There are a lot of VCO geeks on
this site, and I am not one of them, but I am starting
to experiment with them.
If the only measurement
aim is frequency, an important factor is how the
high series resistance is achieved. Often this entails
parasitic inductance and capacitance that places
a highly reactive load on the output of the VCO.
The issue of "load reactance frequency pulling"
arises: what ruggedness does the VCO have against
a highly reactive load. This is sometimes given
in the data sheet.
By placing an impedance across
the output of the VCO that is within its acceptable
range (eg 50 or 500 ohms purely resistive), any
reactance in the test jig (the frequency counter)
becomes insignificant and has virtually no effect
on the free running frequency.
purity of the DC control voltage, ie freedom from
noise, will somewhat effect the measured frequency,
but when integrated over a long enough period, as
most forms of frequency instrument allow, then the
measured frequency is a reliable average center
A VCO from a good OEM source should
produce very low levels of spurious frequenciy components
at the output, so I see no virtue in having a bandpass
filter there. A good 6dB attenuator serves as a
good swamping matched load, and you can hang virtually
anything down-stream of that without effecting frequency.
At bottom, life is all
Sucking in and blowing out.