Hi all,
I received multiple questions about the measuring results for the load of the SOL calibration kit, after calibration. Some people mention that it does not show exactly 50 Ohms. Let me try to explain that this is not an error. Instead it is a common misunderstanding of calibration.
SOL calibration means to cancel out any systematic errors in measurement to a well defined reference plane. Let's assume the end of the BNC-connector of the FA-VA 5 is our wanted reference plane.
Whatever device under test (DUT) is attached to the end of the BNC connector needs to be measured correctly. Unfortunately we don't know exactly what is before the BNC-connector, inside the FA-VA 5. Internal cables and paths may be longer or shorter, parasitic capacitance and inductance does exist, the components of the measurement bridge have tolerances and many more unknowns. All theses imperfections will make it impossible to exactly calculate the right measurement results at the end of the BNC-connector.
There are 2 approaches to deal with these imperfections. One approach is to use components with lower tolerances, compensate parasitic effects. However, at higher frequencies and for a large frequency range it is very hard to do this.
The second approach is to simply accept imperfections and calculate the right measurement results with the help of calibration. The basic idea is to replace all imperfect components before the reference plane (before the BNC-connector) with two networks in series. One network consists of all discussed components, but now it is assumed that they are ideal. All the imperfections are summarized in the second network. It is an error network. All is unknown as we don't know the value of the errors. It is a set of equations with unknowns. If we are able to figure out the unknowns, we can perfectly calculate our measurement result at the reference plane.
Here is the point where SOL-calibration comes in. SOL-calibration means to measure 3 well known standards to solve that error network. It is common to use short, open and load (50 Ohms), but you can use any 3 different well known standards as long as the real values of the standards are known. Devices like the FA-VA 5 allow to enter the real values for the standards, along different models. For low frequencies a short is 0 Ohm, an open is infinite resistance, the load is 50 Ohms. But for higher frequencies again parasitic capacitance, inductance etc. comes in. This is why the professional BNC-SOL-set for the FA-VA 5 has a peace of paper which tells about the real values of the standards along frequency. The PC-software VNWA allows to even enter more sophisticated models to characterize the standards. Always with the same approach: Use the best match for the real values for the standards at a frequency, measure the standards, use the values to calculate the error network and every next measurement at the reference plane (where the standards were attached) will show the right results for the DUT.
Now assume you use ideal 40 Ohm instead 50 Ohm for the load and enter the 40 Ohm as the value for the load into the calculation. The calibration will still work perfectly. After SOL calibration any DUT will be measured correctly as the calculation knows about 40 Ohm. If you measure again the same 40 Ohm load used for calibration, what will it show? It will show 40 Ohm, because the device is calibrated and the real value of the load does show up. Many people expect 50 Ohm as they assume a load is an ideal 50 Ohm standard, but it is not. If you did follow the explanations above it is clear that it is not important whether the load device has real 50 Ohm. More important is that real values are reproduceable and / or are measured for that individual load. This is why the professional BNC-SOL-set has individual measured loads. But they are not perfect ideal 50 Ohm (which is close to impossible).
The net is. If the real values for the SOL-set are entered into the device and if calibration is done correctly the device measures the real values for any DUT. For the load from the professional BNC-SOL-set it is not 50 Ohm, nothing wrong. If you would attach a perfect 50 Ohm load after calibration to the device it would show 50 Ohm. You can test this with a small 50 Ohm SMD resistor soldered to a BNC connector, which comes a little bit more to an ideal 50 Ohm load if done thoroughly. Anyway, as this is not reproducible it is not a solution for many users.
One more comment about the reference plane as explained above. It was assumed that this is the end of the BNC connector. But there is no need for this limitation. If we attach a 10 Meter cable to the BNC connector and do the SOL-calibration at the end of the cable, what will happen? The cable will become part of the error network, it is 'calibrated out'. Whatever device will be attached to the new reference plane (end of the cable) will be measured correctly. This is a very powerful property of calibration. We can move the reference plane, include other cables and devices as needed. For antennas we can calibrate at the end of the feed line once and measure the real SWR at the feedpoint of the antenna.
Hope that helps to understand why the SOL-kit load does not show 50 Ohm.
Regards
Michael
Dieter VK3FFB
