Vector Network Analyzer as an SA - practical? Good? - Page 1
Forum section: electronics / testgear

author: rx8pilot
author_rank: Super Contributor
timestamp: August 19, 2016, 06:51:34 pm
content: Over the past few months I have been studying RF and digital communications systems. It seems that if I go much further, I will benefit from having an Spectrum Analyzer and if I am lucky a VNA as well.Is it practical to use a VNA as a general purpose SA where the signal origin is totally external? Would some models be better at this task than others? Hoping to get a 3-for-1 if it is actually possible. Since I only know the very basics of how these instruments do what they do - I just don't know. It seems that the hardware of a VNA would have no problems, but maybe the software is not setup to work like a general purpose SA with a TG? Right now I am trying to understand how much a should budget and the answers to this question would certainly impact the budget. It will be a few more months before I can buy anything significant, so a nice slow education is what I am after.

author: TimFox
author_rank: Super Contributor
timestamp: August 19, 2016, 08:51:26 pm
content: VNAs are not designed to use with a "freestanding" signal (one independent of the internal source), Naively, I tried that with an -hp- 8753 VNA, but got really bad results. Look at the heterodyning technique used to get the input down to a measurable frequency (< 1 MHz) and you will see that it can suffer badly from images.

author: rx8pilot
author_rank: Super Contributor
timestamp: August 19, 2016, 08:58:29 pm
content: Quote from: TimFox on August 19, 2016, 08:51:26 pmI was afraid that would be the case. Not a show stopper, but hoping to get lucky and purchase fewer boxes if at all practical. Thank you for the response.

author: G0HZU
author_rank: Super Contributor
timestamp: August 19, 2016, 09:22:19 pm
content: Some spectrum analysers can also be used as a VNA. My old Advantest TR4172 spectrum analyser has an internal tracking generator and it has an impedance measuring option fitted . This involves using an external Wiltron/Anritsu return loss bridge that comes with the option.However, it doesn't work very well as a VNA. I wouldn't recommend using it in VNA mode. It's also a huge, heavy and ugly monster of an analyser. I like it as a spectrum analyser but many others will definitely not like it.HP produced a VNA/spectrum analyser combo model in the 1990s, the HP4396A/B. Like the Advantest, this only covers up to 1800MHz. The HP4396 is much, MUCH better than the TR4172 when used in VNA mode. Night and day difference in terms of performance and usability. However, it generally isn't as good as the 4172 in spectrum analyser mode despite having a digital RBW for the narrow RBW settings.

author: KE5FX
author_rank: Super Contributor
timestamp: August 19, 2016, 09:27:46 pm
content: It's extremely hard to design an instrument that does a good job as both an SA and a VNA. Their block diagrams may look superficially similar, but under the hood, different design decisions are called for at almost every stage.Not to say it can't be done, but it's difficult.

author: Bud
author_rank: Super Contributor
timestamp: August 19, 2016, 09:33:50 pm
content: I say no and no. Do not waste your time, find a proper used SA.

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 19, 2016, 09:38:14 pm
content: I know it's not what you are looking if you are out to save some money,but Keysight has the PNA-X series:http://www.keysight.com/en/pdx-x202277-pn-N5242A/pna-x-microwave-network-analyzer-265-ghz?cc=US&lc=engA mind boggling slew of options including built in spectrum analyzer.One nice thing about combining instruments with the VNA is they can do full vector error corrected measurements.

author: Howardlong
author_rank: Super Contributor
timestamp: August 19, 2016, 09:47:45 pm
content: I'd also ask yourself why you think you need a VNA. If you are measuring and adjusting RF filters, amplifiers, attenuators, matching sections, or antennas, then there may be justificaction. An awful lot of this can be done with a spec an with a tracking generator +/- RLB though, and that option avoids you needing to be able to read and interpret a Smith Chart.My work is mixed domain with RF, but the VNA is switched on maybe once a month on average for an hour or so, but as is common in these situations you can go months without needing to make a measurement and then suddenly you spend two full days a slave to the VNA. Most of those measurements I could do with an SA + TG, but occasionally a VNA is very useful, matching SAW filters being one of them, as the data sheets' published impedances for SAW filters are often quite some way off, having both mag and phase helps with this.Some years back I did a lot of antenna design, and a VNA was invaluable for that work when matching my real life designs.A VNA, useful tool as it is, I find is good for R&D but I don't use one for production: I design stuff to be built with reproducibilty in mind with the minimum of adjustment, if any, and any required are made through self-calibration and autonomous sanity checks. This makes sense if you make thousands, but for a hundred or so it might well be more cost effective to simply manually tweak rather than come up with an automated self-test of course.

author: G0HZU
author_rank: Super Contributor
timestamp: August 19, 2016, 09:50:02 pm
content: QuoteIt depends on the budget and the bench space available... The HP4396B will generally outperform the various modern USB VNAs for many tasks. I'd happily use/trust the 4396B for general (non critical) VNA tasks.It performs 'OK' as a spectrum analyser with fairly poor far out phase noise and only average dynamic range etc. I used the 4396A a lot as a VNA and spectrum analyser in the early 1990s when we bought our first one at work. It was a very popular instrument in the labs and we soon bought another. I think we ended up with over a dozen of them because they are also a cheap combo option for use in production testing in ATE racks.We probably still have about 8 of them on the test equipment shelves at work. They sit there unloved and gathering dust today because they only go to 1800MHz. Every couple of years or so we throw one or two of them in the skip at work if they fail calibration or have any kind of issue such as a minor fault or missing buttons etc. But they still fetch good prices today because I think lots of companies also used them for ATE work. So they are still in demand by companies who need them for legacy ATE work.I would still class it as an excellent instrument for general purpose RF design up to 1800MHz. It's no star but it is good enough for most taskshttp://www.keysight.com/en/pd-1000000865%3Aepsg%3Apro-pn-4396B/network-spectrum-impedance-analyzer?cc=US&lc=eng

author: rx8pilot
author_rank: Super Contributor
timestamp: August 19, 2016, 09:54:18 pm
content: Quote from: rfeecs on August 19, 2016, 09:38:14 pmThat is breathtakingly outrunning my skill setQuote from: KE5FX on August 19, 2016, 09:27:46 pmThis is the value of community support. So I was considering a new USB SA like Signal Hound BB60C or Tek RSA's while the VNA would be a used model from HP/Agilent or similar. The ability to record the USB SA's for later analysis seems interesting and they have more capability for demodulation (so it seems) than the old HP/Agilent models.I was initially thinking that around $5k or so would get the USB SA and a reasonable but older VNA with an additional $1k for the various accessories - cal kit, cables, adapters, and whatever other little starter bits. That is around $3k for the VNA itself - does that seem too low or is that a reasonable range for 3Ghz+ ?

author: rx8pilot
author_rank: Super Contributor
timestamp: August 19, 2016, 10:05:08 pm
content: Quote from: Howardlong on August 19, 2016, 09:47:45 pmTo be honest - I don't need one. At least not for sure. As I have been reading and learning about RF and communications at a fundamental level - a VNA seems like it will allow me to dive deeper as I wish instead of being limited by the instruments on the bench. If I allocate the resources needed early - I will be ready when the time is right. The SA with TG is a for sure purchase in the relatively near future. Like all the other things on the bench - I tend to buy them only knowing basically what they are used for and then I can hands on learn by doing immediately following some book study. It is hard for me to learn from a book without being able to reinforce the information with action.Right now, I am spending some time figuring out what I can measure with a VNA that could not me measured with any other method. Fun!

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 19, 2016, 10:40:19 pm
content: With that wiz bang 6GHz scope you have, you can pretty much do what a VNA can do. Just a lot slower. (You would also need a signal generator, maybe a power divider and/or impedance bridge) With the scope, you can measure the magnitude and phase relationship of two signals. By changing the scale, you can do it over quite a big dynamic range.But I'm guessing you can't easily do a swept measurement and display the results in dB over say a 100 dB dynamic range. You could do that with your scope, either manually or with the aid of automation. But it would be much slower than the VNA, which could display the response of an amplifier or filter in real time while you tune it, for example.

author: rx8pilot
author_rank: Super Contributor
timestamp: August 20, 2016, 12:05:48 am
content: I am fiddling through some setups to do Bode plots of the feedback loop on my SMPS designs. For higher frequency and dynamic range would be marginal at best I think. Because the scope was free, I can now go after instruments that were previously pretty far down on priority list.My first 'high-speed' design is coming relatively soon. I will largely be copying a reference design to prevent too steep of a challenge. That project will me see for myself the various challenges of dealing with 6Ghz signals.Return loss, insertion loss, impedance, frequency response, etc will be fun and interesting parameters to learn how to measure.Sent from my horrible mobile....