# What is and how to measure the tuning port capacitance of a VCO

I was asked to check all the specifications of this Voltage Controlled Oscillator. I might have an idea of how to measure all the parameters except for one, the Tunning Port Capacitance. I think my problem stems from the fact that I don't really know what this parameter means for a VCO. At first, I thought it was kind of an input (or output) capacity, but this seems wrong.

What does this parameter mean for a VCO and how to measure it? simulate this circuit – Schematic created using CircuitLab

There are many ways to measure Port Capacitance (70pF) which may change with DC bias voltage (0.5 to 25Vdc). With this method above you can expect the above to resonate at 5MHz when the Capacitance is connected in close proximity. You can use any input square wave that modulates the collector voltage enough to see edge ringing and compute C from the LC resonant frequency.

Just in case, you can change the Vcc bias to see if the capacitance changes with Port DC bias. C1 can be a fixed capitor to test your circuit and replaced with the VCO test for port capacitance.

CLK1 might also be used for testing the BW of 10MHz of the output modulation spectrum with a 50 Ohm instead of L1.

• but what is exactly the Port Capacitance of a VCO? Can't I use some of the standard impedance measurement equipments like the LCR meter, VNA or RF Vector Impedance Meter? would the DC bias be a problem? – diegobatt Aug 2 '18 at 1:48
• Read "Input Capacitance" for modulation Port. Yes you if you can vary DC bias to confirm minimal effect. It is designed to yield > 10MHz with a 50 Ohm source, yet modulation BW is limited by internal circuit. – Tony Stewart Sunnyskyguy EE75 Aug 2 '18 at 1:49
• following these definition: "VCO INPUT CAPACITANCE: The total equivalent capacitance seen at the tuning port of the VCO. This capacitance is a function of the DC component of the test signal applied to the tuning port, and depends mainly on the varactor diode capacitance vs. voltage characteristic. " Can I just connect an RF vector impedance meter like the HP4815A to measure it directly? – diegobatt Aug 2 '18 at 1:56
• If you isolate the VNA from DC , sure – Tony Stewart Sunnyskyguy EE75 Aug 2 '18 at 2:00