FreeLists / si-list / [SI-LIST] Re: Balun measurement

[ronald miller <ron@xxxxxxxxxxx>]

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Hi Steve

Very tight doesn't ring true to me for SI applications, especially with 
baluns which I have designed and used
for years.   For Radio, and microwave they are acceptable.

Also, 100Mhz to 4 Ghz will not wok for models in data.  The 4-Port 
network analyzer will work or you can
use a TDR/TDT to model differential lines.

If you have a balun with 2 to 3 degree phase and less than .1 db gain 
across the whole band you may have
something useful.

Incidentally, 20 years ago we used a model for SAW device which 
according to the gain and phase across
a limited bandwidth should have worked fine for an IF filter, but it did 
not.  Had to go back to an LC filter.

Why?  The reflections on which SAW devices depend for operation 
introduced delays that were outside
the normal measurements.  The steady state measurement of the devices 
frequency response across the band
looked good and for a steady signal it was fine.  However, as soon as 
modulation was added widening the band
of the signal spectrum, the SAW screwed up the data.  Cause??  Excessive 
group delay, but only visible with
modulated signal.  This is a time domain device being used in the 
frequency domain.  Similar problems occur
when using frequency domain devices in the time domain.... especially 
measurements for models.

The point?  SI is concerned with time domain data modulated across an 
extremely wide bandwidth and with
harmonics as much as 3 to 5 times the fundamental.  Baluns cannot be 
designed forthe accuracy needed for
models in digital data transmission systems.  Each adjustment they make 
to broaden the bandwidth of the
balun will cause unintended distortions in delay even if  magnitude and 
phase.seem to be OK across the band
you haveselected.

Ron


steve weir wrote:

>Ron generally that is true.  However, PSPL has an interesting Tx line 
>balun: 5310-104 that has very tight gain matching and phase linearity 
>over the 100MHz to 4GHz range.
>
>Best Regards,
>
>
>Steve.
>ronald miller wrote:
>  
>
>>dont use baluns for measurements.... using network analyzers.
>>They are only useful for narrowband applications...
>>
>>ron
>>
>>john wrote:
>>
>>  
>>    
>>
>>>All,
>>>
>>>
>>>I measured a 1:1 balun as a three port network, port 1 is the unbalanced 
>>>input, port 2 is the 'through' connection and port 3 is the indirect
>>>connection. I only have a two port VNA so the unused ports were 
>>>terminated with 50 Ohms during the measurements
>>>
>>>
>>>Port 1 -----OOOOOOOOO--------o Port 2
>>>           o-OOOOOOOOO--o
>>>           |            |
>>>           |            |
>>>          ---           |
>>>          GND           |
>>>                        |
>>>                        ----o Port 3
>>>
>>>
>>>
>>>With this 1:1 Balun the Balanced port has differential impedance of 50 
>>>Ohms so terminating with the VNA/terminator into 50 Ohms on say ports 2 
>>>and 3 gives a large impedance mismatch. The the application circuits for 
>>>the chip to which the balanced input connects shows a 100 Ohm resistance 
>>>directly across the pins of the balance port (2-3) which is then tracked 
>>>at 100 Ohms differential to the device where it is internally terminated 
>>>with 100 Ohms. Now using the same test board, I solder a resistor 
>>>(infarct three 300 Ohms in parallel) across ports 2 and 3 and re-measure 
>>>I have should have the correctly terminated case.
>>>
>>>I took the balun off the test board and measured the terminator across 
>>>ports 2 and 3.
>>>
>>>I then simulated the terminated case using the s-parameters from the 
>>>first measurement and those from the resistor only measurement and 
>>>compare them to the terminated case measurement.
>>>
>>>In general there is reasonable correlation in shape between S11, S22 and 
>>>S33. However the simulated S11 has less loss than the measurement of the 
>>>terminated case. the simulated S22 and S23 again have the same overall 
>>>shape as the measured results although the anti-resonances are not as great.
>>>
>>>The through S21/S12 S13/31 however show little correlation between the 
>>>simulated and measured results having large anti resonances where the 
>>>measured results do not.
>>>
>>>As this is part of an overall simulation I would like to get the Balun 
>>>measured and simulating correctly before addressing the rest of the 
>>>simulation.
>>>
>>>
>>>Can anyone suggest a better way of measuring the balun ?
>>>
>>>
>>>Regards
>>>
>>>
>>>John
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>>
>>>
>>> 
>>>
>>>    
>>>      
>>>
>>  
>>    
>>
>
>
>  
>

-- 
Ronald Miller
Ghz Data, Signal Integrity Consulting
7721 Sunset Ave.
Newark CA  94560
tel     510-793-4744
cell    510-377-9380
fax     510-742-6686
www.ghzdata.com




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