[time-nuts] Re: RCB-F9T Adapter PCB with USB and 50 Ohm Timepulse SMA Connectors

[Lux, Jim]

On 8/24/22 1:26 AM, Bruce Griffiths via time-nuts wrote:
> I don't see any explicit jitter specs on the BUF602 datasheet either.
> Have you measured it?
> How consistent is it from part to part?
> The jitter of most CMOS families has been measured.
> For example 74HC buffers typically have ~4ps jitter at room temperature.
> Even this is much lower than the jitter of typical FPGAs when the effect of cross coupling from other clock domains such as internal oscillators etc are taken into account.
> Faster logic families such as 74AC buffers (~ 1ps) have even lower jitter.
> CMOS devices have a typical propagation delay tempco of around 0.4% of the delay per/C. FPGA gates have similar delay tempcos.
> Neither the delay tempco nor the jitter is typically specified on CMOS device datasheets nor are they on the BUF602 datasheet so you have to measure them or leverage the results obtained by others.


This is true, as far as I know, of anything where you're pushing the 
limits of performance.  Buy them, measure them, and use them if 
acceptable.  And don't trust that buying the same part will still work 
in the future.

Whether it's low jitter, low noise, prop delay, or whatever.

I could easily see Brand X 74AC having different characteristics than 
Brand Y 74AC, while both meet the specs in the datasheets (which are 
identical).

That's sort of the challenge faced by people who want to make one-off, 
vs small volume, vs mass production.

For the first, you can use parts that happen to work that you happen to 
have.  For the second, you can do a "buy and screen" sort of process.

For the latter, you either have to have "data sheet guarantees" (perhaps 
as a specialty order from the MFR and they do screening)

In all cases you have to be willing to discontinue a product when the 
parts you need are no longer available.

This is incredibly frustrating when you're trying to duplicate a 
published state of the art design (even if "state of the art" was 1999) 
and you have limited time (so you can't just buy a dozen different parts 
and try them all)


By the way, don't forget the fast parts from Potato Semiconductor - 
http://www.potatosemi.com/