[TangerineSDR] GPSDO Thoughts

[Scotty Cowling]

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
>