QUISK Help File
Author: James C. Ahlstrom
Description: Documentation for Quisk VNA
Keywords: quisk, vna, vector network analyzer, ham radio

### 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.