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

[Carsten Andrich]

Hi John, John,

On 23.08.22 00:50, John Ackermann N8UR via time-nuts wrote:
>
> On 8/22/22 15:59, John Sloan via time-nuts wrote:
>
>>> Have you had any issues with unmatched transmission line effects?
>>
>> Being an embedded software engineer in the telecom field, I’m 
>> probably not qualified to answer that question. But I did cause the 
>> board to reset when I tried putting 50 Ohm terminators on the TP1 and 
>> TP2 SMA connectors. When I did this I checked the SparkFun schematic 
>> and dimly recall that there is a resistor between each TP output pin 
>> and the corresponding output pin (if you choose to solder on a 
>> header) on the board, and that this connection is in parallel with 
>> the SMA connector, which has no resistor at all. So I am inclined to 
>> agree with you (despite my ignorance).
>
> I have had issues with various NEO-M8 and ZED-F9 boards when running 
> the TP at a high (RF) rate, and driving more than a foot or two of 
> coax into a digital, presumably high impedance, input.
>
> I think there are two related problems:
>
> 50 ohm coax driving a high impedance load is going to ring all over 
> the place if given a chance.  I found that putting an in-line 50 ohm 
> terminator at the far end cleaned the waveform up very nicely.  That's 
> not unique to u-blox, it's just a fact of life.
>
> But with a terminator, the amplitude becomes marginal to trigger the 
> logic at the far end.  I suspect that's the u-blox not able to source 
> enough current to get a solid logic transition through the terminator, 
> and buffers should help.
Yep. Its digital outputs are rated for 5 mA max.
>
> There are newer and maybe better solutions, but on TAPR PPS projects 
> we've stuck with a design that Tom Clark came up with, paralleling 
> several (usually 3) 74AC04 inverters each with a 22 ohm resistor on 
> its output.  [ Originally Tom used 47 ohms thinking that would be 
> better for 50 ohm coax, but all the higher resistance does is lower 
> the voltage available. 22 ohms works well and gives more oomph at the 
> output. ]
>
> The resistors may not do any one thing perfectly, but they provide 
> something resembling a source termination as well as equalize current 
> if the gates don't all trip at exactly the same time. Whatever the 
> theory, it seems to provide a fairly robust output that can drive coax 
> and deliver a useful output at the far end. As far as I've ever been 
> able to measure, the 74AC delay/jitter doesn't have any noticeable 
> detriment to the real world rise time.
>
> As Bob said, this topic has been discussed a lot over the years and 
> it's really tough to reach an optimum solution because there are so 
> many different use cases.  The parallel AC04 gates have worked well 
> for us.

Thanks for the suggestion. My general use case requires very low noise 
and high slew-rates. The line drivers shouldn't substantially 
deteriorate the performance of an LTC6957 (320 ps typ. output rise 
time). I've narrowed usable parts down to AD8000, TI BUF602, and TI 
BUF608. For now I've picked the BUF602, because it's hand solderable.

>
> Note that a potential issue with modern hardware is 3.3V GPS systems 
> trying to drive 5V logic inputs.  That doesn't give you a whole lot of 
> margin for termination loss.  Fortunately, everything I need to plug 
> in to these days seems to be happy with 3.3V levels.
I've made the same observation. The RF ICs that I need to deliver 10/100 
MHz and pulse signals to are even fine with 3.3V / 2 = 1.65V.
>
> John
> ----
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Best regards,
Carsten