Transmission/Reflection Vector Network Analyzer

with calibration and compensation

Support Docs: McDermott TAPR VNA Article

Hardware setup:

(Nov. 15, 2005) Finally added calibration and compensation routines to the magnitude and phase measuring scripts. Instead of trying to make one complete do-it-all application I've broken the tasks down into a set of individual scripts that collect data, process it, and format and display data.

Transmission Measurements

Calibration for transmission measurements is pretty easy. With the test jig wired, we put a 'bullet' or short through-connect in place of the device to test, then run a freq scan with and save the data to a calibration file for later use. Then we remove the 'bullet' and connect the test device, a filter or what have you, in it's place and run another scan and save the data to a device data file. Once we have those two, calculating the actual transmission response is easy. The theory is:
	s21(actual) = s21(measured) / CAL21  (See McDermott)
s21(actual), (measured) and CAL21 are all complex mag/phase values collected for each frequency with is the script that applies the calibration compensation to the data. Say we have run with a connect-through 'bullet' and collected data into a file "calib_trans". Then removed the bullet and connected a device, ran and collected data into a file "dut_data_raw". We can then apply compensation with:
	$ ./ calib_trans dut_data_raw dut_data_comp
Once the compensated data is obtained, you can display it on a static Smith chart by converting it to rectangular form:
	$ ./ dut_data_comp dut_data_comp_smith
and putting the file name in smith_chart and displaying with:
	$ gnuplot smith_chart
OR displaying it on a dynamic Smith chart with Powermate knob control:
	$ ./ dut_data_comp

Reflection Measurements

Reflection is a bit more involved as we need to make 3 calibration data files, one with the DUT disconnected (open), one with a short for the DUT (short), and one with a 50ohm load resistor for a DUT (load). Once we have created those three files with, we can connect the test DUT and collect the test device data. Once we have the test device data and the calibration data we apply compensation with:
	$ ./ calib_open calib_short calib_load dut_data_raw dut_data_comp
and then plot or display it as above. The compensation math is more complicated and I refer you to McDermott above for all the gory details.


Transmission test: This just shows that "Anything / Anything = 1" where Anything does not equal zero, and demonstrates that transmission calibration actually works. With a through-connect 'bullet' in place we run (remembering to start it with a -tt command line option) from 250kHz to 25MHz and save the data to "calib_trans". Then I run it again (testing repeatability) and save the data to "calib_trans_2". Then I use the first file as calibration data, the second file as device data and plot the results. Here is a plot of the calibration data:

and of course the test data, "calib_trans_2" looks very similar. After processing:
	$ ./ calib_trans calib_trans_2 test_data
and when the conpensated 'bullet' test data is plotted:

which shows phase and magnitude at unity across the spectrum from 250kHz to 25MHz. Next we connect an actual device, my 80mtr ham bandpass filter, run and save the collected data as "80mtr_pass-filter_raw_data", then process it:
	$ ./ calib_trans 80mtr_pass-filter_raw_data 80mtr_pass-filter_comp_data
which, when plotted:

which looks somewhat like the uncompensated plot but the slow 90 degree or so phase shift error over a 25MHz scan has been magically removed!

Next we try a similar process with reflected measurements. First run and get data for open (calib_refl_open), short (calib_refl_short) and with a 50ohm load (calib_refl_load). This is how the calib_refl_short data looks:

again with the slow phase shift. When we run conpensation on the "short" file:
	$ ./ calib_refl_open calib_refl_short calib_refl_load calib_refl_short calib_refl_short_comp
it turns out like this:

showing a nice even magnitude of 1 and a phase of -1 (180deg) across the spectrum. Finally, plugging in the filter, repeat the above, we get this plot of it's reflection characteristics:

We can convert the polar data (in "freq mag phase sphase" format) to rectangular:
	$ ./ 80mtr_pass-filter_refl_comp_data 80mtr_pass-filter_refl_comp_data_smith
and plugging "80mtr_pass-filter_refl_comp_data_smith" into smith_chart and running
	$ gnuplot smith_chart
it comes out:

and even more fun is to load the polar data into and play with the Powermate knob. With the knob adjusted to the outside of the middle lobe (corresponding to the middle of the 'saddle' in the above plot) it looks like:

Notes: The calibration files and the device data files MUST match in frequency. The calibration files must start at the same frequency and include the entire range in the device data file. That is, if the device data is collected from 5 to 15MHz, the calibration files MUST also start at 5MHz and go up to at least 15MHz, altho it can go higher (as soon as device data runs out compensation processing stops). If the frequencies do not match up you will get an error message.