Calibration loads?

[kdavis]

Bob,

It is interesting to me that you supply a 200 ohm calibration load with the 4170 analyzer.
Would you share the reason you chose this value rather than 50 ohms?
(I know it makes no difference as it is calibrated out)
In a few other posts you discuss using 50 ohm loads and larger loads 200 to 2K ohms.
Would you comment on applications where using different load values other than 200 ohms would be beneficial?

Thanks
kdavis

[Bob]

The AIM is able to make accurate measurements of very large impedances (several K ohms). When measuring very large values at modest frequencies (HF band), it may be more accurate to cal with a larger resistor so the difference between the cal resistor and the measured impedance is less. Accuracy still depends on how closely the cal resistor is to the stated value. If the cal resistor has too much stray capacitance or inductance, the larger value may not give results that are as accurate as using a good quality 50 ohm resistor. Try it both ways to see which is better for a particular application. For high frequencies (greater than 100 MHz) a smaller 50 ohm resistor will probably be better.  Note that when doing the open circuit calibration step, it is important to attach the open circuit load. Don't leave the RF connector open.  The open circuit load has several picofarads of capacitance that affect the the final result.  Leaving out this small capacitance upsets the calculations so the final answer is less accurate.  You can see this effect by calibrating with and without the open circuit load and then measuring a known good quality resistor.  The equivalent capacitance will be different.

Note that the 50 ohm cal resistor should be made using two 100 ohm resistors in parallel. This combination tends to cancel the inductance and capacitance over a wide frequency range and it gives much better results than a single 50 ohm resistor. Ideally, R/L = R*C. With realistic values for L and C, setting R=100 gives good results over the AIM freq range.

--73/ Bob

[kdavis]

When evaluating the resistance of common mode chokes, is there any value in using a large calibration load such as 1000 ohms or 2000 ohms to calibrate the 4170?

[Bob]

Calibrating with a larger resistor may be better. However, the most critical parameter for testing baluns is the stray capacitance. Even a few tenths of a pf can affect the max impedance and resonant frequency. If the balun is going to be mounted in a metal case, do the final testing with it in the case. Nearby metal will add stray capacitance.

-73/Bob