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a free running vco will have a lot of frequency jumping, and it will be hard to make a measurement. I would go with a delay line discriminator type phase noise measurement system, and try to loosely couple into the VCO and reamplify with a decent RF amplifier. You want to loosely couple, because if you load down the VCO in a way that the circuit does not usually work, you can change the phase noise.
If you are talking about a phase locked vco, it will be a little easier to measure.
a free running vco will have a lot of frequency jumping, and it will be hard to make a measurement. I would go with a delay line discriminator type phase noise measurement system, and try to loosely couple into the VCO and reamplify with a decent RF amplifier. You want to loosely couple, because if you load down the VCO in a way that the circuit does not usually work, you can change the phase noise.
If you are talking about a phase locked vco, it will be a little easier to measure.
Thanks for the replay.
Yes, im talking about a free-running VCO. Could you suggest a good document taking about the application of this technique in PN measurement.
Due to theoretical reasons, a delay line discriminator will not have as good a sensitivity close to the carrier as other methods. But in a free running VCO, that is jumping around +/-500 Hz anyway, you do not need a good close to the carrier phase noise floor. You can buy systems commercially:
**broken link removed**
Some of the fancier systems, like agilent's, can be configured to also use a delay line externally.
Other methods require some sort of 2nd oscillator or external synthesizer loosely "locked" to the device under test. This is often hard to do with a free running oscillator, as it jumps around and breaks "lock".
Depending on how bad your VCO phase noise is, and if you can use a measurement far from the carrier, you might get away by just using a modern spectrum analyzer with a phase noise measurement option. NOt as accurate, but can work. If you are going to go that way, make sure the power supply to the chip is very low noise (linear supply, not a switcher, and external capacitors bypassing right at your probes).
Also, be aware that at the chip level, light can modulate the oscillator. You want to turn off any microscope optical illuminators, keep air conditioner air currents from hitting the chip, etc.
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