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How to measure LC oscillator phase noise.

A

Andrew VK3BFA

Jan 1, 1970
0
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.
73 de VK3BFA Andrew
 
A

Allan Herriman

Jan 1, 1970
0
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.
 
D

Don Pearce

Jan 1, 1970
0
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.

This is all very well, but unfortunately the noise floor of the
analyser is unlikely to be the limiting factor here. The phase noise
displayed on the analyser is the sum of the phase noises of the device
under test and all the various local oscillators in the spectrum
analyser. Analyser local oscillators (the first anyway) are designed
for broad pulling range, not good phase noise. They tend to be
mutually exclusive.

A free-running oscillator at 30MHz or so should be capable of very
good phase noise at the odd few kHz offset - far better than that of
the analyser oscillator, so the measurement is not likely to be very
convincing.

The best way to get the measurement is to build a crystal oscillator
within the range of the VCO, and phase lock them together with a very
narrow loop (a few Hz maximum). Feed both outputs into the LO and RF
ports of a mixer, and examine the output. It will be baseband noise
with the characteristics of the noise of the oscillator. You can
examine this very nicely with a sound card on your PC. A low noise
op-amp will make a good pre-amplifier to get a decent sized signal.

d
Pearce Consulting
http://www.pearce.uk.com
 
I

Ian Buckner

Jan 1, 1970
0
Andrew VK3BFA said:
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.
73 de VK3BFA Andrew

Do a google search on "phase noise measurement", there's lots of
good stuff there on how to do this. However, you will need a low
frequency spectrum analyser. I'm not sure of the HP141 low frequency
limit.

Basically, you use a PLL to demodulate the phase noise using
a second oscillator, either identical to the one you are testing, or
much
quieter. For SSB work you are likely to be interested in offset
frequencies down to a few hundred Hertz, hence the need for a LF
spectrum analyser.

Regards
Ian
 
J

James Meyer

Jan 1, 1970
0
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

Use an FM receiver to listen to the oscillator (or better, one of its
harmonics). An AC voltmeter on the receiver output will give you some numbers.

Jim " KISS * " Meyer

* Keep It Simple Sir
 
L

Leon Heller

Jan 1, 1970
0
Andrew VK3BFA said:
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.
73 de VK3BFA Andrew

Receiver oscillator phase noise measurement is discussed in the 2001
Handbook - page 14.9. All you need is a +10 dBm crystal oscillator,
attenuators and an AC voltmeter. You only get the value at the crystal
oscillator frequency, of course.

73, Leon
 
M

Mike Engelhardt

Jan 1, 1970
0
VK3BFA,
...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?...

You can get a feel for the phase noise of a VCO with
just an oscilloscope. Set up a delayed sweep so that
you're looking at a section of the CW wave long after
the trigger. Check your scope's delay stability with
a Xtal oscillator and then you can qualitatively see
how much worse a VCO is than your scope. When I was
doing this in the 70's, good scopes had delay noise
floor around maybe 50 or even 20ppm, much lower than
most LC VCO's. Don't know current values.

--Mike
 
A

Andrew VK3BFA

Jan 1, 1970
0
Mike Engelhardt said:
VK3BFA,


You can get a feel for the phase noise of a VCO with
just an oscilloscope. Set up a delayed sweep so that
you're looking at a section of the CW wave long after
the trigger. Check your scope's delay stability with
a Xtal oscillator and then you can qualitatively see
how much worse a VCO is than your scope. When I was
doing this in the 70's, good scopes had delay noise
floor around maybe 50 or even 20ppm, much lower than
most LC VCO's. Don't know current values.

--Mike

Thank you to all who responded - much apprecaited. Now know how to
start looking at the problem, so will try out the various techniques
described.
73 de VK3BFA Andrew
 
M

Mike

Jan 1, 1970
0
VK3BFA,


You can get a feel for the phase noise of a VCO with
just an oscilloscope. Set up a delayed sweep so that
you're looking at a section of the CW wave long after
the trigger. Check your scope's delay stability with
a Xtal oscillator and then you can qualitatively see
how much worse a VCO is than your scope. When I was
doing this in the 70's, good scopes had delay noise
floor around maybe 50 or even 20ppm, much lower than
most LC VCO's. Don't know current values.

Assuming your scope timing circuit is much better than your VCO, this
measurement should provide a jitter result that is proportional to the
square root of the delay. It also provides no information about the power
spectrum of the noise - something that is usually implied when talking
about phase noise, even if only a single point is specified (as in, -110dBc
at 30kHz offset).

-- Mike --
 
J

John Larkin

Jan 1, 1970
0
VK3BFA,


You can get a feel for the phase noise of a VCO with
just an oscilloscope. Set up a delayed sweep so that
you're looking at a section of the CW wave long after
the trigger. Check your scope's delay stability with
a Xtal oscillator and then you can qualitatively see
how much worse a VCO is than your scope. When I was
doing this in the 70's, good scopes had delay noise
floor around maybe 50 or even 20ppm, much lower than
most LC VCO's. Don't know current values.

--Mike

20 PPM is about right for a good analog or equivalent-time-sampling
scope. Some digital scopes are better, but many have an uncompensated
+-1 sample time uncertainty. Few have enough delay range to be really
useful here.

We've been playing with triggerable LC oscillators lately. Jitter of a
couple of PPM is not too hard to get. We test this by triggering the
LC oscillator from a digital delay generator, then using a DDG delayed
output to trigger a scope, which in turn looks at one cycle of the
oscillator at a fast sweep rate. So the long-delay jitter is dominated
by the DDG, which has jitter in the 1e-9 range.

John
 
J

John Miles

Jan 1, 1970
0
Thank you to all who responded - much apprecaited. Now know how to
start looking at the problem, so will try out the various techniques
described.
73 de VK3BFA Andrew

One suggestion: get an HP 8553A plugin for your 141T, if you don't
already have one. In the US at least, it will cost about as much as a
nice steak dinner. This is a very good plugin for the 0-110 MHz range.
Use it (following Allan H's setup instructions) to compare the noise
profile of your VFO to a decent crystal oscillator at a nearby
frequency.

You shouldn't see much of a difference. If you do, it means your VCO is
worse than the 8553A's, which should not be the case considering you're
working at HF and the 8553A's VCO is working in the VHF/lower-UHF range.

You will not be able to use "minimum" resolution bandwidth setting for
this measurement as Allan suggests, by the way. The 8553A/8552B
combination goes down to 10 Hz RBW, which won't yield a calibrated
response at 10 kHz/division at any sweep speed you'd be willing to wait
for. As a rule of thumb, shoot for 10X narrower RBW than the channel
bandwidth your oscillator's application will eventually require. Don't
forget to normalize your phase noise reading to dBc/Hz by subtracting 10
*log(RBW).

-- jm
 
R

Richard Hosking

Jan 1, 1970
0
This would seem to be a good way to get reasonable measurements assuming
You allow for the mixer loss (7-10dB for a diode mixer)
The mixer is driven appropriately (eg +7dBm for the LO and 0dBm for the
RF ports)
The post mixer amp does not add noise and is flat enough (you will need
some gain)
You have a calibrated soundcard/software (?would Linrad do here?)

Assuming the crystal oscillator is much better than the LC oscillator,
the phase noise of the oscillator will not be attenuated outside the PLL
bandwidth by the loop filter. The phase detector should not add
significant phase noise at this frequency

With the 8553 plugin for the 141 you could get crude measurements, but I
suspect the spec an LO would be worse than your LO, which would negate
any measurements below a certain level.

Richard
 
D

ddwyer

Jan 1, 1970
0
Andrew said:
Thank you to all who responded - much apprecaited. Now know how to
start looking at the problem, so will try out the various techniques
described.
73 de VK3BFA Andrew
2 othe ways

not too difficult.
1 Needs an HP 8640 sig gen or crystal oscillator.


Get minicircuits douple balanced mixer. and 10.7 MHZ ssb crystal filter.
Calibrated attenuator. and low noise 30db amplifier.
drive the mixer with your VCO and the crystal osc or sig gen
With the atten at 40db in the VCO path Adjust the VCO or Siggen so that
the difference freq is 10.7 MHz and goes through the crystal filter
followed by the amplifier.
Set that as 0db on the 141T.
tune your VCO or sig gen by 5+kHz so that the mixed carrier disappears
in noise/reduces a lot now reduce the attenuator step by step.

The above transfers the noise floor limit from the 141T to the crystal
osc or sig gen
Used to measure -110dbc this way 19kHz from carrier. (using HP 840C
could do better with a crystal.


Alternatively split the o/p of your VCO send one half to the lo port of
the mixer the other port is fed via a 100 metres of coax to delay.
Mixer o/p will be proportional to noise. Note again low noise amplifiers
and attenuators will come in handy.

[email protected]
 
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