VNA advice

[eurofox]

I would like to get advice from VNA users since I'm interested to buy one, I have never use a VNA before.

1/ It is an expensive instrument for hobby use 
2/ I'm not an RF dude  just like to play with ultra fast pulses, cables, connectors, spliters .... are RF grade and would like to measure components as well with respect frequency reflection .....
3/ Option 1: an old heavy second hand HP, more than 20 kg on my shelf and bulky  and 25-30 years old and still expensive
4/ Option 2: an USB VNA, many options from 500 Euros to 20.000 Euros ...., I can forget the last one (http://www.coppermountaintech.com/)
5/ I have a Rigol 815 + TG that I use mainly with the TG to measure cables, filters, spliters but is limited in bandwidth, I suppose that I can use a VNA to do this job as well, this way I could use the money from selling the Rigol to invest in a VNA
6/ I read positive and negative feed back from http://sdr-kits.net/VNWA3_Description.html, I know there are some users on the forum, input from them is very welcome. What I dislike is the limit of 1,3Ghz ... I know for the price 
7/ There are many other USB version but more dedicated to HAM users 
8/ I just came across this link today http://www.megiq.com/ not cheap as well but ...., any experience with this one?

Thanks in advance 

[XFDDesign]

I have to ask, if you've never used a VNA, how do you know you need one?  Practically speaking, the VNA is mainly useful for collecting small-signal like scattering parameters; nothing like pulses and such which might be better off with a TDR Scope.

Can you clarify what it is you're wanting to do specifically? Your wants are all over the place. If your SpecAn w/TG isn't sufficient for higher freq bandwidths, you could always upmix an then down mix with a fixed-frequency LO and some mixers. This would certainly be cheaper and easier than a full-on VNA route.

How often do you find yourself caring about return losses, matching, discrete device stability, etc?

[eurofox]

I have to ask, if you've never used a VNA, how do you know you need one?  Practically speaking, the VNA is mainly useful for collecting small-signal like scattering parameters; nothing like pulses and such which might be better off with a TDR Scope.

Can you clarify what it is you're wanting to do specifically? Your wants are all over the place. If your SpecAn w/TG isn't sufficient for higher freq bandwidths, you could always upmix an then down mix with a fixed-frequency LO and some mixers. This would certainly be cheaper and easier than a full-on VNA route.

How often do you find yourself caring about return losses, matching, discrete device stability, etc?

I read a lot about VNA's and the use of it and possibility to use it for components analysis, I already build power spliters and could find out my basic mistakes with the SA + TG.

With respect to pulse I have a sampling scope but you can only measure the results of the design.

[G0HZU]

the VNA is mainly useful for collecting small-signal like scattering parameters;

Yes, that's the main reason I have a big old HP VNA here. I use it to create 1 or 2 (or n) port (small signal) s parameter models that can be exported via GPIB direct to an RF simulator on a PC. Usually I design something on an EM simulator and then mill a PCB and then test it on the VNA. This is often research stuff eg recently I've been doing lots of research into microwave filters or oscillators or couplers based on defective ground structures.

The modern low cost (and very small!) alternatives based on DDS technology seem to offer excellent performance up to VHF but by UHF the performance declines markedly on the ones I've looked at. So they aren't really suitable for me because most of my work is in the GHz region and I need to generate models with fairly high precision.

My advice to anyone is to only buy something like a big old HP VNA if you really need to maintain high performance up into the GHz region. For many people the modern DDS alternatives will be fine and they only take up a small amount of space.

[Howardlong]

I've had an HP 8753A for about ten years. It gets used sporadically, and I've had to repair it twice, the screen is reknowned for being a bit touchy. I also have an Anritsu S331A which is basically a one port S11 VNA which is more convenient to set up if all I need is a one port measurement.

Remember you'll need a calibration kit for your VNA, they can be expensive too.

For most of the HPs you'll need either an S Parameter test set or a T/R test set, you'll have an expensive papaer weight with out one. A few HPs, like the 8714, have a built in T/R test set. T/R test sets are cheaper than S parameter test sets, but you can do everything you need with a T/R test set except you'll need to turn the DUT around manually to do all the tests, although most of the time I'm only interested in S11 and S21. I use a T/R test set, in fact it's a 75 ohm one, designed for CATV, so I use a minimum loss pad to convert it to 50 ohm at the expense of a few dB of sensitivity.

If you do a lot of antenna stuff, phasing harnesses and RF amps they're great bits of kit to have. I was doing an awful lot of antenna stuff when I bought mine, and it was very worthwhile for that. I haven't used the new PC based units so I can't comment on them.

[Co6aka]

6/ I read positive and negative feed back from http://sdr-kits.net/VNWA3_Description.html, I know there are some users on the forum, input from them is very welcome.

I have one and recently used it to construct a three channel star combiner on 450 MHz; loss in the star cable worked out to be under 0.25dB, impedance on all ports was within +/- 2 Ohms of 50 nominal, with similarly-minimal reactances. Paid for itself on that one job. For what it is, it's pretty slick. The design is dated, but I imagine DG8SAQ is playing with newer DDS chipsets and ADCs by now. (And FWIW, there's a post somewhere on the 'net by a guy who re-routed the VNWA's audio out to his high end soundcard with interesting-looking results.)

[XFDDesign]

I read a lot about VNA's and the use of it and possibility to use it for components analysis, I already build power spliters and could find out my basic mistakes with the SA + TG.

With respect to pulse I have a sampling scope but you can only measure the results of the design.

I'm still not seeing a compelling reason to justify the need for a very expensive piece of test gear. Again, you're not going to get Pulse response information off of a VNA, pulse measurement specifically will be done with a Communications Sampling Scope, not any old "Oscilloscope" but a ridiculous piece of kit (http://www.tek.com/datasheet/csa8000b). Pulses are a "time-domain" kind of thing. VNAs are, in a terrible simplification, a highly glorified multi-channel Spec-An with phase measurement; a frequency domain device.

Let me use the following as a metric:

Q. Why is a return-loss bridge insufficient for your needs?

[edavid]

Again, you're not going to get Pulse response information off of a VNA, pulse measurement specifically will be done with a Communications Sampling Scope, not any old "Oscilloscope" but a ridiculous piece of kit (http://www.tek.com/datasheet/csa8000b). Pulses are a "time-domain" kind of thing. VNAs are, in a terrible simplification, a highly glorified multi-channel Spec-An with phase measurement; a frequency domain device.

Of course you can get time domain information from a VNA!

http://cp.literature.agilent.com/litweb/pdf/5989-5723EN.pdf

[Bud]

Moreover, professional VNAs use in time domain so called gating feature to de-embed parts of the circuit. For instance, in a circuit consisting of a RF connector, piece of transmission line and a actual device being tested, gating can be set to remove influence of the connector and transmission line and leave only parameters of the device.
While it can also measure distance to fault, transmission line length, mismatch, etc, it is still not a right tool to measure timing characteristics though.

But to the original poster's question, I'd say you could start with a inexpensive USB VNA. May be your interest will fade out and then you will not feel bad for spending $K for a industrial VNA that you not using.

[XFDDesign]

Of course you can get time domain information from a VNA!

http://cp.literature.agilent.com/litweb/pdf/5989-5723EN.pdf

Well now, "Today I learned..."

I still wouldn't use a VNA to perform that kind of measurement. It's like a Frankenstein's monster of mess to pull it off, like a poor grade FFT from a scope to do spectral analysis.

[edavid]

It's like a Frankenstein's monster of mess to pull it off, like a poor grade FFT from a scope to do spectral analysis.

When you learn about TDNA your head will explode

[eurofox]

I read a lot about VNA's and the use of it and possibility to use it for components analysis, I already build power spliters and could find out my basic mistakes with the SA + TG.

With respect to pulse I have a sampling scope but you can only measure the results of the design.

I'm still not seeing a compelling reason to justify the need for a very expensive piece of test gear. Again, you're not going to get Pulse response information off of a VNA, pulse measurement specifically will be done with a Communications Sampling Scope, not any old "Oscilloscope" but a ridiculous piece of kit (http://www.tek.com/datasheet/csa8000b). Pulses are a "time-domain" kind of thing. VNAs are, in a terrible simplification, a highly glorified multi-channel Spec-An with phase measurement; a frequency domain device.

Let me use the following as a metric:

Q. Why is a return-loss bridge insufficient for your needs?

The meaning to use a VNA in this pulse measures is not directly related to the pulses itself but to the components used to design, connect .... it

Below my latest test on my sampling scope but would like to improve it.

[KJDS]

The key to VNA performance is the directivity in the couplers and the source and load match. Some of the affects of these can be calibrated out. I've no idea where the USB tools are, but I do know that I'm happy with my HP8753C with 6GHz option.

I've spent many years in the past tweaking performance of components using time domain on an 8720 20GHz analyzer. For improving component performance then using time domain then 6GHz just never gave me enough resolution to isolate things.

Whether it is worth it is down to you, but I wouldn't be without mine.

[Mr Simpleton]

Just to make it clear, any VNA is only as good as your calibration kit and cables.... these can cost more than just your VNA, especially if you are doing stuff over 3GHz.
You say you are not an RF-guy still find 1.3GHz limiting...
BTW the MegIQ starts at 400 MHz, not good in my book!

I have access to a 8753E, and it is an amazing machine, still big, bulky and expensive. At home I use my NVNA v2 and I am more than happy! This is an excellent tool. Now if you do need higher dynamic range and precision why not build the N2PK variant, you can get blank PCB's and start soldering This one goes only up to 70 MHz but offers dynamic range that equals or better the HP!

BTW the 8753A have an infamous CRT that more than often fails. HP did trade in our A for an E, as they really wanted the A to disappear, and not show up as 2:d hand, I kept the s-parameter test set but this did not affect the trade-in Now what to do with that box?? 

[thewyliestcoyote]

VNA is only as good as your calibration kit and cables

So true, but it does depend on what type of measurements you are doing. I my day job mostly everything is free space measurements like in a chamber. There a calibration can be nothing more than isolation and response (Admittance tunnel). This can be done with just a placement of a metal plate.

you're not going to get Pulse response information off of a VNA

That is not exactly true, pulse response can be calculated from VNA measurement, its just a matter of some fourier mathematics. There is a python module that maybe of some interest to you called scikit-rf. It can do time domain gating, time domain transform and so on. At The heart of what's going on is a IFFT for time domain transform, and a IFFT, vector multiply with a windowing function, and a FFT to return to the frequency domain.

eurofox what is it you would to measure? frequency? accuracy and precision? It can make a BIG difference in the price of the instrument. Maybe a 2 channel software defined radio could meet your needs if they are simple.

[Howardlong]

I'm not quite sure how I forgot this, but aside from the antenna work I did about a decade ago, I also use the 8753A extensively with my crusty T/R test set to design and test dozens and dozens of RF filter prototypes, both lumped and SAW. When matching SAW filters in particular, it's difficult to think of a better tool to use: they are notoriously picky to minor aberrations from optimum matching impedances, and the manufacturers' specs on SAW filter matching are routinely complete bolleaux.

[eurofox]

eurofox what is it you would to measure? frequency? accuracy and precision? It can make a BIG difference in the price of the instrument. Maybe a 2 channel software defined radio could meet your needs if they are simple.


My target is to get a pulse with a rising edge as fast as possible and amplitude the highest possible.

[G0HZU]

When matching SAW filters in particular, it's difficult to think of a better tool to use:

Yes, one neat approach with many filter types is to take a raw 2 port model of the (non 50R) bandpass filter using the VNA (i.e. with direct 50R connections to the BPF) and then export the 2 port model to a computer to then model/optimise the required matching network on a simulator.

I find that this is the most powerful approach specially if you want to model and optimise the matching network across the whole passband of the BPF to get the best match and/or lowest ripple on the simulator.

That way you usually only have to build the matching network once and then test it with the VNA once built. You can also use the (accurate) models for the BPF and the matching network in an overall system simulator if required.

[Howardlong]

...export the 2 port model to a computer...

Ah, but a real engineer would use a Smith chart, apparently. :-)

[thewyliestcoyote]

My target is to get a pulse with a rising edge as fast as possible and amplitude the highest possible.

A couple of assumptions about what you are testing, strictly linear(can be a big if for LNA's PA's and such). This is a function of the maximum frequency of the VNA is going to be about equal to the BW TDR or scope being used to measure the pulse. Resolution of the pulse can be much greater for VNA derived pulse response then TDR. If you increase the number of sample points it is equivalent to upping the sample rate of the TDR or scope, more detail! But the part of highest possible amplitude I am not sure why? unless you are looking for distortion vs. voltage of pulse or something like that. To my knowledge this can not be done on a VNA using, maybe there is a way but never used it.

Ah, but a real engineer would use a Smith chart, apparently. :-)

Hahahaha I think I have never used the smith chart for anything other then education and talking point in presentations. Solving real world problems(not single frequency or amplitude) is almost impossible and best done by just using the measurement values in complex number form.

[Howardlong]

Ah, but a real engineer would use a Smith chart, apparently. :-)

Hahahaha I think I have never used the smith chart for anything other then education and talking point in presentations. Solving real world problems(not single frequency or amplitude) is almost impossible and best done by just using the measurement values in complex number form.

To be fair, this is the best analysis I've seen of the scenario, and there's more than a fair bit of Smith chart goodness going on: http://pdfserv.maximintegrated.com/en/an/AN742.pdf

It's one of those "break glass in case of fire" application notes that needs to be kept safe somewhere for when you just might need it.

All of which brings us back to the question of whether the OP needs a VNA or not. As a generalisation, and I welcome others' opinions, I'd say if you can't read a Smith chart, then a VNA is probably not what you need, and a scalar network analyser would be a more cost effective solution, or, indeed, for-two port thru measurements, a spectrum analyser with tracking generator would be just fine.

This treatise may help,the OP: http://cp.literature.agilent.com/litweb/pdf/5965-7917E.pdf

[thewyliestcoyote]

To be fair, this is the best analysis I've seen of the scenario, and there's more than a fair bit of Smith chart goodness going on: http://pdfserv.maximintegrated.com/en/an/AN742.pdf

SCFTW! wow nice thanks for the link! It is a great tool to get a idea of what is happening. It also show the 4 types of passive devices resistors, capacitors, conductors, and transition lines and there relationship. I am not counting mem-resistor because you I can go to mouser, digikey, or where ever and order one. They show the equations! and when design something that is what you need.

A couple of assumptions about what you are testing, strictly linear(can be a big if for LNA's PA's and such). This is a function of the maximum frequency of the VNA is going to be about equal to the BW TDR or scope being used to measure the pulse. Resolution of the pulse can be much greater for VNA derived pulse response then TDR. If you increase the number of sample points it is equivalent to upping the sample rate of the TDR or scope, more detail! But the part of highest possible amplitude I am not sure why? unless you are looking for distortion vs. voltage of pulse or something like that. To my knowledge this can not be done on a VNA using, maybe there is a way but never used it.

I had a talk with the professor I will be getting my masters under and the best ideas in 10 mins for measuring these kind of devices with a signal source and 2 channel vector signal analyzer. To same thing about math of S-parameters of these kind of devices are Fourier coefficients or transform not just a complex value for each of the parameters. This is a very complex type of measurements. So if you can stick to "linear devices" and work in the small signal range of LNA's, PA's and other non linear devices.

[G0HZU]

...export the 2 port model to a computer...

Ah, but a real engineer would use a Smith chart, apparently. :-)

I still use a PC based smith chart matching program for simple narrowband matching tasks but for wideband low loss circuits the most efficient way to do it is by taking an accurate 2 port model of the filter (or coupler or RF switch etc) from the VNA and doing it all on a simulator. This way you can analyse and optimise the matching across hundreds of frequency points in real time and the computer takes into account S11 S21 S12 and S22.

I've even used this approach when matching narrowband stuff like a high impedance crystal filter. i.e. measure it unmatched with direct connections to the 50 ohm VNA and then export a 2 port model (S11 S12 S21 S22) to a simulator and then design/optimise a match circuit (to 50 ohm) for this model on a simulator.

The oldschool smith chart method is OK for narrowband matching to known/fixed impedances but it's not as powerful as the above method. You also get the bonus of getting accurate n port models to use in a system simulator.

[KJDS]

I still use a smith chart when matching in microwave office or ADS. It makes it easier to see what's happening as you build up a matching circuit whether it's broad or narrow band. I can't imagine trying to match to the input of an RF power transistor, where the input impedance may be 0.5ohms with a chunk of series inductance without the aid of a smith chart.

[PA4TIM]

F@$#g Windows explorer
Spend an hour typing, typed a few backspace and the browser leaves this page and away is my text. This was the last time, Now I'm sure, I will replace it by Linux on this laptop too. 

http://www.pa4tim.nl/?p=1594 I made a series of tutorials because there was/is not much literature aboutt NA for beginners. I had to learn it by myself the hard way.

I have  2 VNWA's, 2 HP-8407 and a lot of accessoires.

Calkits are the most important together with cables. My best cables are 279 euro a piece (I got them for free from a friend), besides that mil grade RG58 (the see through version), home made extra shielded belden/radial cables for bnc and GR cables.

I use calkits from Amphenol, rosenberger (sma), homemade (N and bnc) and from GR (GR874 and GR900)

You need fixtures and adapters too. All those things can be much more expensive.

The results depent very much on the way you measure and calibrate. You can not make good measurements with a bad calkit.

My interest in VNA's is component behaviour related.
I like the VNWA a lot, it has a lot of functions. OK, it is limmited to 1500 MHz (only with my best cables and call kit) but if you want to play at 6 GHz the calkit you need for that will cost a lot more as a complete VNWA.