Quisk VNA Help (February 2012)
This is the Help file for Quisk VNA, a program that turns the
Quisk2010 and HiQSDR transceiver hardware into a Vector Networ Analyzer
(VNA).
This
Help appears when you press the Help button. Quisk is written by Jim
Ahlstrom, N2ADR,
www.james.ahlstrom.name. Mail to jahlstr at gmail.com. To run the
Quisk VNA program, use "python quisk_vna.py" or set up a
shortcut. This program only works with my hardware that is based
on UDP. It does not work with SoftRock hardware.
You need to update your firmware to Version 1.3 or later. The
new firmware locks the phase of the RF output to the phase of the RF
detector. There is a time delay in the path, but this is removed
by the calibration proceedure. The calibration graphs will show a linear phase change with frequency due to this delay.
There are two ways to use your hardware. You could connect the
RF output through attenuators, through a device under test and then
back to the RF input. This is called transmission mode. It is
used to plot the response of filters and to measure the electrical
length of cables. Or you could connect the RF input and output to
a resistive return loss bridge. This is called reflection mode. It is
used to measure SWR and impedance. You must choose a mode and
calibrate for that mode and hardware before you take any data.
The hardware generates RF output only when one of the "Calibrate"
buttons or the "Run" button is turned on. When calibrating, press
the calibration button and wait for the screen to show a steady
trace. Then press it again to turn the RF off before changing
cables. The calibration routines save data every 15 kHz from zero
to sixty megahertz. You can change the measurement frequency span
at any time without recalibration. Although you can set a
frequency span up to sixty megahertz, my original hardware has a low
pass filter cutoff of 35 megahertz, so the upper frequency range is
smaller and will appear noisy.
Since the transmit and receive frequencies are equal, the data is at
DC, and is averaged and effectively low pass filtered. This
provides immunity from interference when measuring an antenna.
Input Protection
Do not connect the RF output to the RF input without inserting
attenuators. The output will overload the input and result in
clipping in the ADC and possible damage. If your device under
test is an amplifier be especially careful to add additional
attenuation to avoid damage. Add enough attenuation to avoid
clipping, but not so much that you lose dynamic range. The
calibration screens show the ADC level. These attenuators also
help to stabilize the input and output impedance and increase
accuracy. I use the HAT series of attenuators from Mini-Circuits.
Thansmission Mode
You must perform a calibration before you can take data. First
set the mode to "Trans" and leave it there. Connect attenuators
and a cable between the RF input and output. Press "Cal Remove"
to remove any prior calibration data, then press "Cal Short". The
screeen will show the voltage and phase at the input. You can
remove the cable and press "Cal Open" to correct for signal leakage,
but this is optional. With the cable connected, press "Run" and
you should see a flat line at zero dB level and zero phase. Now
insert your test device in series. The test device could be a
filter, an amplifier (be careful) or an additional length of
cable. You can press "Run" again to stop taking data, and the
most recent data will be retained.
Reflection Mode
This mode is used with a resistive return loss bridge.
Connnect the RF
output to the generator terminal, and the RF input to the detector
terminal. Use attenuators as required. You must
perform a calibration before you can take data. First set the
mode to "Refl" and leave it there. Press "Cal Remove" to remove
any
prior calibration data. Connect an open circuit (or nothing) to
the impedance
terminal of the bridge, and then press "Cal Open". Connect a
short circuit to the bridge, and then press "Cal Short". Connect
a 50 ohm termination to the bridge and then press "Cal Load".
If you do not have a set of Open/Short/Load standards, you can just
leave the connector open and use "Cal Open" alone. Using "Cal
Load" will attempt a baseline correction, but this requires all three
calibrations be used. So your calibration options are: Open; both
Open and Short; or all three.
Connect a 50 ohm termination to the bridge, and
press "Run". The graph will show the magnitude and phase of the
reflection coefficient, and the return loss is the drop in magnitude
below zero dB. Ideally your bridge will have a directivity of 30
dB or more. The phase may be noisy if the magnitude is very
small. Now connect an
unknown impedance to the bridge; for example, an antenna. The
graph will plot the return loss, reflection coefficient and SWR.
The status line will display the impedance and the equivalant
capacitance or inductance.
You can attach any impedance, such as an unknown capacitor or inductor,
and read the value directly. The value may seem to vary with
frequeny due to stray inductance, variation of permeability with
frequency, and bridge imperfections; so choose a reference frequency
wisely. Remember that the bridge measures the impedance relative
to fifty ohms, so accuracy suffers if the impedance is outside the
range of 5 to 500 ohms or so.
Fun
In transmission mode, add an extra length of cable and see the phase
change. When the phase change is ninety degrees, that is a
quarter wave. The effect of velocity factor is included, and can
be measured. Use a bare wire (with attenuators) as the test
fixture, and then add a ferrite bead to the wire to measure its
properties. Insert a filter to see its response.
In reflection mode, measure your antenna from zero to sixty
megahertz. If it is a dipole, you will see the drop in SWR at its
third harmonic. Add a cable to the bridge to see its impedance at
a quarter and half wave length. Measure the length of your
transmission line by replacing your antenna with a 100 ohm
resister. The bridge will read 100 ohms at multiples of a half
wavelength. If you short out your antenna at the far end of your
transmission line, the bridge will read zero at half wavelengths, and
infinity at quarter wavelengths.