VCO testing and impedance - RF Cafe Forums
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Post subject: VCO testing and impedance
Posted: Thu Jan 08, 2009 10:43 pm
Joined: Thu Jan 08, 2009
(1MOhm VS. 50Ohm in testing)
probably comes around often, I didn't see too many
full explanations that might help some of the other
readers and listers. Most know that at low frequencies
the 1MOhm works well at not disturbing the circuit).
I suppose there is difference too if one is 'probing
a functional line' vs. taking the direct output
of the VCO.
That said, thequestion is what
are the important criteria to consider between a
1MOhm load vs 50Ohm load) when making frequency
measurements of a VCO (say using a frequency counter)
in a manufacturing test setup of volume testing?
(say 2 MHz VCO)
Most equipment vendors lean
towards a '1 MHz high impedance' direct connect
via cable, some others say since the cable is 50Ohm
and the equipment is cable of providing either 50Ohm
or 1MHOhm load impedance, use 50Ohms throughout
to keep entire path consistent.
A good 'FET
probe by Agilent' actually is 500Ohm load but since
it is active it provides a 50Ohm connection to whatever
...All this said, it seems
that in a manufacturing test situation, using an
active filter of some sort (provides high Z to VCO
and 50OHm to counter) would be the most accurate,
what are some others point of view on this?
Posted: Fri Jan 09, 2009 5:29 am
Joined: Fri Feb 17, 2006
Location: London UK
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.
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