Author Topic: Plot Smith Charts with Rigol DS1054Z?  (Read 710 times)

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Online RoGeorge

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Plot Smith Charts with Rigol DS1054Z?
« on: June 03, 2019, 12:59:09 am »
I searched 'Smith Chart with DS1054Z' and didn't find anything.

Are there any PC programs to make a Smith Chart using a digital oscilloscope (or at least to use the raw data from a CSV file with the ADC values coming from the channels of a digital oscilloscope)?

Offline Radiosonde

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #1 on: June 03, 2019, 07:41:11 am »
I just dont get it.
You have some sort of VNA thing that outputs some sort of signal, now you want the scope to take the Signal and display a SmithChart?

Thats surely not possible


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Online RoGeorge

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #2 on: June 03, 2019, 08:04:39 am »
I don't have a VNA, and I want to use a generator + the oscilloscope as a VNA.

The generator is feeding the device under test, and the oscilloscope is measuring the response.  Oscilloscope is used only as an ADC, at least two channels in order to measure amplitude and phase.  The ADC values are to be used to calculate the complex impedance and plot it on a Smith Chart (for the given frequency).

Online Yansi

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #3 on: June 03, 2019, 09:54:20 am »
Won't simply happen. At all!

Scope is just scope. Either YT mode or XY drawing mode.

At best you could draw yourself a S-parameter plot as a function of frequency - of course you would need an external mag/phase detector adapter to do that.
 

Offline David Hess

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #4 on: June 03, 2019, 10:40:49 am »
It could certainly be done but DSOs which support VNA operation are very rare and I am only aware of microwave crossover instruments which will produce a Smith chart.  Personally I think any DSO which has a signal generator output should support this and all thee modes (swept, impulse, noise) of VNA operation.
 

Offline T3sl4co1l

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #5 on: June 03, 2019, 12:22:32 pm »
I mean, with custom firmware, it sort of could; but who knows if anyone is working on that.  It'll be a long time in development.

Also, it's not going to give s11 or s22 without adding couplers.  Basically you're looking at getting only s21.  It's not much!

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Offline Bud

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #6 on: June 03, 2019, 01:00:58 pm »
VNAs perform systematic error correction (calibration) by applying specific math. Non-VNA devices do not do that.
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Offline Sferix

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #7 on: June 03, 2019, 11:45:17 pm »
It is of course possible, but it's probably not going to give you good results. 

You would need to use the scope with a signal source, directional bridge and computer to acquire the data.  Then you can capture the transmit reference signal, the forward signal, the reflected signal, and the thru signal which you can then process digitally to calculate the network parameters s21 and s11.  You essentially need to implement a digital receiver using the scope and computer software.  You will also need to implement all the VNA calculations and error correction.  Not a trivial task but doable.

Issues include:  The DS1054Z is not going to be a high performance receiver.  You may want to adjust the Y channel sensitivity and reacquire the data to get useful dynamic range.  The DS1054Z computer interface/processor is not particularly quick or reliable in my experience and at one acquisition per frequency, it won't be quick.

There is a poor mans solution which I have tried.  The DS1054Z can measure amplitude and phase using the measurement utilities and you can recover those measurements over the computer interface.  You can use those measurements directly to calculate the network parameters.  It's very slow and you have to handle the phase wrap around.  Also there appeared to be an error (discontinuity) in the phase estimate from my DS1054Z at low values.

Good luck.
 

Offline rf-messkopf

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #8 on: June 04, 2019, 01:02:13 am »
You would need to use the scope with a signal source, directional bridge and computer to acquire the data. 

You do not necessarily need a directional bridge. You could measure voltage and current trough an unknown two-terminal device and find its complex impedance. This could be plotted in a Smith chart as a function of frequency. You could even perform vector error correction, like a VNA does.

Many of the cheap one-port low-frequency antenna/impedance analyzers work according to this principle.
 

Offline Bud

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #9 on: June 04, 2019, 01:48:27 am »
The most shitty cheap VNA gadget is going to do a better job. Do not waste your time dancing around your scope.
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Offline MarkF

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #10 on: June 04, 2019, 04:05:52 am »
Have you seen this in lieu of a smith chart


 

Offline David Hess

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #11 on: June 05, 2019, 01:26:20 am »
The most shitty cheap VNA gadget is going to do a better job. Do not waste your time dancing around your scope.

But not down to DC and usually with compromised performance at low frequencies.

An oscilloscope is not a substitute for an RF VNA (1) but it could be suitable for less demanding and low frequency network analysis with the proper firmware.

There is nothing which prevents making a fully bidirectional 2 port (or more) analyser which operates down to DC although I do not know of anybody who has bothered.  (2) Most designs are like Cleverscope, Compsys Design, or Picotest Bode 100 VNA which use a low impedance output and two high impedance inputs like an oscilloscope.

(1) Unless it is a crossover instrument like a microwave transition analyser but performance is compromised compared to a real VNA.  How do VNAs which support non-linear operation work and perform?

(2) Impedance analyzers could be designed this way but the ones I have studied were not.
 

Online RoGeorge

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #12 on: June 05, 2019, 03:25:07 am »
I don't want to change any oscilloscope's firmware, this will be very unrealistic.

All I am looking for is a free open source computer program, or at least a library able to plot a complex impedance value (or a list of them) to a Smith Chart.

In theory it is possible to read the channels' data by SCPI (for digital oscilloscopes).  From there, it should be possible to calculate the phase shift relative to the reference signal.  Knowing the phase and amplitude, it should be possible to calculate the complex impedance.  Then change the frequency, or change the port, and take a new set of measurements so to represent the next point on the Smith chart, and so on.

Of course, extra cables or circuits might be necessary (e.g. external directional coupler) depending on wanted measurement.  Apart from the SCPI oscilloscope connected to the computer, we will also need a SCPI signal generator.  The computer will do all the logic and the calculations, including the chart display.  The oscilloscope and the generator will function as SCPI computer controlled instruments, no firmware to modify.

It will be very slow, and probably not very precise, but it will be better than nothing, I guess.  And the same program can be easily adapted to any SCPI oscilloscope/generator, so it will be a long term tool.  No idea why nobody did that yet.

Offline MarkF

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #13 on: June 05, 2019, 10:57:06 am »
I have been wanting to give GNU Octave a try. 
But, I haven't had time to play with it.  We useMatlab at work but don't have it at home.

You might check out this Toolbox for Octave.
It claims to have Smith chart plotting.

Maybe you can write an Octave interface to talk directly to the scope?
 

Online RoGeorge

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #14 on: June 05, 2019, 12:56:24 pm »
Very good idea, thank you!

Offline OwO

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #15 on: June 05, 2019, 02:28:10 pm »
A good USB VNA costs about half of a DS1054Z so I'm not sure why you would bother.

In theory it is possible to read the channels' data by SCPI (for digital oscilloscopes).  From there, it should be possible to calculate the phase shift relative to the reference signal.  Knowing the phase and amplitude, it should be possible to calculate the complex impedance.  Then change the frequency, or change the port, and take a new set of measurements so to represent the next point on the Smith chart, and so on.
There are a number of problems with this, first of all you have to do calibration if you want usable measurements. The best directional coupler will not give you impedance readings without calibration because there are amplitude and phase variations between the coupled and reflected ports (you will also likely need two couplers to get a reference channel).

Then you have to terminate your scope ports with 50 ohms, a high impedance input will not do and you will get huge ripples relative to frequency.

Finally the amplitude and phase readings from a scope are usually based on very few data points and will be nowhere near as stable as what you would get from a real vector detector (implemented in DSP and taken from many thousands of samples).
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Offline Bud

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #16 on: June 05, 2019, 03:14:40 pm »
  No idea why nobody did that yet.

Because people realize what you have not realized yet - that this is waste of time.
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Online RoGeorge

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #17 on: June 05, 2019, 03:47:51 pm »
- Any VNA requires calibration, that's why any VNA owner also has a calibration kit.
- Yes, cheaper scopes (including DS1054Z), will need feed-through 50 ohms BNC terminators.  Feed-through terminators are common items in any RF drawer, on par with 50 ohms cables.  At low RF frequency, very easy to DIY.
- Amplitude and phase can be calculated with much greater precision than looking at the waveform on the screen, by downloading a longer waveform and post-processing it in the PC.  Thousands of samples can be used for oversampling, in order to increase the ADC resolution, and DSP techniques over so many samples can get a very precise phase shift measurement.  Repeat this for each Smith chart's point, and we can get decent performance.

The only major drawback I see is the speed.  Downloading the data can take many seconds for each corresponding point on the final Smith chart, but that is strongly dependent of the requested number of points.  Not all measurements need to have 3 decimal places.

Alternatively, the speed can be greatly improved considering most of the generators have sweeping frequency capabilities.  So, instead of point by point measuring at each frequency, a frequency sweep can be made at once, and the full sweep response can be memorized by the scope then transferred later to the PC, by a single SCPI transfer for the whole frequency sweep.  From there, the whole batch of samples can be DSP-ed together in the PC, in order to find the complex impedance evolution over time, and plot that on the chart.  Sweeping like that could make a Smith chart almost in real time, or at least reasonably fast to allow "live" tuning of the DUT.

Trying to implement that will be a great learning experience, and this is the whole point.  This idea is not meant to be a commercial product, it is not for the money, it's just for the fun of it, as a hobby.

You all just envy for when everybody with a DS1054Z will be able to do VNA measurements.   :D
« Last Edit: June 05, 2019, 04:05:23 pm by RoGeorge »
 

Offline OwO

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Re: Plot Smith Charts with Rigol DS1054Z?
« Reply #18 on: June 05, 2019, 04:04:54 pm »
Yeah but only up to 100MHz, and with a lot of external hardware required (2 couplers, signal generator, lots of cables and adapters). There are now 200MHz VNAs on taobao for less than $50, and I'm sure even that beats the accuracy of what you are trying to do.
« Last Edit: June 05, 2019, 04:06:55 pm by OwO »
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