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

### Quisk VNA Help (December 2016)

This is the Help file for Quisk VNA, a program that turns the Quisk2010 and HiQSDR transceiver and the Hermes-Lite transceiver into a Vector Network 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 hardware that is based on UDP. It does not work with SoftRock hardware.

For HiQSDR you need to update your firmware to Version 1.3 or later. For Hermes-Lite update to 32 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 procedure. 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 through attenuators 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 calibration is saved and will be restored the next time the program starts. This is convenient when making repeated measurements, but you should calibrate frequently for best accuracy. Hermes-Lite requires a power on/off and a calibration before taking data, but the calibration is good until it is powered down.

The hardware generates RF output only when one of the "Calibrate" buttons or the "Run" button is turned on. The calibration routines save data every 15 kHz from zero to sixty megahertz (thirty for Hermes-Lite). You can change the measurement frequency span at any time without recalibration. Although you can set a frequency span up to sixty (thirty) 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.

### Transmission Mode

You must perform a calibration before you can take data. First set the mode to "Transmission" and leave it there. Connect attenuators and a cable between the RF input and output. Press the "Calibrate.." button. With the cable connected press "Short". The "Short" calibration is required for transmission mode. For the optional "Open" calibration, disconnect the cable and press "Open". Then press "Calibrate" to save the calibration. Connect the cable and 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. Press "Run" to take data.

### Reflection Mode

This mode is used with a resistive return loss bridge. Connect the RF output to the generator terminal, and the RF input to the detector terminal. Use attenuators on both. You must perform a calibration before you can take data. First set the mode to "Reflection" and leave it there. Press the "Calibrate.." button. Connect an open circuit (or nothing) to the impedance terminal of the bridge, and then press "Open". Connect a short circuit to the bridge, and then press "Short". Connect a 50 ohm termination to the bridge and then press "Load". Then press "Calibrate" to save the calibration. If you do not have a set of Open/Short/Load standards, you can just use "Open" alone, but it is highly recommended to use Open, Short and Load.

Now 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 frequency, the reflection coefficient and SWR, the impedance, the equivalent capacitance or inductance for that impedance, and the values for the parallel equivalent circuit.

You can attach any impedance, such as an unknown capacitor or inductor, and read the value directly. The value may seem to vary with frequency 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 (thirty) megahertz. If it is a dipole, you will see the drop in SWR at its fundamental and 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.