145 lines
6.9 KiB
HTML
145 lines
6.9 KiB
HTML
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<!DOCTYPE html>
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=utf-8">
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<meta name="description" content="Documentation for Quisk VNA"><meta name="author" content="James C. Ahlstrom"><meta name="keywords" content="quisk, vna, vector network analyzer, ham radio">
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<title>QUISK Help File</title>
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</head><body>
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<h3>
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Quisk VNA Help (December 2016)
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</h3>
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<p>This is the Help file for Quisk VNA, a program that turns the
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Quisk2010 and HiQSDR transceiver and the Hermes-Lite transceiver into a Vector Network Analyzer
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(VNA). This Help appears when you press the Help button. Quisk is written by Jim
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Ahlstrom, N2ADR,
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www.james.ahlstrom.name. Mail to jahlstr at gmail.com. To run the
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Quisk VNA program, use "python quisk_vna.py" or set up a
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shortcut. This program only works with hardware that is based
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on UDP. It does not work with SoftRock hardware.
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</p>
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<p>For HiQSDR you need to update your firmware to Version 1.3 or later.
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For Hermes-Lite update to 32 or later. The
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new firmware locks the phase of the RF output to the phase of the RF
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detector. There is a time delay in the path, but this is removed
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by the calibration procedure. The calibration graphs will show a linear phase change with frequency due to this delay.
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</p>
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<p>There are two ways to use your hardware. You could connect the
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RF output through attenuators, through a device under test and then
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back to the RF input. This is called transmission mode. It is
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used to plot the response of filters and to measure the electrical
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length of cables. Or you could connect the RF input and output through
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attenuators to
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a resistive return loss bridge. This is called reflection mode. It is
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used to measure SWR and impedance. You must choose a mode and
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calibrate for that mode and hardware before you take any data.
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The calibration is saved and will be restored the next time the
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program starts. This is convenient when making repeated measurements,
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but you should calibrate frequently for best accuracy. Hermes-Lite requires a power
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on/off and a calibration before taking data, but the calibration is good until it is powered down.
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</p>
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<p>
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The hardware generates RF output only when one of the "Calibrate"
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buttons or the "Run" button is turned on.
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The calibration routines save data every 15 kHz from zero
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to sixty megahertz (thirty for Hermes-Lite). You can change the measurement frequency span
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at any time without recalibration. Although you can set a
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frequency span up to sixty (thirty) megahertz, my original hardware has a low
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pass filter cutoff of 35 megahertz, so the upper frequency range is
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smaller and will appear noisy.
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Since the transmit and receive frequencies are equal, the data is at
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DC, and is averaged and effectively low pass filtered. This
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provides immunity from interference when measuring an antenna.
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<h3 id="g0.0.1">Input Protection </h3>
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Do not connect the RF output to the RF input without inserting
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attenuators. The output will overload the input and result in
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clipping in the ADC and possible damage. If your device under
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test is an amplifier be especially careful to add additional
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attenuation to avoid damage. Add enough attenuation to avoid
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clipping, but not so much that you lose dynamic range. The
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calibration screens show the ADC level. These attenuators also
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help to stabilize the input and output impedance and increase
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accuracy. I use the HAT series of attenuators from Mini-Circuits.
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<h3 id="g0.0.2">Transmission Mode
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</h3>
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You must perform a calibration before you can take data. First
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set the mode to "Transmission" and leave it there. Connect attenuators
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and a cable between the RF input and output. Press the "Calibrate.." button.
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With the cable connected press "Short". The "Short" calibration is
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required for transmission mode. For the optional "Open" calibration, disconnect
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the cable and press "Open". Then press "Calibrate" to save the calibration.
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Connect the cable and press "Run" and
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you should see a flat line at zero dB level and zero phase. Now
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insert your test device in series. The test device could be a
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filter, an amplifier (be careful) or an additional length of
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cable. Press "Run" to take data.
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<h3 id="g0.0.3">Reflection Mode
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</h3>
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<p>
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This mode is used with a resistive return loss bridge.
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Connect the RF
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output to the generator terminal, and the RF input to the detector
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terminal. Use attenuators on both. You must
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perform a calibration before you can take data.
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First set the
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mode to "Reflection" and leave it there. Press the "Calibrate.." button.
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Connect an open circuit (or nothing) to
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the impedance
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terminal of the bridge, and then press "Open". Connect a
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short circuit to the bridge, and then press "Short". Connect
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a 50 ohm termination to the bridge and then press "Load".
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Then press "Calibrate" to save the calibration.
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If you do not have a set of Open/Short/Load standards, you can just
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use "Open" alone, but it is highly
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recommended to use Open, Short and Load.
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</p>
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<p>
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Now connect a 50 ohm termination to the bridge, and
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press "Run". The graph will show the magnitude and phase of the
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reflection coefficient, and the return loss is the drop in magnitude
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below zero dB. Ideally your bridge will have a directivity of 30
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dB or more. The phase may be noisy if the magnitude is very
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small. Now connect an
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unknown impedance to the bridge; for example, an antenna. The
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graph will plot the return loss, reflection coefficient and SWR.
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The status line will display the frequency, the reflection coefficient and SWR, the impedance, the equivalent
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capacitance or inductance for that impedance, and the values for the parallel equivalent circuit.
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</p>
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<p>
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You can attach any impedance, such as an unknown capacitor or inductor,
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and read the value directly. The value may seem to vary with
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frequency due to stray inductance, variation of permeability with
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frequency, and bridge imperfections; so choose a reference frequency
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wisely. Remember that the bridge measures the impedance relative
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to fifty ohms, so accuracy suffers if the impedance is outside the
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range of 5 to 500 ohms or so.
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</p>
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<h3 id="g0.0.4">Fun</h3>
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<p>
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In transmission mode, add an extra length of cable and see the phase
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change. When the phase change is ninety degrees, that is a
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quarter wave. The effect of velocity factor is included, and can
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be measured. Use a bare wire (with attenuators) as the test
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fixture, and then add a ferrite bead to the wire to measure its
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properties. Insert a filter to see its response.
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</p>
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<p>
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In reflection mode, measure your antenna from zero to sixty (thirty)
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megahertz. If it is a dipole, you will see the drop in SWR at its fundamental and
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third harmonic. Add a cable to the bridge to see its impedance at
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a quarter and half wave length. Measure the length of your
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transmission line by replacing your antenna with a 100 ohm
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resister. The bridge will read 100 ohms at multiples of a half
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wavelength. If you short out your antenna at the far end of your
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transmission line, the bridge will read zero at half wavelengths, and
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infinity at quarter wavelengths.
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</p>
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</body>
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</html>
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