Saturday, December 5, 2009

N2PK VNA - LLC calibration standard

The RF-IV sensor is very useful for performing measurments of high-Q devices. To improve the accuracy of such measurments, an extra calibration step is required - LLC or Low-Loss Capacitor calibration. It "provides reference of impedance with an accurate 90 degree phase angle in place of the 50 Ohm LOAD standard". A good read about the RF-IV method is this Agilent document. There is also a very informative application note from Agilent about High-Q measurements. MyVNA software allows for this calibration step since ver. 0.47. LLC calibration requires a special calibration standard - capacitor with very low ESR and very high Q (>10 000). Paul, N2PK recommends the use of a few loads which are "impedance of -j50 at the center of frequency range and maybe a 4:1 freq range - i.e. from Fc/2 to 2*Fc" or 3, 10 and 30 MHz for example. "Another option is to use a cap that has about the same reactance as an inductor whose Q is to be measured".
MyVNA software has a built-in model for the ATC 100B series (Porcelain Superchip Multi-layer) capacitors by American Technical Ceramics . These capacitors are ultra-stable, low ESR, High Q and low noise, specifically designed for microwave use. The accuaracy of the capacitance value is not extremely critical since the software allows you to enter the frequency at which the cap is expected to have -j50 impedance and correction can be made there. I followed Paul's example and made 3 calibration standards centered at frequencies close to 3, 10 and 30 Mhz using standard values available by ATC. The construction technique for the standards is exactly the same as the one I used to prepare my other BNC calibration standards (described here).

This picture shows the "30 Mhz LLC standard", using two 47 pF (tolerance 2%) ATC 100B capacitors in parallel for a total value of 94 pF (the impedance is -j50 at proximately 33.85 MHz).

1 comment:

Paul - K0EET said...

Thanks for the e-bay tip on the Male BNC / PCB mount connectors. I just picked up a set