[time-nuts] DIY VNA design

Brooke Clarke brooke at pacific.net
Tue Aug 23 11:44:18 EDT 2016

Hi Dave:

I worked on the HP/Agilent 4380S test system software.  The 4380A test set has 8 ports and 3 receivers (R, A & B) as 
well as a built-in Short - Open - Load to speed up the calibration.  Uses bridges.
Used for measuring both ends of CAT5 cable and Firewire where each wire gets a test port. S-parameters transformed into 
Z-parameters to balanced parameters.  Note this system can test BALUNS where one port is coax and the other balanced 

Have Fun,

Brooke Clarke
The lesser of evils is still evil.

-------- Original Message --------
> 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.
>> Anyway,  it is an interesting project, but personally if I were going to
>> go to the effort of building a 2-port VNA, I would build one with 4
>> receivers.
>> Dave
>> _______________________________________________
> We used to have a lot of arguments at Agilent about how many
> receivers were needed.  The most I ever heard advocated was 5,
> and the least was 1 or 2.  I had to intervene in some of these
> arguments to bring up what I call the "back door reference"
> fallacy.  If you were making a "scalar" network analyzer that
> only dealt with amplitude, you could make various arguments
> about why you don't need so many receivers.  In principle,
> 1 receiver could work.  (The achilles heel of this idea
> turns out to be imperfect repeatability of switches, and
> very long settling times and thermal tails in switches.
> None of these calibrate out).
> In any event, as soon as you start talking about vector
> network analyzers, you are measuring phase.  Unlike amplitude,
> phase is always a relative measurement.  That is why you
> need a reference ("R" channel).  You compute A/R.  This
> requires a minimum of 2 receivers, an "A" and an "R".
> Concurrently, not consecutively.  Architectures that skimp
> on receiver count, or ostensibly omit the reference channel,
> are really a cheat.  There will be some back channel between
> the instrument clock and the sampling clock in the ADC that
> in essence acts as a reference channel.  If there is any
> warm up drift in the phase of this channel, you will get
> non-correctable errors if you try to multiplex a single
> receiver.  It is also another source of crosstalk on the
> PC board.
> Another problem with skimping on receivers is that you
> can't do full 2 port calibration, I used to
> have people show me "proof of concept" why they don't need
> full 2 port calibration.  They would compare a test of
> some simplified architecture to some top of the line VNA
> and show that the measurements were the "same".  Just like
> the graphs you see comparing low cost VNA's to Agilent
> VNA's (it always seems to be Agilent, not one of the other
> name brands).  It would often turn out that these "benchmarks"
> were not good tests of the analyzer.  Changing to more
> challenging tests would reveal the true superior design.
> For example, if you calibrate with a short, open, and load,
> and then measure the short, it always looks perfect.  But
> if you add a short length of transmission line in front of
> it, the simplified architecture may not work so well any
> more.  This is called a "remote short" test.
> Rick
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