Re: How to measure LC oscillator phase noise.
From: Allan Herriman (allan.herriman.hates.spam_at_ctam.com.au.invalid)
Date: 06/15/04
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Date: Tue, 15 Jun 2004 19:35:06 +1000
On 15 Jun 2004 01:30:30 -0700, ablight@alphalink.com.au (Andrew
VK3BFA) wrote:
>I am in the process of building a 20m SSB transceiver using a LC VFO -
>how can you measure phase noise of the device without a roomfull of
>complex expensive test equipment - is this possible with simple
>techniques even if only in a relative good/bad way? - I have a decent
>CRO and a tempermental HP141 which works if thumped in the right
>place. I am not an engineer, and the only references I have here are
>in the ARRL hanbook for 2002 - they talk about pahes noise with
>respect to DDS synthesisers but give no practical help. Or am I asking
>an impossible question - can it be done by "good" oscillator design
>and it flows from this - the groups comments appreciated.
You need both good design and good measurements.
There are many simple design mistakes that can have an adverse affect
on phase noise, and it's easy to make those mistakes.
The simplest way to measure phase noise is with a spectrum analyser.
They might be good down to about -90dBc/Hz to -100dBc/Hz or so,
depending on offset freq.
Better than that requires a phase noise test set.
I'm not sure how good the HP141 is, but I suspect it will have a noise
floor higher than your oscillator under test at certain offset
frequencies.
How to measure the phase noise floor of your spectrum analyser:
1. Get a clean source, e.g. a crystal oscillator. No, you can't use
that old sig gen you have, it's probably noisier than the spec an.
2. Plug source into spec an. Adjust so the peak is in the middle of
the display.
3. Set span (or start and stop frequency limits on the display) to
about 3 times the offset frequency at which you are trying to measure
the noise. (E.g. if the spec is so many dBc/Hz @ 10kHz, set the span
to 50kHz.)
4. Reduce the resolution bandwidth to minimum. This will reduce the
video bandwidth and increase the sweep time, if they are coupled. If
not coupled, make the appropriate video bandwidth and sweep time
adjustment manually.
5. Adjust the reference level so that the peak is at the top of the
display.
6. Wait for a sweep (slow).
7. Use the cursors (in delta mode if available) to measure the
difference in level between the peak and a point at the offset
frequency (e.g. 10kHz) away from the peak.
8. If your instrument has a noise mode for its cursors, use that,
otherwise, normalise the level at the cursor to a 1Hz bandwidth. This
can be done (roughly) by subtracting 10.log10(rbw) dB from the level
at the cursor, where rbw is the noise bandwidth of the resolution
bandwidth filter. (If you don't know the noise bandwidth, just use
the resolution bandwidth.)
(Sorry, I don't think the '141 has cursors. You'll have to try to
read levels against the graticule.)
10. Take this value away from the reference level, and you have your
phase noise at a 10kHz offset.
E.g. 100MHz +10dBm signal. Cursor measures -75dBm at 10kHz away from
the carrier. Resolution bandwidth is 10Hz.
Phase noise is (+10) - (-75 -10.log(10)) = -95 dBc/Hz @ 10kHz
Regards,
Allan.
- Next message: Rheilly Phoull: "Re: unregulated power supplies"
- Previous message: Hans Summers: "Re: Soldering surface mount components"
- In reply to: Andrew VK3BFA: "How to measure LC oscillator phase noise."
- Next in thread: Don Pearce: "Re: How to measure LC oscillator phase noise."
- Reply: Don Pearce: "Re: How to measure LC oscillator phase noise."
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