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Adding a compensation function for use with RF active probes

By VK3NH •  2017-02-07 08:05:13 •  267 clicks

You have a range of near field probes which are good for emissions analysis and also avoid overload issues but they cannot really identify the location of circuit faults.

Active probes have a high impedance input allowing signals to be analysed with minimal effect on the operation of the circuit being observed. Typically they may operate to 500MHz or even into the GHz range if they use differential input design and typically have an input impedance in the 100 Kilohm to 1 Megohm range and an output into 50 ohms. The maximum output levels of these probes are normally less than 10dBm and many probes have optional input x10 and x100 dividers which further increase the input impedance and attenuate the signals if necessary.

They are extremely useful when tracing equipment faults because you can observe the stage to stage progression of signals in a device and see when an amplifier is not working or find a break in the RF signal path due to a dry joint.

The portability of the KC901V makes it attractive to use with such probes in the field.

The active probes gains however vary with frequency and it is this response that I wish to compensate for to give more accurate results.

For some applications you could use the probe with the KC901 in the S21 mode to sweep circuits and indeed compensate for the frequency response but the real levels would not be correct because you are following the generators varying output level. Also, in practice the input being swept into an active device like a small signal amplifier may then be driven into overload at some frequencies giving you a distorted view of the devices real performance. In any event the S21 mode only gives you amplitude levels relative to the generators output at any frequency.

The most useful mode to use with the probe is Spectrum analysis. Knowing that the accuracy of this function is quite good, it would be very useful if the KC901V had the option of activating the generator in the Spectrum analysis mode in order to implement a user calibration function for the probe like the user calibration function of the S21 mode.

The above suggestion is based on the fact that you could do the first sweep with a Port 1 to Port 2 connection with a normal 50 ohm cable over the working frequency range of the probe and then a second sweep with a special 50 ohm termination on Port 1 which allows the probe to monitor the voltage across the termination and store the difference as correction information to adjust the observed result when making measurements with the probe.

Such an arrangement should work well to at least 1GHz.

regards Noel

operation  function  identify  operate  active 

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