[SI-LIST] Re: Measuring characteristic impedance of transformer

Comment 1: a transformer has no fixed impedance as such.  However, they
can be designed to have best efficiency and bandwidth when connected to a
target impedance, and in that sense are often said to have a certain
impedance.  As a rule of thumb you can use a transformer in systems from
half the target impedance to twice, i.e. anywhere from 25 to 100 ohms for
a 50 ohm design target, with only moderate degradation in performance.
 
Comment 2: when you say 1:4 do you specifically mean impedance ratio or
turns ratio?  A 1:2 turns ratio is a 1:4 impedance ratio.  A 1:4 turns
ratio is a 1:16 impedance ratio.  Are you cascading 1:1 and 1:4
transformers as you have implied?  If so, why?  Is the 1:1 used as a true
transformer, i.e. the primary terminates one transmission line, the
secondary drives the next, or is it rotated 90 degrees so that you are
using like a common mode choke or 1:1 balun (actually an unun for single
ended microstrip)?
 
Comment 3: your TDR result will not look like a pure resistance.  When
the step reaches the transformer, at the rising edge you will see a
narrow spike of high impedance caused by leakage inductance in series
with the primary winding.  Parasitic capacitance and other effects may
cause ringing -- transformers are more complex than simple inductors. 
Then you will see the reflected impedance of the secondary termination,
which is what you are looking for.  Meanwhile, the shunt impedance of the
primary inductance will exponentially decay from an approximate open
circuit in shunt to an approximation of a short circuit like any other
L/R time constant.  The most valid impedance reading will be after any
initial ringing dies down and before the L/R time constant causes
significant droop of the waveform.  It sounds like you have the baseline
shift of many TDR designs when AC coupling is present.  In that case you
have to eyeball the reflection magnitude and convert it to ohms yourself
rather than trust the on-screen measurement.  Also, standing wave
terminology doesn't really fit the context of time domain measurements,
but I presume you are talking about the reflected waveform.  
 
You have very naughty and disoriented pcb traces.
 
Orin Laney
 
On Thu, 25 Jun 2009 21:34:13 -0700 (PDT) padma gundala
<pgundala2001@xxxxxxxxx> writes:
> Hi,
>  
> We are measuring the characteristic impedance of transformer.(TDR)
> Path is
>  
> SMA- 50 ohms trace- 1:1 transformer- 1:4 transformer- 200ohms 
> (resistor soldered across)
>  
> We find as soon as the co-ax is connected to the SMA, immediately a 
> standing wave exisists on the co-ax i.e insteAd of measuring 50ohms 
> measures the 174 ohms)
>  
> And the complete wave(volt ) even at the transformer, pcb traces 
> shits upwards.
> Can anyone explain this phenomena.
> We also tried measuring a 68 ohms inductor alone+pcb trace.
> the co-ax measures 50ohms , no standing wave on the co-ax.
>  
> Would like some input from anyone who tried this.
>  
> Thanks,
> Padma
>  
>  
> 
>       
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