[Collins] Any Frequency

[Kees & Sandy]

I ask you to please check the results of testing done by Jack Smith of Clifton Labs and check the specs SiLabs has on the part. The lack of spurs on the Si570 output relative to very frequency agile DDS chips is very impressive. It's as good/better than the HP 8640 signal generators.
73 Kees K5BCQ 

---------- Original Message ----------
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: "james.liles" <james.liles at comcast.net>
Cc: collins at mailman.qth.net
Subject: Re: [Collins] Any Frequency
Date: Tue, 06 Oct 2009 11:04:10 -0600

On Tue, 2009-10-06 at 10:19 -0500, james.liles wrote:
> Has anyone measured or even detected the phase noise that an Si570 chip has 
> added to an HF radio???  Can the consequence be distinguished while being 
> used in a pristine natural low noise environment?

That's not the problem in many parts of the world. The problem is
reciprocal mixing from nearby BC transmitters, worst on 40 meters.
Though in the really quiet environment (VHF through microwave) a pretty
good LO can affect the system MDS, so I claim. I have yet to prove that.
At HF, atmospheric noise will override a great deal of receiver internal
noise.

In reciprocal mixing the phase noise of the LO puts the same phase noise
sidebands on strong signals and they supply noise at the signal
frequency, typically stronger the closer the frequency spacing. A
typical none synthesized oscillator has a phase noise spectrum (called
Leeson by the name of the first one to quantize it) that declines about
10 dB per decade of offset. A PLL spectrum often is flat out the control
loop bandwidth, then tapers. That's mostly from obnoxious phase noise of
the excessively agile VCO.

In the past, a DDS has needed some sort of narrow band filter like a PLL
to clean up spurs from the clocking process. Adding more bits to the
logic improves but doesn't cure that situation. Often the choice of
clock frequency is critical for having some clean spectrum where needed
where the spurs can be suppressed with LC filters.

>  Have used an HP8664A 
> w/opt 4 to sub as the LO in other HF receivers and could not distinguish any 
> difference between the crystal or 8664A with option 4 on or off.  When used 
> in a shielded dummy load environment with the 8664A, phase noise becomes 
> apparent to measurement but not to the ear when compared to HF noise.

I have an experiment planned to test for phase noise effect on MDS, but
other projects have interfered with it and now buying a house will put
it off several months or longer. If the 8664A is an improvement on my
8640 its really good for phase noise, but not as good as a really good
HP frequency standard.

> If 
> the plan is to use the radio for general coverage, experimenting with a chip 
> sounds like a novel idea.

Has the possibility of performing better than a conventional PLL
synthesizer, but with occasional spurs in the LO signal that may prove
annoying but are movable by using different clock frequencies. 

> Kindest regards Jim K9AXN
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer

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