A common VCXO specification is the Vc input impedance. A typical value might be 100KOhm or 10MegOhm. What does this actually mean?
Although there may be exceptions, it generally does not mean there is a 100KOhm resistance to ground as the spec might imply. The typical Vc input circuit looks like this:
Resistor R1 couples Vc into the oscillator to control the bias on the Varactor tuning diode D1. If the VCXO is fully integrated there will also be some type of protection diode, D2, on the Vc pin. The datasheet for this oscillator input may say 100K referring to R1, but the actual impedance is much higher at low frequency. Some VCXO designs will also include a high input impedance buffer between the Vc pin and R1 and quote an impedance spec in the Meg Ohm range. In this instance a value of 5 or 10Meg Ohm may mean “a really big number.”
Using an impedance to characterize the Vc input dates back to an era when loop filters often used Op Amps to drive the Vc pin. Today charge pump type phase detector outputs are most common. Regardless of the type of detector output used, a more detailed input model will always give the most accurate result when modeling the loop response.
With the advent of charge pump detectors there is an additional concern over the DC leakage current of the Vc input. This current can be in either direction and has been known to vary from 10nA to 10uA. While these current levels are small compared to the typical charge pump output, the phase alignment of the input signals to the phase detector will be altered to compensate for the leakage. This phase skew may result in increased reference spurs in the PLL output or even trigger an out of lock condition.
Back to Vc impedance specification, a high value such as 5Meg Ohms may indicate the addition of a Vc input buffer or it may represent a typical leakage current at mid Vc. If leakage is a concern the device in question should be fully characterized.