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

Sat Feb 3 21:03:14 EST 2007

Hi Dave,

I have built VE2ZAZ GPS standard. It was powered form a wall transformer. 
This prevents the transformer magnetic field from disturbing the circuit and 
saves me from building a power supply !
Clock jitter should not be a problem if you have at least 1 Volt peak to 
peak at the LCT1485 input.
I built a buffer circuit for sampling the 10 MHz inside my HP5328A counter, 
which uses an HP 10544A VCXO. The buffer output uses a low Q resonant 
circuit that cancels the connecting cable capacitance and provides filtering 
against possible spurious far away from 10 MHz. This means that the LCT1485 
input sees a very clean 10 MHz waveform.
The only spurious that I could detect on the 10 MHz output were the heater 
switching transients at 3.7 KHz and a few multiples. These were - 80 dB down 
from the carrier, too low to worry about.
This is documented on my web site:  www.geocities.com/ve2_azx

73,

Jacques Audet
VE2AZX

>From: David Garnier <dgarnier at wi.rr.com>
>To: w9sz at prairienet.org, gps_standard at mailman.qth.net
>Subject: [GPS_Standard] Re: a low jitter Sine to Square-wave circuit.
>Date: Sat, 03 Feb 2007 14:55:49 -0600
>
>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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