|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 10:27 pm
(1MOhm VS. 50Ohm in testing)
This topic 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 equipment...
...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 12:07 pm
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.
Obviously the 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 frequency.
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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