[GPS_Standard] Re: a low jitter Sine to Square-wave circuit.

[David Garnier]

Hi Zack,

Been wanting to get back to you on your comments on my concern.

Unfortunately Digikey isn't replacing stock on the MC10ELT21 part,
I haven't called them to ask why, I would like to have a couple of
these devices handy.  (Mouser didn't stock it and Newark was out
of stock at that time.)

I did look at the data sheet on the LCT1485 chip after your comment. 
Further reading on these receivers devices backed up your point.
I bet this isn't a significant source of clock jitter compared to the
recovered 10 MHz clock from the GPS receivers.  I bought some
mini-circuit 1 to1 transformers to further isolate any possible
ground loops.  We have a expensive Tek communication jitter
analyzer scope at work, I can't really make these kind of measurements
during a lunch hour, this has to be a weekend project, I have
seen this instrument only used a couple of times. I also have the
added problem that the engineering lab is in the middle of the
building, way to far for running a GPS antenna with coax. :-(

I recently purchased a Trimble Resolution T receiver, after I
received the order acknowledgment from the factory, they
indicated that their minimum factory order is now $100 dollars.
(The receiver was $75.)  However, I wasn't discouraged in
placing the order with local sales rep.

Have you noticed that Bert and others have opted not to place
the 120 volt power transformer in the same box as the controller,
oven oscillator and receiver???  I wonder if people are concerned
or have noticed 60 Hz hum being induced into the control loop?
To be on the save side I have twisted all of my power and signal
leads and maintained a separate ground return back to their
respective voltage regulators.

I have been on a number of recent engineering projects that
twisting the DC power supply leads had reduced image artifact
and EMI noise in the far field.  I suppose a simple fix to the
installed transformer would be wrapping a one turn flat copper
strap around the exterior of the transformer thus forming a 1 turn
short circuit for radiated mag fields.  Ever see power transformers
done this way for high end audio equipment???

This is turning out to be an interesting project.  73's neighbor!

Dave Garnier - wb9own


Thanks

Hi Dave,

On Tue, 26 Dec 2006, David Garnier wrote:


> > Hello,
> > 
> > I have not thought this question out completely but maybe someone
> > more knowledgeable on PLL's or interfacing sine-wave oscillators to
> > TTL interfaces could answer these basic questions.
> > 
> > I have a Isotemp OCXO 134-10, 10 Mhz sine wave oven oscillator
> > http://www.isotemp.com/docs/OCXO134-10Spec.pdf
> > 
> > 1) I need a Sine wave to TTL (circuit or IC) to interface this oscillator
> > to an existing high quality RF signal generator. 
> > 
> > _Question._ It seems to me that this application would require a "low 
> > jitter"
> > sine to square-wave circuit. But, wouldn't the PLL in this RF signal 
> > generator
> > filter out this added jitter?
> > 
>   

The chip Bert uses is an LTC1485.  This is a Schmitt-trigger device, so I 
imagine the jitter is fairly low.

In another application, I was looking for a low phase-noise oscillator to 
drive an Analog Devices AD9835 DDS chip.  Analog has an application note, 
AN419, with just such a circuit.  It uses the MC10ELT21 chip as a low 
phase-noise sine-to-TTL converter. Digi-Key stocks these. These chips are 
good to at least 150 MHz!


> > 2) _Inside the loop of a disciplined GPS oscillator_, is it less 
> > critical to have
> > a "low jitter" sine to square-wave converter?  ((Does the excess jitter 
> > in this
> > converter "come out in the wash" being within the disciplined GPS 
> > oscillator?))
> > 
>   

It would depend on how narow the loop filter is.  The narrower the 
better, but the longer it takes to lock up.

In Bert's circuit I don't think it's critical but I haven't done any tests 
yet.


> > 3) Measuring pico-second jitter isn't trivial.  Any ideas on how to do 
> > this easily?
> > 
>   

Jitter translates into phase noise in the frequency domain.  The OCXO spec 
sheet you listed gives the phase noise for three frequencies, the close-in 
and the farther-out phase noise.  It looks respectable to me.

You will need a spectrum analyzer with the capability of looking at least 
145-150 dB below the carrier.


> > 4) Maybe these questions cannot be initially answered but need elaborate
> > "jitter measuring equipment" to make these determinations?
> > 
>   

The spectrum analyzer I mentioned above should do the trick.


> > 5) There's some lab equipment available that maybe could help answer this
> > question - but this is vacation time!!!
> > 
> > Thanks in advance for any thoughts you have on the subject.
> > 
> > Regards & 73's
> > 
> > Dave Garnier - wb9own
>   

What are you going to use the circuit for?  I'm going to use it to either 
phase-lock or directly synthesize LO's for microwave transverters for 24, 
47, 76 GHz and up.  You lose out on phase noise each time you multiply the 
LO frequency, so when you get up around 76 GHz it becomes a factor to 
consider.  I'm trying to avoid spending as much as I'd spend for a new HF 
rig for a rubidium standard, as WA1ZMS did. We'll see how well it works. 
It should do OK at least on 24 and 47 GHz.

73, Zack W9SZ



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