[time-nuts] DIY VNA design
brooke at pacific.net
Tue Aug 23 11:44:18 EDT 2016
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
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
> 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.
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