What is benefit of 4 port VNA? - Page 1
Forum section: electronics / rf-microwave

author: fonograph
author_rank: Frequent Contributor
timestamp: July 28, 2018, 08:30:58 am
content: What are pros of 4 ports? Is it same like two 2 port VNAs? I mean is it just more channels like on oscilloscope or can you do something magical on 4 port that two 2 port VNAs cant do?

author: jungle vegetable
author_rank: Contributor
timestamp: July 28, 2018, 11:51:47 am
content: Differential device measurements become easier with 4-port

author: CopperCone
author_rank: Super Contributor
timestamp: July 28, 2018, 11:53:31 am
content: you can either do one direction differential measurement or bi-directional single ended measurement with 1 step.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 28, 2018, 11:53:41 am
content: There are a few reasons.Let's start with the obvious: multi-port networks. A two-port VNA can do, obviously, two-port measurements. This means that two-port filters and amplifiers, or simple antennas can be done easily.But what if I need to measure and verify a 3-port device, such as aWilkinson power divider? Or a 4-port device, such as arat-race or quarter-wave coupler?In all of these cases, we need more than one port. Now, in theory you can do this with a two-port analyzer and using very high-quality 50 ohm terminators on every other port, and then work the ports in groups of two to get all our S-parameters (port 1&2 for S11, S21, S12, S22, port 1&3 for S13, S31, S33, port 2&3 for S23, S32, port 1&4 for .... you get the idea). But this would be a pain, and engineers cost a lot of money, so if you do this often, you should get a 4-port or multi-port VNA system.Another classic example of why we need multi-port VNAs is datacables and backplanes. The newest displayport standard offers up to 4 pairs at 8.1 Gbit/s datarates. These cables and connectors should be measured in order to be certified. This means that we need to know not just the transmission of one pair, but also the crosscoupling of the pairs, and all of this requires a multiport VNA. Actually, a normal 4-port would not even suffice here (but there are actually test-port extenders from most major VNA manufacturers, such asKeysightor our German friends atRohde & Schwarz)In addition, in differential networks, you could use a balun to measure the system but you can also often use two channels of a VNA to make up the 'positive' and 'negative' signal of the differential line (and in a sense treat it as a four-port network, I guess). This way you can study things like odd/even mode impedance of the transmission line, which you can't do with a balun.But there are other interesting reasons why you might want to use more than two ports. One of them, but I'm not that familiar with it, and I don't know what VNAs can/can't do this, isloadpulling. In this case you use one port for the measurement and the other for sending a wave back, and this way you can 'sweep' the apparent impedance at the port. This is important when designing amplifiers because it allows us to figure out what point we have to match for maximum power or efficiency.Another common purpose is non-linear networks such as mixers. Again, not that familiar with how much of this is limited to the top-of-the-line VNAs, but for some you can output different frequency and power signals from different ports at the same time, and this allows you to characterize something like a mixer fully (note that this is not the same as what you can do with some 2-port VNAs that offer external mixers! In those cases you are not bring out two ports you can measure S-parameters with, but an LO port and a IF port, and the LO port will then be multiplied inside the frequency extender).I'm sure there are some more applications that I am currently not thinking of.

author: rfdave#gmail.com
author_rank: Contributor
timestamp: August 02, 2018, 01:13:03 am
content: You need a VNA to do a load pull on a PA, for calibration, but the actual Load Pull requires an impedance tuner. Focus Microwaves and Maury Microwave both make automated versions. High $$$ specialist equipment.