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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