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Measurements >> Phase Noise and Jitter Measurements >> Phase noise for a large linewidth, low power, unstable free-running oscillator
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Message started by sandman on Oct 20^th, 2008, 5:49am

Title: Phase noise for a large linewidth, low power, unstable free-running oscillator
Post by sandman on Oct 20^th, 2008, 5:49am

Hi all!

I have an oscillator that runs at nearly 8GHz and has a -3dB bandwidth of about 40 MHz. It's not a traditional LC or CMOS oscillator and which is why I'm trying to characterize it's phase noise. The carrier power is low (@ -40dBm) and is above the noise floor of the measurement setup by say 30 dB (which I agree is noisy!).

I'm using the direct measurement (method) to measure the PN profile of this free running oscillator (using a R&S FSU26 with a software PN add-on kit available from the same company). The oscillator at the moment has a carrier frequency drift, to the tune of a say 10-15MHz. I assume that because of this fluctuation the machine measures frequency noise and not just the intrinsic phase noise of my oscillator. I still don't understand this fully and it would be great if you or anyone would have some inputs or comments on this and what steps I would need to take to measure this noisy, very low Q, very low power oscillator.

Additionally, the machine fails to track the oscillator signal even if I increase the error tolerance (in frequency drift and signal peak power) to large percentages. So, I manually specify the level and frequency of the carrier for the PN measurement. I'm willing to live with some percentage of error in this measurement assuming it is - say, below 10%.

Any inputs are welcome. Thanks !

Title: Re: Phase noise for a large linewidth, low power, unstable free-running oscillator
Post by Mazz on Jan 11^th, 2009, 12:21am

You have 2 possibilities:

1. Use the same method used in specific phase noise instruments (that is complex to build up, you'll need a mixer, a low frew LNA, a clean FM mod RF source and so on)

2. Lock the VCO in an external PLL with very low loop bandwidth.

In any case you'll need an external LNA to rise up the VCO level to -10 - 0 dBm.

I hope it can help.

Mazz