# Measurement and Technical Data

**Practical Measurements**

First of all a disconnected DUT has to be used and no loaded capacitors incl. electrolyte type please.

Measurement of capacitance and inductance is done by selecting the right mode, calibrate with leads open (C – Mode) or shorted (L – Mode) and attaching the DUT to the clamps.

The LCQ-Meter is capable to measure capacitance below 1 pF to larger than 10 nF. Very large capacitors prevent the oscillator from swinging as the internal feedback capacitor in the oscillator is too small for that. Inductance can be measured below 1 uH to larger than 100 mH.

Measurements on parallel tuned circuits start with selecting the right operating mode and directly attaching the DUT to the clamps. The display will alternate between showing capacitance, inductance and projected resonance frequency and quality Q. The last one has to be switched on with the ‘Toggle Q’ button.

The measurement range is the same as in the other modes but care has to be taken with very small capacitors in the tuned circuit. Below 10 pF the accuracy will decrease due to not perfectly compensated parasitic capacitance. Also the frequency will become relatively high. The oscillator will easily cover 50 MHz but the microcontroller prescaler will exit somewhere between 40 MHz to 50 MHz. Also the parasitic capacitance overall will prevent higher frequencies.

The range for quality Q measurement works well towards Q of 300 if the capacitance CX is not too low and of a low loss type. If possible it should be above 100 pF to reduce the influence of the parasitic capacitance which attenuates the tuned circuit to some extent. With CX at 1000 pF Q measurements can even go higher to above 500 if you have such high quality components at hand.

Frequency measurement is straight forward. After selecting the F-Mode an external supplied signal’s frequency will be displayed. The measurement range depends on the prescaler and ends somewhere between 40 MHz to 50 MHz. The signal has to have a peak voltage of about 1 Volt (depending on R8). The input resistance is around 150 Ohms. Also please note that the gate time for the measurement is quiet small, so accuracy is limit. It is just a basic additional feature which will solve most quick measurement needs.

**Accuracy of The LCQ-Meter**

Even with a low cost and small device design there has to be some measurement accuracy which fits to amateur needs. With the methodologies and implementation described nobody could expect any 0.1 % accuracy.

But in practice the goal was to achieve 5 % accuracy for capacitance and inductance measurement over a wide range and 10 % - 20 % accuracy for quality Q measurement.

Picture 21: Inductors and capacitors to test the LCQ-Meter

Some available components shown in picture 21 were used for test measurements. The components were reference measured with a Vector Network Analyzer with and without RF-IV methodology.

Q for inductors and/or parallel tuned circuits was measured as well with the VNWA on different methodologies including the ‘Notch Method’ with serial and parallel resonant traps as described in [4]. The last method showed up to be the most accurate for quality Q over a wide range of measurements.

However, measuring high Q in the range of Q > 300 was vulnerable towards parasitic resistance and capacity. Picture 22 shows 2 different front ends for the Notch measurement. The lower, little PCB is from a HF test-set for the VNWA. Even with careful calibration the highest measurable Q for C1 & L5 combination (1nF Mica with 12.6 uH toroid) was about Q = 280 using that board, whilst the LCQ-Meter showed a Q better than 300 all the time.

Doing Q calculations by hand on the scoped envelope plot from the LCQ-Meter it was clear that there is no error within the LCQ-Meter and Q has to be higher than 280. Finally parasitic resistance of the connectors on the test board showed up to be the bad guy. Milliohms matter here! With careful calibration and direct soldering of the DUTs to the SMA connectors (picture 22 upper part) Q increased to 306 both with the parallel and serial trap notch method.

Picture 22: Measuring parallel notch with VNWA

Q-measurement becomes even more difficult if very low capacities are used as picture 23 will prove.

Picture 23 shows all measurement results for L-mode, C-mode and L||C-mode. It includes also Q references from both notch measurement types.

The notch measurement shows differences between both types up to 60 % if only 10 pF is used for the capacitor in the tuned circuit! This shows that measuring Q of parallel tuned circuits with very low capacity could become a challenge in general.

Picture 23: Measurement results

But let’s start with capacity and inductance measurement. For singular capacity and inductance measurement the LCQ-Meter does show good results within expected accuracy. For very large inductors like the 100 mH one it was not possible to create any reference value with the available VNWA.

The parallel tuned circuit measurements of capacity parallel to inductance show good results as well. The PCB version 2.3 with much lower parasitic capacitance in the oscillator section pays off here. Even capacitance down to 10 pF can be measured with reasonable accuracy. A little bit more fine tuning on the parasitic capacitance approximation function would probably turn all yellow cells to green, going below 5 % measurement error. But in HAM Radio practice this small differences in capacity and inductance will very seldom matter.

On Q measurement, for practical combinations of L and C, the measurement results are better than expected if Q is high enough. If Q is much below 100 the decay of the envelope is too fast to get measured correctly.

On the other side this means Q measurement will become more accurate on higher Qs. This is a big advantage of the LCQ-Meter compared to other measurement types as discussed earlier.

The original goal was to measure Q up to 10 MHz. Low Q and very low capacitance are challenging.

The question is if those limitations are of any practical relevance for HAM Radio? Who builds a parallel tuned circuit with a 100 uH inductor and a 10 pF capacitor? Well, you never know.

The net is if there is a choice for the tuned circuit capacitance (for example if measuring inductors only with a high quality capacitor in parallel) the tuned circuit capacity should be taken as large as possible.

Another test not shown here is to introduce extra loss by resistors. By adding a 1 Ohm or 2 Ohm resistors is can be shown that high quality Q circuits loose quality within expected results.

**Technical Data**

Measurments done on and with the LCQ-Meter finally let do the technical data shown in picture 24.

Picture 24: Technical Data

**Summary**

The LCQ-Meter proves that there is definitely much more functionality possible than currently available in the low cost small device area of amateur measurement instruments. It also demonstrates how powerful current combinations of microcontroller, software and hardware could become even for complex measurement tasks.

All of this is possible in a very affordable way. All used components are relatively low price and have good availability. If someone does not need multiple power supply options or all functionalities a lot of components can be saved.

All files including EAGLE PCB files, software etc. are available on an as is base on [5] for non-commercial use.

Special thanks to DJ6EV and DD1KT for testing some of the prototypes and giving a lot of good hints and recommendations for improvement.

I hope you enjoyed this article and got some interesting insides and ideas.

Michael Knitter, DG5MK

References

[1] AADE L/C Meter, http://www.aade.com/lcmeter.htm

[2] HP/Agilent Impedance Measurement Handbook,

http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdf

[3] Broadband amplitude-stabilized oscillator by JULIUS FOIT, Proceedings of the 5th WSEAS Int. Conf. on Microelectronics, Nanoelectronics, Optoelectronics, Prague, Czech Republic, March 12-14, 2006 (pp1-5)

[4] Experimental Methods in RF Design; Hayward, Campbell, Larkin; 1^{st} edition published by ARRL; chapter 7.9

[5] Author’s web page, http://www.dg5mk.de