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

[Carsten Andrich]

Hi Bob, Uwe,

thanks for the extensive summary and the ITU G.703 reference!

As an RF engineer by profession, I'm compelled to not (knowingly) 
mistreat transmission lines :D

3.3V LVCMOS output level is determined by the ZED-F9T, so the 
straightforward solution was to add a hand-solderable buffer opamp (TI 
BUF602) that can drive +17 dBm into a 50 Ohm load. +17 dBm is already 
quite a strong signal for most 50 Ohm RF measurement gear, so another 
reason for being able to drive 50 Ohm loads is the option to use 
standard 50 Ohm attenuators. Alternatively, the buffer amp enables 50 
Ohm source termination to reliably drive 3.3V into high-impedance loads. 
A good coincidence that the resulting output levels are ITU G.703 
compliant :)

Best regards,
Carsten

On 22.08.22 21:00, Uwe via time-nuts wrote:
> Hi Folks,
>
> read carefully ITU G.703 for 1PPS 50 Ohm or v.11 interface on RJ45 Con.
>
> Cheers.
>
>
> Uwe
>
>
> Gesendet mit der mobilen Mail App
>
> Am 22.08.22 um 20:47 schrieb Bob kb8tq via time-nuts
>
>> Hi
>>
>> The issue of terminating PPS signals and how to do it “right” has been debated
>> a lot on this list. A quick summary:
>>
>> 1) Some sort of termination is a really good idea. That termination should match
>> the coax being used.
>>
>> 2) Some devices you might wish to drive already *have* a 50 ohm termination in
>> them.
>>
>> 3) Most “modern” devices expect you to hit some sort of logic level with your
>> drive signal. Is that 50% of 5V, 3.3V, 2.5V? Is it 0.8V TTL? Unfortunately each
>> device and design is a bit different.
>>
>> 4) Will your “gizmo” that is looking for 3.3V ( or worse 2.5V) logic be ok with a
>> 5V peak signal? One would *hope* it will. There are some examples where the
>> answer is no …
>>
>> So what to do?
>>
>> 1) If you terminate with 50 ohms at the source, you can drive unterminated
>> loads pretty well. Logic levels tend to line up. “Un-terminating” pieces of commercial
>> gear simply to allow a terminated drive might not be a fun thing.
>>
>> 2) You can terminate both ends of the coax. This makes for the best match
>> and the least ring on the line. It also means quite a bit of drive current. If you want
>> 5V logic levels, you need 10V drive. Good luck with your 3.3V unterminated device
>> plugged into that output by accident ….
>>
>> 3) You can just terminate the load *and* drive the line from a low impedance / high
>> current driver. Hitting a full ( so 90% ) logic swing this way at 5V into a load termination
>> takes a pretty hefty driver. It also has to be fast and low jitter. Unfortunately this is
>> what a lot of commercial gear seems to expect you to do.
>>
>> If you have control over all the sources and loads, then picking a logic voltage is the
>> first step. Assuming there is a common ground, power all the sources with that
>> voltage. Make sure all the loads can tolerate / accept that voltage. Source only
>> 50 ohm termination likely would be the rational way to set things up.
>>
>> One can get “buffer” rated logic gates. Be sure to check things like the package
>> dissipation / current before buying a SOT-23 sized gizmo with 8 gates in it ( and
>> good luck mounting that BGA part … yikes …). Yes the duty cycle is low with
>> 1 us pulses. Not so much with 50% duty cycle. Either way, package current may
>> limit you.
>>
>> Parallel up as many gates as needed to hit the required current. If you are source
>> terminating a source load resistor goes in series with each gate output. With load
>> only termination, very small resistors are typically used (or no resistors at all). To hit
>> full 5V logic into load only terminated 50 ohms, you will use a lot of gates ….
>>
>> Fun !!
>>
>> Bob
>>