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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.