicates 
phase distortion.  There will likely be a low frequency and a high frequency 
where phase shift begins to occur.  If you keep going up in frequency you 
will likely find a  frequency where the pattern is a  circle.  That 
indicates 90 degrees of phase shift from one side to the other (not good).  
A reasonably good transformer should show a  clean straight diagonal line at 
least down below 100 cps, and up to 6 or 7 kc.

Even the best transformers will eventually show phase shift.  I have noticed 
many UTC LS transformers begin to shift around 11 kc.  I have never tested 
one that went all way to 20 k without distortion, despite their specs that 
say flat to 20 kc and above.

Don K4KYV

_________________________________________________________________
Join the world’s largest e-mail service with MSN Hotmail. 
http://www.hotmail.com