testing low power VCOs, 50 ohm line on a board

We have designed a low power VCO and to test it we are bringing out the VCO outputs to be connected to 50 ohm line on a board. We cannot match the VCO output nodes to 50 ohms since this would be too much of a load on the VCO and can affect its frequency and amplitude. So we thought we could connect a 2000 ohm resistor in between the VCO output and the 50 ohm line. This would of course attenuate the VCO signal on the line by 2000/50. VCO phase noise is -105dBc at 1MHz.

The concern is whether the measurement on a spectrum analyzer could be impacted in some way because of the 2000 resistor in series (other than the fact that it will be attenuated)?

Any other comments on testing low power VCO's would be greatly appreciated.

Thank you in advance.

thats not really a good way of attenuating the VCO output
and it isnt going to produce a 50 Ohm impedance match.

You should use a proper attenuator pad like a "Pi" or "T" attenuator consisting
of 3 resistors of appropriate values so that any load ie a Spec. An. or next stages
see 50 Ohm at their input. And conversely that the VCO will be driving into a
50 Ohm load (ie. the attenuator)

Look here for a calculator for selecting resistor values.

..... http://www.microwaves101.com/encyclo...attenuator.cfm



cheers
Dave
VK2TDN

Using a pad attenuator is ok, but the formula provided is for the same input/output impedances.
You have to use the attached formula to match the impedance of the VCO output to the input impedance of the Spectrum Analyzer, using two resistors.

yes thats good Vfone for different impedances
I was thinking at the time of a decent commercial VCO with the nominal 50Ohm impedance

If the person has built their own VCO its ooutput impedance could be anything
who knows what ;)

cheers
dave

You really need a decent probe. A passive Tektronix 20:1 probe is one option, although that only gets the impedance up to 1k ohms. To minimise loading you really need an active, low capacitance FET probe.

Keith.

Hello Deve,
attenuators at higher frequencies will be costly affair.
Also it is not easy to built GHz attenuator.
So I think you need to use resistors for higher frequencies.

Thanks to all for their replies and suggestions!