[time-nuts] Re: Ublox NEO-M8T-0-10​as a LO question

[John Ackermann N8UR]

Hi Chris --

The difference is that the ublox module is outputting a signal that's 
being generated without much concern for its phase noise.  The digital 
process used creates a  lot of jitter in the edges of the output 
waveform.  It simply wasn't intended for the TIMEPULSE output to be used 
in RF (analog) circuits.

The Bodnar GPSDO follows the GPS output with a synthesizer chip that 
also functions as a "jitter attenuator" which in addition to allowing a 
wide range of frequency outputs also cleans up the input signal.   The 
synth chip output has much lower phase noise than the GPS by itself.

But the synth output is still noisier than a simple crystal oscillator 
and might not be sufficiently good to be multiplied up to the 10 GHz 
range.  That's why others have suggested starting the LO chain with a 
higher frequency XO that is locked (in a narrow loop bandwidth) to the 
GPS signal.  That will result in significantly lower phase noise when 
multiplied up to 10 GHz.

73,
John
----

On 3/31/21 8:09 AM, Chris Wilson wrote:
> Hello everyone
> 
>   Monday, March 29, 2021
> 
>   Thanks for the replies, some went a little over my head, which is not hard :) I am wondering how people successfully use a Leo Bodnar programmable GPS source to provide an external LO for an LNB as opposed to its internal xtal. What is different, in simple terms please, between my Ublox output and the Leo Bodnar one?
> 
> 
> Best regards,
>   Chris        mailto:chris at chriswilson.tv
> 
> 
> LJ> On 3/29/21 10:14 AM, John Ackermann N8UR wrote:
>>> That's probably a really bad idea.  The phase noise from the TIMEPULSE
>>> output is pretty bad compared to a "real" RF source, and by the time
>>> it's multiplied up to 10 GHz you'll have more noise than signal.
>>> Attached are some phase noise plots and a couple of spectrum analyzer
>>> captures to give you some idea what to expect.
> 
>>> BTW, even the Bodnar unit may not look too good at 10 GHz -- remember
>>> that you increase phase noise by 20 dB for 10 times multiplication.
> 
>>> John
> 
> LJ> What's the signal bandwidth from Es'Hail?
> 
> LJ> The optimum strategy is a *very quiet* crystal oscillator that you
> LJ> discipline with the 1pps, and choose that oscillator so its frequency is
> LJ> what you need.
> 
> LJ> What we've done in the past is use the reference to clock a NCO in FPGA,
> LJ> and use one of the well known spur reduction techniques that pushes the
> LJ> spurs away from the center before running it to the DAC. This degrades
> LJ> the performance at, say, 100kHz away, but improves the performance
> LJ> within 1 kHz. This relies on knowing what the loop bandwidth is in your
> LJ> 10GHz LO PLL, since inside that bandwidth it's the reference, but
> LJ> outside it's the DRO or GaAs oscillator.
> 
> 
> LJ> There might be some DDS chips that implement this kind of thing - the
> LJ> latest chips from ADI are pretty sophisticated.
> 
> 
> 
> 
> 
>>> On 3/29/21 12:25 PM, Chris Wilson wrote:
> 
> 
>>>>     29/03/2021 17:20
> 
>>>> Can I use my Ublox NEO-M8T-0-10​  as a LO for  a  modified satellite
>>>> LNB on 10 GHz? It needs 25 MHz and the Ublox is my only GPS locked
>>>> source for such a frequency. I want to receive the Es Hail downlink
>>>> with excellent stability. I can lock the receiver to 10 MHz which is
>>>> available from my Trimble Thunderbolt. If the Ublox would do I would
>>>> not have to buy something like the Leo Bodnar GPS. Thanks.
> 
> 
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