Great plan, John. I only had access to the 5052A for a day, so was
pretty limited in what I could do; it's great that you'll be able to
carry things forward.
The one addition I would suggest is that you duplicate the DDS output
measurements using the 10MHz source as well as the VF oscillator. That
would add to my sketchy info about the effect of the multiplier on phase
noise.
I was not happy with the results I got, I think due to the way I was
coupling out of the DDS, which was essentially a x1 scope probe with
less-than-perfect grounding. After the fact, I redid the connection
with a 50 ohm series resistor at U1 pin 6 (actually, mounted into a via
on that line) and a short piece of RG-174 feeding a buffer amplifier. I
think something like that will give better results.
By the way -- I looked at the AD9854 data sheet and it includes several
phase noise plots. From those, you would gather that there is a cost in
using the internal multiplier, but that it doesn't scale with DDS output
frequency. Also, from a quick read you could get the impression that
the phase noise of the reference clock doesn't really matter (within
reason, of course); they show the output phase noise plot without any
reference to the noise of the clock.
73,
John
----
[EMAIL PROTECTED] wrote:
This is cool. It's great to be able to discuss the nuances of what
really goes on within our radios. Spreading the word is a very good
thing. I checked out John's(N8UR) web site and I'm inspired. I have
an E5052A Signal Source Analyzer setting here and plan to make a few
measurements.
Here's the plan so far:
i. measure phase noise of a 10MHz crystal
(it will probably be limited by the E5052A)
ii. the 200MHz VF1611
iii. DDS output with VF1611 as the clock at:
1MHz
5MHz
10MHz
50MHz
Anyone have any additional ideas? I'm all ears.
73,
k2ox
-----Original Message-----
From: richard allen [mailto:[EMAIL PROTECTED]
Sent: Wednesday, January 04, 2006 5:39 PM
To: John Ackermann N8UR
Cc: [EMAIL PROTECTED]; FlexRadio@flex-radio.biz
Subject: Re: [Flexradio] DIP OCXO for SDR
Gentlemen,
The DDS does Direct Digital Synthesis hence the name. It does not
divide
back to anything but runs an phase accumulation engine at 200 MHz (or
whatever
the clock rate) that produces output values to the dac at that rate.
No division is performed.
Richard W5SXD
John Ackermann N8UR <[EMAIL PROTECTED]> wrote:
(01/04/2006 16:09)
[EMAIL PROTECTED] wrote:
First of all, any osc multiplied up widens its sidebands
(phase noise) by the multiplication factor and the inverse is
also true. The DDS does a 20X to 200MHz and a divide by 20 to
get back to 10MHz. I guess it's academic at this point how
much jitter is added by the DDS until someone measures it.
You're only dividing back to 10MHz if that's the operating frequency.
At higher operating frequencies, the division doesn't equal the
multiplication (and of course, at lower ones it exceeds it). Measuring
the DDS output lets us see the phase noise where it counts, taking into
account both multiplication and division, rather than just at the
fundamental frequency of the reference.
Second, Rubidium standards are not intended to be used as local
oscillators. They have terrible phase noise. They are intended
to be used in timekeeping. It is their long term drift that
excels, not short term phase noise.
Agreed in general -- though there's a wide difference in performance
between different types of Rb; some use FM modulation of the xtal, which
results in horrible phase noise, while others, like the 5065A, don't.
My post wasn't suggesting that you use an Rb as the primary reference.
However, the DDS output when driven by 10MHz shows the effect of the
multiplication, which is all I was trying to do. My web page also has a
plot of the HP 5065A phase noise at 10MHz, so you can see the difference
in noise between the raw and multiplied frequencies.
John
http://www.febo.com/geekworks/sdr1k/sdr1k_phase/index.html has
screenshots that show the phase noise at the output of the DDS for the
standard 200MHz oscillator, and an HP Rubidium frequency standard at
10MHz multiplied by 20 in the DDS. You can clearly see the phase-noise
hit caused by the multiplication.
John
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