[time-nuts] DIY VNA design

jimlux jimlux at earthlink.net
Mon Aug 22 23:35:39 EDT 2016

On 8/22/16 5:01 PM, Richard (Rick) Karlquist wrote:
> On 8/21/2016 3:59 PM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
>> That said, I don't know why the author is using directional couplers.  A
>> bridge is much wider bandwidth.  It is more lossy though.
> In general, a resistive bridge will always require a
> transformer/180 degree hybrid/differential amplifier
> to make it work.  If you are going to go to the trouble
> of making a broadband transformer or hybrid, you might
> as well just build a traditional directional coupler,
> because it is no more difficult.  All the resistive
> bridges I have seen are followed by broadband differential
> amplifiers.  The resistive bridge itself has a minimum of
> something like 15 to 20 dB loss, and the differential
> amplifier has a minimum NF of 7 dB or so.  This results
> in a great loss of sensitivity, but you can always get
> the sensitivity back by using a narrow IF bandwidth and/or
> lots of averaging, or (rarely) a high drive level from
> the source.
> Having said that, one of the putative advantages of a resistive
> bridge is accuracy.  However, with today's calibration techniques,
> this is no longer all that important, so a traditional coupler
> might be more practical than it used to be.  I remember attending
> the retirement party of Agilent's last great designer of couplers
> (pre-calibration) and let me tell you, this guy was a total guru.
> He was one of greatest practitioners in this area of all time.
> He freely admitted that he was now obsolete due to calibration.
> Any old coupler is good enough.

these days, what you want is repeatability, so that you can "calibrate" 
and have the calibration remain stable.

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