Some recent articles discussed some effects that in part are a result of Zo having a complex value (ie a non-zero imaginary part). Continue reading On working with complex Zo
Category: Measurement
On the concept of that P=Pfwd-Prev
The article On negative VSWR – Return Loss implications raised the question of the validity of the concept of that P=Pfwd-Prev.
The Superposition Theorem is an important tool in linear circuit analysis, and is used to find the combined response of independent sources. Superposition applies to voltages and currents, but not power. Continue reading On the concept of that P=Pfwd-Prev
Measuring balun common mode impedance – #3
A correspondent having read my series Measuring balun common mode impedance – #1 related difficulties with his Rigexpert AA-230Zoom.
The articles showed some techniques for measuring common mode impedance of a current balun.
The following examples are of a test choke wound on a BN43-202 binocular core, and the results are quite similar to what might be expected of a broadband HF current balun. The measurements were made with a Rigexpert AA-600.
Above, the measurement result using RigExpert’s newest software Antscope2. Continue reading Measuring balun common mode impedance – #3
Small common mode choke for analyser antenna measurements using 2843000202 (BN43-202)
The project is design, implementation and test of a small common mode choke for use with an analyser for antenna measurements.
The choke must have medium to high Zcm from 1 to 30MHz. It is intended to be used with analysers supporting SOL calibration, so effectively any impedance transformation within the fixture is compensated and the reference plane is the load side terminals of the device.
The candidate core is a low cost #43 binocular ferrite core that is fairly easy to obtain.
Above is a first pass check of the likely Zcm at 1.8MHz using a Fair-rite 2843000202 (BN43-202) binocular core. These chokes have relatively low self resonance frequency so a value for Cs is supplied that delivers self resonance at around 5MHz. Zcm at 1.8MHz needs 8-9t, 8.5t will be used (ie the twisted pair enters one end of the binocular and leaves the other end for convenient layout). (8.5t is not strictly correct, but it is a close approximation in this case.)
Continue reading Small common mode choke for analyser antenna measurements using 2843000202 (BN43-202)
Equivalent noise bandwidth – IC-7300 CW Rx Filter2 – (500Hz sharp)
For a lot of experiments, knowledge of the Equivalent Noise Bandwidth (ENB) of a receiver is necessary. The ENB is the bandwidth of an ideal rectangular filter with the same gain as some reference frequency.
Though filters are often specified in terms of bandwidth at x dB down, that metric is of relatively little value, the x is often 6dB but not always, the filters depart significantly from ideal or even common response.
In brief, a white noise source is connected to the receiver input, Filter2 (nominal 500Hz bandwidth sharp response) selected and set to standard PBT, and the audio output captured on a PC based audio spectrum analyser, Spectrogram 16 in this case.
Spectrogram is set to integrate over 30s to average the variations due to the noise excitation. The resulting graph and text spectrum log are saved.
The method is explained in detail at Measure IF Bandwidth.
Above is the spectrum plots, as receivers go this is relatively flat.
Continue reading Equivalent noise bandwidth – IC-7300 CW Rx Filter2 – (500Hz sharp)
Equivalent noise bandwidth – IC-7300 SSB Rx Filter2 – (2400Hz sharp)
For a lot of experiments, knowledge of the Equivalent Noise Bandwidth (ENB) of a receiver is necessary. The ENB is the bandwidth of an ideal rectangular filter with the same gain as some reference frequency, 1kHz is usually specified for SSB telephony receiver sensitivity measurement.
Though filters are often specified in terms of bandwidth at x dB down, that metric is of relatively little value, the x is often 6dB but not always, the filters depart significantly from ideal or even common response.
In brief, a white noise source is connected to the receiver input, Filter2 (nominal 2400Hz bandwidth sharp response) selected and set to standard PBT, and the audio output captured on a PC based audio spectrum analyser, Spectrogram 16 in this case.
Spectrogram is set to integrate over 30s to average the variations due to the noise excitation. The resulting graph and text spectrum log are saved.
The method is explained in detail at Measure IF Bandwidth.
Above is the spectrum plots, as receivers go this is relatively flat, lacking the usual tapering off above 1kHz (a technique to cheat on sensitivity specs).
Continue reading Equivalent noise bandwidth – IC-7300 SSB Rx Filter2 – (2400Hz sharp)
DSZH WK-6889 – temperature sensor
The specifications of the DSZH WK-6889 – temperature sensor do not give accuracy or type of the temperature sensors used.
They are most likely to be one of: RTD, thermocouple, or NTC thermistor. Continue reading DSZH WK-6889 – temperature sensor
Post implementation review R134a replaced with HyChill Minus 30
I am considering replacing the R134a refrigerant in my car aircon system with a hydrocarbon refrigerant. The candidate is Hychill Minus 30 (HC-30).
Comparison of R134a and HyChill Minus 30 gave a limited comparison of R134a and HC-30 from the point of view of pressure temperature behavior as it impact practical implementation and measurement.
This article is a post implementation report, and baseline for future system evaluation.
The vehicle uses a TXV and variable displacement compressor, so low side pressure should be controlled by the variable displacement compressor, and evaporator superheat controlled by the TXV.
The system was evacuated and charged with 240g of HC-30, being 30% of the R134a charge as advised by Hychill, and leak tested.
After settling, on the driveway with no supplemental air flow, fan on full, OAT 22°, on a digital manifold set for R134a, the low side pressure was 255kPa, evaporator outlet 12.9°, displayed superheat 7.2°. At this pressure, R134a calibration reads 2.0° high, so evaporator superheat is corrected to 5.2° which is quite within expectation for a TXV controlled expansion. Continue reading Post implementation review R134a replaced with HyChill Minus 30
Geometry factors for some common Fair-rite binocular ferrite cores
Designing with some common Fair-rite binocular ferrite cores can be frustrating because different parameters are published for different material types, and some are controlled for different parameters.
An approach is to derive the key geometry parameter from the published impedance curves and published material complex permeability curves.
For example, the above curves for a 2843002402 (also common known as a BN43-2402) were digitised and iteratively Calculate ferrite cored inductor (from Al) used for find the value of Al that gives the observed value for Z at 10MHz on the chart above. Continue reading Geometry factors for some common Fair-rite binocular ferrite cores
Measuring trap resonant frequency with an antenna analyser – measurement of a real trap
Finding the resonant frequency of a resonant circuit such as an antenna trap is usually done by coupling a source and power sensor very loosely to the circuit.
Above is Fig 1, a diagram from the Rigexpert AA35Zoom manual showing at the left a link (to be connected the analyser) and the trap (here made with coaxial cable).
Above is the trap measured, the wires were connected as a bootstrap trap as in Fig 1. The coupling link is a 60mm diameter coil of 2mm copper directly mounted on the AA-600 connector, and it is located coaxially with the trap and about 10mm from the end of the trap.
Above is the ReturnLoss plot of the trap very loosely coupled to the AA-600.
Of course this technique will not work on a trap that is substantially enclosed in a shield that prevents magnetic coupling. Note also that many traps used in ham antennas are simply a coil wound on an insulating rod and each end connected to the adjacent tubing, possibly with an overall aluminium tube that may or may not be bonded to the element tube at one end. The latter really become part of the element and measurement separate to the element is not simply translated to in-situ.
Equivalent circuit / simulation
The inductor has previously been carefully measured to be 3.4µH. We can calibrate a model of the coupled coils to the observed resonant frequency and ReturnLoss.
Above, the equivalent circuit. We can calculate the flux coupling factor k from the model, it is 2.3% so this is very loosely coupled to avoid pulling the resonant frequency high.
Above is the simulated ReturnLoss response over the same frequency range as measured.
Conclusions
It is practical to measure the resonant frequency of a trap by loosely inductively coupling an antenna analyser, depending on the structure of the trap and the capability of the analyser.
Practical measurements can be explained with a theoretical model of the measurement setup.