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SDG2042X Timebase Calibration

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rf-loop:

--- Quote from: 2N3055 on November 02, 2020, 12:22:54 pm ---@rf-loop

Well said. You might get a crystal that will be tight enough but rest of the circuit has to temp compensated..
But ordering crystal with such tight specs will be expensive. VCTCXO are cheap and apart from cheapest crap most have amazing specs for the price.

As for timekeeping, i had my share of work on precise clocks for motorsports (rallying and hillclimb is popular here in my hometown) previously, some years ago..
Spend lot of time devising clever compensation techniques to achieve some pretty tight timing requirements comparable to Tag Heuer Sport timers at the time... Today, you would simply slap a nice DCTCXO, tune it up, and put GPS module for control and calibration...

--- End quote ---

Yes.

Btw, bit challenging to keep xtal tightly in freq due to G and vehicle angle changing and including even pressure change...  as we know from time nuts, all is changing, everything and all is affecting, everything... all drifting, all are off... no one know what is perfect time or freq. But I have stopped this all long time ago because it is waste of time. My time is proportional to my age... more age...more fast time goes so where I need precise time or precise frequency... what do not even exist, just as there is not real DC, also it exist only in theory books.

World with exactly one clock or what ever time reference have perfect time with zero error.

mawyatt:
How about a Chip-Scale Atomic Clock ::)

https://www.defenseworld.net/news/25331/DARPA_To_Develop_Advanced_Tiny_Chip_Scale_Atomic_Clocks_For_Military_PNT_Applications#.X6AmLC2ZPUI

Best,

Johnny B Good:
@rf-loop and @2n3055,

 Thank you for your very considered responses. Our differences of opinion over what exactly that VCXO/VCTCXO actually is within the SDG2000X model range seems to be over the terminology used.

 I think the VCXO used by Siglent in this case would be better described as a "Low TempCo" VCXO rather than a VCTCXO with a best temperature stability of 0.1ppm (limited by the retrace effect), typically applied over the 0 to 25 deg C range versus a Low TempCo VCXO optimised to stay within the wider but still impressively good 1 to 2ppm limits achieved by a wise choice of crystal cut and components to minimise the overall XO temperature drift. There's no specific temperature compensating circuit involved, just a combination of components that passively minimise the overall tempco effect.

 I'm sure that if the copy writers had decided they could legally describe such a low tempco VCXO as a VCTCXO, they would have done so (they're not in the habit of missing such golden opportunities to legally use such higher value descriptions in their feature lists or specifications whenever possible).

 The fact that they'd pointedly avoided the description "VCTCXO", relying only upon "Letting the figures speak for themselves", suggests that my interpretation is the more reasonable one in this case. I'm not trying to "score points" here, just trying to set the record straight since a VCTCXO would typically offer an order of magnitude improvement in temperature stability over the low tempco version of VCXO Siglent have elected to use here.

 It might seem that I'm "nit picking" over the terminology but since Siglent clearly understand the distinction between a low tempco VCXO and a VCTCXO by avoiding any possibility of being accused of making false claims by sticking only to the actual facts and figures of the frequency stability parameters, I felt it was a point worth making over the unjustified use of the description "VCTCXO" when not even Siglent themselves were prepared to use such a description.

 I have no issues with Siglent's careful precision in their description of the frequency stability of the internal clock oscillator. Indeed, they're to be lauded for their honesty, whilst at the same time, admired for the effectiveness of their copywriter's art in persuading prospective buyers into reading more into the description than actually exists, i.e. the presence of a VCTCXO when only a high quality low tempco VCXO is involved. :)

 In this case, Siglent have quite reasonably included an external clock reference socket to render the distinction between a low tempco VCXO and a VCTCXO rather academic. Although the "Export the internal clock" option when operating from the internal clock can be a handy way to synchronise other test kit in the absence of an actual 'Lab reference', it's just a shame that this export feature couldn't be toggled on and off in a separate menu option to avoid polluting an otherwise perfectly fine daisy-chained lab reference distribution setup.

 Far better to have provided this internal clock export function over a separate dedicated BNC socket but that adds to the BoM costs, hence this second best compromise of using a single BNC socket for both functions. It's just rather unfortunate that the user's options choice is so poorly implemented (as is the case for so many other aspects of the front panel UI which remains so unconscionably far from being a mature interface considering that they've had over four years in which to correct all the shortcomings which make it such a pain to use - notably the frequency parameter setting UI).

 You might question why I even bothered to buy a Siglent AWG in the first place but in spite of this, this still remained the best affordable option and the issue is essentially just one major firmware revamp away from being fully resolved. The only worrying aspect being that Siglent have been sitting on their hands in fixing the blatant deficiencies in the UI for the past four years or so.

 I'm an optimist and since the fix is do-able as a firmware update, I still have hopes that this will ultimately be resolved, especially in view of their record with other of their T&M model ranges regarding timely responses to bug fix requests and user wish lists. I'm just a little puzzled at the AWG division's apparent "Poor Man's" status in regard of UI developer talent.

 Apropos of bug fix requests, I've not been able to find any topic threads relating specifically to the SDG2000X series as exists for the SDS2000X DSO related topic thread so I'm wondering whether anyone here knows of such a thread or am I simply going to have to start one of my own? :-\

John

mawyatt:
According to Vectron's (well respected OEM, used their oscillators in many applications) App note, see Figure 1, a +-2ppm XO per Siglent's data sheet for the SDG2042X is in the TCXO category.

https://www.vectron.com/products/literature_library/tutorial_on_tcxos.pdf


From Siglent data sheet.


That's good enough for my understanding of the boundary between a XO, TCXO and OCXO. And since the Siglent data sheet shows a 0~40C temp range, which implies (assumes linear change) a TC of +-2ppm/40C or +-0.05ppm/C which places it well within the TCXO category shows this actually a quite good TXCO indeed ;)

I can say that the stability relative to the 10MHz reference in the SSA3021X Plus which is the best reference I have available is outstanding :-+


Best,

Johnny B Good:

--- Quote from: mawyatt on November 02, 2020, 06:06:13 pm ---According to Vectron's (well respected OEM, used their oscillators in many applications) App note, see Figure 1, a +-2ppm XO per Siglent's data sheet for the SDG2042X is in the TCXO category.

https://www.vectron.com/products/literature_library/tutorial_on_tcxos.pdf
(Attachment Link)

From Siglent data sheet.
(Attachment Link)

That's good enough for my understanding of the boundary between a XO, TCXO and OCXO. And since the Siglent data sheet shows a 0~40C temp range, which implies (assumes linear change) a TC of +-2ppm/40C or +-0.05ppm/C which places it well within the TCXO category shows this actually a quite good TXCO indeed ;)

I can say that the stability relative to the 10MHz reference in the SSA3021X Plus which is the best reference I have available is outstanding :-+
(Attachment Link)

Best,

--- End quote ---

 Thanks for providing a link to the Vectron app note. Page 3 provides a more detailed look at the temperature compensation curves typically applied in TCXOs. As you can see, you cannot safely assume a linear change of frequency over its total temperature range (at least in the case of a TCXO - it might be a less 'lumpy' curve for a low tempco VCXO as used in the Siglent SDG2000X series but still not the linear change you've assumed).

 In the case of the 50MHz TCXO I'd used as a reference to compare the Siglent's 10MHz internal clock reference to, I've always assumed its given 0.1ppm stability over the 0 to 25 deg C range to be a maximum deviation from its calibrated frequency (+/-0.1ppm).

 In any case, it had never occurred to me to calculate an equivalent frequency shift with temperature in terms of ppb/K since I knew there was little to be gained in knowing this figure. It was what it was and all I could do was to do my best to avoid operating outside of this specified 0 to 25 deg C range as much as I could possibly arrange and also try to arrange that it never suffered any more change of temperature than those mediated by changes in room temperature, hence my choice of mounting location within the FY6600-60M, right above the small cooling fan to act as both a deflector and benefit from the incoming room temperature air flow to hold it within two degrees above ambient (typically within a degree or two of the 25 deg upper limit of its 0.1ppm temperature range specification.

 If I had bothered to calculate a per deg frequency change figure to get a sense of the stability involved, I would have arrived at a 4ppb/K for a simplified linear curve and perhaps multiplied it by 2.5 to approximate the sinuous curve that's typically applied to realise such a tight temperature stability over a 25 K range.

 The biggest issue I'd had with that clock module in the FY6600 was that of the backlash in its trimcap, making it virtually impossible to calibrate it any closer than 30ppb of frequency other than by dumb luck. In the end, when I'd gotten hold of a bunch of 10MHz OCXOs for cheap, I decided to replace it with one of these OCXOs and a 3n502 clock multiplier to save any further frustration.

 That OCXO upgrade had given me a whole new insight into the value of precise frequency measurement which has since allowed to observe phenomena that had previously been masked by the previous lax standards of "Good Enough" typical of AWGs ranging from Feeltech cheapies right through to expensive Keysight and not so pricey Siglent AWGs.

 Funnily enough, now that I have finally built myself a modern day version of the James Miller GPSDO design to give me a stable GPS based frequency reference, re-calibrating that 50MHz TCXO for this latest testing session, though still a pain went a lot smoother and, in fact, some 4 hours after powering up yesterday's test setup late this afternoon, it's now settled to within -200ppt of 50MHz (10mHz low) and I'm just about to cool it down with that USB desk fan... within a few seconds it had increased by 4ppb in frequency before going back down to within 300ppt short of where it had been until I shut the fan off when it slowly resumed its drop in frequency before reversing back to around 1ppb high courtesy of the retrace issue which rather spoils the TC game plan which brings us back to that chart on page 1.

 I'm surprised that Vectron should even think there's any overlap between high end TCXOs and low end OCXOs, especially when they show a gap between VCXOs and low end TCXOs. To my mind, there's no justification for having a gap between TCXOs and OCXOs any less than an order of magnitude of temperature stability rating.

 I mean, why would anyone go to all the additional power requirement costs of a sub standard OCXO when a less power hungry TCXO of higher frequency stability with temperature could do the job without all that expense and additional heat loading? It makes no sense that such an overlap should even exist.

 Perhaps I'm overlooking the possibility of cheaper AT cut crystal based OCXOs operating at a lower oven temperature (~60 deg C?) with lower peak power demand being a cheaper alternative to the highest specced TCXO designs as a possible reason for this overlap?

 A 0.1ppm over the 0 to 25 deg C range is pretty close to the limit before retrace starts making a complete mockery of any such improved temperature compensation attempts. If you're going to make all that extra investment required to support an OCXO, you'll certainly be looking to get better than just one magnitude of improvement with more likely a two magnitudes or better improvement to justify such an expense.

 If you've no choice but to bite the bullet on account not even the best available TCXOs can meet your stability requirement,  I should imagine it will be a case of, "In for a penny, in for a pound" and "You might as well be hanged for a sheep as for a lamb." and all that.

 In other words, you'd be more inclined to go for a 1ppb rated OCXO than for a barely adequate 10ppb rated one unless your company's chief bean counter feels he can get away with doing ten dollar's worth of damage to the company's reputation for the sake of a 50 cent saving on the BoM costs without risk of being given his marching orders.

 Am I right in thinking that Siglent datasheet image relates to the SSA3021X Plus? It looks almost, but not quite, identical to the frequency characteristics section in the SDG2000X's datasheet. Unfortunately I can't easily check images whilst in the 'reply' window but I seem to recall a 20 year ageing figure versus the 10 year ageing figure in the SDG2000X's frequency specifications.

 If that's currently your best frequency standard then be prepared to "Have your socks blown off" by a thousandfold improvement should you ever invest in a decent 10MHz GPSDO lab standard to plug into the Ext Ref input socket. :)

John

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