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Output buffers for VCOs

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iVenky

iVenky

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Hi! I am designing an LC VCO and would want to design output buffers for driving the output for testing. What's the general architecture for the output buffers? Can it be differential? I have seen some architectures where they use class A amplifier but I would like to know what is used generally.

Thanks!
 

iVenky said:
Hi! I am designing an LC VCO and would want to design output buffers for driving the output for testing. What's the general architecture for the output buffers? Can it be differential? I have seen some architectures where they use class A amplifier but I would like to know what is used generally.

Thanks!
Click to expand...

Depends of frequency and power output needed. In RF VCO, a class A type buffer is preferred as it does not load the VCO output, and it also generates the lowest harmonic content.
 

iVenky said:
Hi,

It's an RF VCO. My frequency range is from 3 GHz to 6 GHz
Click to expand...

Then I would use a wideband buffer amplifier. There are many on the market. MiniCircuits offer connectorized amplifier modules, RF MIcro Devices offers MMICs. Now it depends on how much power you need at the output. Choose a suitable gain and output power. If you designed your VCO, you should be able to build your buffer amplifier with a MMIC.
 

If the VCO is an IC internal VCO it may be incapable
of driving a 50-ohm load regardless of whether it's a
MMIC or a piece of test equipment. Presumably any
off-chip port will have to be a sane impedance. So
is the need really an impedance transformation like
500 ohms on-chip pullups, to 50 ohms PCB trace?
If so and at these frequencies, maybe a series R
(to minimize de-Qing), series C (to DC-block the
signal from ground referred termination), shunt R
(the needed or unavoidable termination) divider
may be the way to go, and work the scale factor
out of the data later.
 

I would add that you probably want to choose a buffer amp with the smallest S21 magnitude value for frequency pulling reasons.
 

An easy way to test the VCO is to use some inverter stages(ideally two) followed by CML which can drive a 50ohm. Phase noise meters will have a 50 ohm input port to measure phase noise. Note that the first inverter is going to add noise and it preferable to use it in the same LDO as used for VCO (to minimise supply pushing). Generally noise > 10MHz will be effected but I guess you will be interested in 10k - 10M range so that should not be a problem.
 

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