[TangerineSDR] GPSDO Thoughts

John Ackermann. N8UR jra at febo.com
Mon Sep 30 07:31:01 EDT 2019


Yes on all counts, I think.  Cheaper, simpler, and better performance. All we lose is frequency agility -- which I still very much want, but as an option.

On Sep 29, 2019, 9:41 PM, at 9:41 PM, Scotty Cowling via TangerineSDR <tangerinesdr at lists.tapr.org> wrote:
>Hi John,
>
>So does it look like the Crystek oscillator with a 4-way output buffer 
>is the best solution for the stand-alone (no CKM) version of the DE?
>
>The oscillator is about $15 in quantity, with the 4-way buffer (Si Labs
>
>SI53341-B-GM) at $1.17 in quantity. This seems to be the cheapest way
>to 
>get the best non-GPSDO performance, and if we can get the Hermes method
>
>to work we could use it as the basis of the GPSDO CKM, couldn't we?
>
>73,
>Scotty WA2DFI
>
>On 2019-09-29 16:27, John Ackermann N8UR via TangerineSDR wrote:
>> Thanks for that Lyle.  The 570 is a neat device, but as you said is
>> stand-alone.
>>
>> We were looking at the SiLabs 543x series clock
>generator/cleanup/driver
>> chips which are basically complex synthesizers with virtually
>arbitrary
>> input frequency and multiple independent outputs settable to anything
>> from 1 Hz to over 1 GHz.  The niftiest versions use a 48 MHz crystal
>> oscillator as a phase noise cleanup and can get to a pretty
>impressive
>> noise floor (something around -150 at 144 MHz) but not as good as the
>> Crystek 122.88 MHz VCXO.
>>
>> But they're in the $13-18 price range (depending on type), have some
>> fiddly layout requirements (want a six layer board), and require
>> ultra-low-noise regulators at significant current, so as much as I
>love
>> the idea, I'm not sure it's the right choice, at least for a
>first-out
>> version.
>>
>> For just a 122.88 output, the Crystek with a four-way output buffer
>is
>> cheaper and cleaner.  And we've learned what caused the PLL
>performance
>> problem in the Hermes-derived boards, so hopefully can avoid that
>this
>> time around.
>>
>> 73,
>> John
>> ----
>>
>> On 9/29/19 6:27 PM, Lyle Johnson wrote:
>>> We used the Si570 in the K3S and KX3 synthesizers.  Excellent noise
>characteristics but can’t be slaved to an external reference.  We use a
>SiLabs synth in the KX2 that can use an external reference but noise
>performance is much worse.
>>>
>>> FWIW,  Lyle KK7P
>>>
>>> Sent from my iPhone
>>>
>>>> On Sep 29, 2019, at 2:39 PM, John Ackermann N8UR via TangerineSDR
><tangerinesdr at lists.tapr.org> wrote:
>>>>
>>>> To the TangerineSDR list --
>>>>
>>>> I've spent the week since DCC thinking about GPSDO questions and
>getting
>>>> things in place to do some experiments.  I was going to put
>together a
>>>> starting-point paper and send it to you and a few time/gps-nuts but
>>>> thought it was better to get some data first.
>>>>
>>>> In quick summary:
>>>>
>>>> A GPSDO is nothing more than a crystal oscillator ("XO") with an
>EFC
>>>> input that is steered to frequency by reference to the precise time
>>>> available from a GPS receiver, usually in the form of a
>pulse-per-second
>>>> signal.  The crystal oscillator can have excellent short-term
>stability,
>>>> but will drift (age) over time and is subject to environmental
>>>> variables, particularly temperature.  The GPS system has excellent
>long
>>>> term stability and accuracy, ultimately tracking USNO(UTC) but
>short
>>>> term usually requires long averaging times to reach that
>performance.
>>>>
>>>> The task of the GPS designer, given the known performance of the XO
>and
>>>> the GPS constellation, is to optimize the control loop to extract
>the
>>>> best of both.
>>>>
>>>> A better XO allows the time constant of the control loop to be
>longer.
>>>> A quieter GPS implementation allows the time constant of the
>control
>>>> loop to be shorter.  What's of interest to me is that a shorter
>control
>>>> loop implies lower performance requirements on the XO, and that
>might
>>>> result in a way to lower overall GPSDO cost.
>>>>
>>>> This wouldn't be very interesting except that there are now some
>low
>>>> cost GPS receiver modules available that might -- maybe, perhaps,
>>>> possibly -- provide a lower-noise GPS time reference.  u-Blox
>>>> (https://www.u-Blox.com) has released a bewildering variety of
>>>> navigation and timing modules with varying capabilities.  I've
>attached
>>>> a table that I put together by extracting data from the u-Blox web
>site.
>>>>
>>>> I'll soon have my hands on five of these modules with different
>>>> capabilities (and price points).  Once I've had a chance to take
>some
>>>> initial measurements, and verify some specs that aren't clear from
>the
>>>> documentation, I'll provide an updated report that might serve as
>the
>>>> basis for some design discussions.
>>>>
>>>> I'm also reaching out to a few friends in the time-nuts world to
>get
>>>> some recommendations for readily available 10 MHz XOs at a couple
>of
>>>> different price/performance points that we can at least use to
>provide
>>>> cost information.
>>>>
>>>> Finally, I'm thinking about whether the SiLabs frequency
>synthesizer
>>>> chip is the most cost-effective way to get the low-phase-noise
>122.88
>>>> MHz performance we need.  It might be cheaper, and better, to use
>the
>>>> Hermes scheme, where a very low jitter 122.88 VCXO is locked to the
>10
>>>> MHz reference -- provided we fix the known problem with the Hermes
>>>> implementation.  The synthesizer offers flexibility, but I think
>its
>>>> cost (with required supporting components) will be greater than the
>>>> Crystek 122.88 oscillator module, and its phase noise performance
>not
>>>> quite as good.
>>>>
>>>> Anyway, more to come.
>>>>
>>>> 73,
>>>> John
>>>>
>>>>
>>>>
>>>> <uBlox_GPS_Comparison_v2.pdf>
>>>> -- 
>>>> TangerineSDR mailing list
>>>> TangerineSDR at lists.tapr.org
>>>> http://lists.tapr.org/mailman/listinfo/tangerinesdr_lists.tapr.org
>>
>
>
>
>
>-- 
>TangerineSDR mailing list
>TangerineSDR at lists.tapr.org
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