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

[dk4ww at gmx.net]

Hi Folks,

read carefully ITU G.703 for 1PPS 50 Ohm or v.11 interface on RJ45 Con.

Cheers.


Uwe


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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
> 
> > On Aug 21, 2022, at 11:56 PM, Carsten Andrich via time-nuts <time-nuts at lists.febo.com> wrote:
> > 
> > On 22.08.22 00:24, John Sloan via time-nuts wrote:
> >> I’ve been using this SparkFun ZED-F9T board with pretty good luck for a while now. It has an SMA for the active
> >> antenna, two SMAs for TP1 and TP2, a micro-USB for power and serial port, and SparkFun’s “Qwiic” I2C connector
> >> (which I’ve never used on any of their boards). I’m currently running this board flashed with TP1 set for 1PPS and
> >> TP2 set for 10MHz. However, I don’t really have any instruments good enough to render an opinion on how well this
> >> works.
> >> 
> >> https://www.sparkfun.com/products/18774 <https://www.sparkfun.com/products/18774>
> > 
> > Sparkfun's ZED-F9T breakout board interfaces look like a step up from u-blox' RCB-F9T. However, my reason to use the RCB-F9T is simple: I've had a bunch of these for years and don't want to throw them away. Also, I need to feed the timepulse signals into a 50 Ohm load and the ZED-F9T output pins are only rated for max. 5 mA [1]. Except for a 33 Ohm resistor, the Sparkfun board routes both timepulse pins directly to the SMA connectors [2]. I doubt that results in 50 Ohm source impedance required to properly drive a high impedance load via a 50 Ohm transmission line.
> > 
> > Have you had any issues with unmatched transmission line effects?
> > 
> > Best regards,
> > Carsten
> > 
> > [1] https://content.u-blox.com/sites/default/files/ZED-F9T-00B_DataSheet_UBX-18053713.pdf#page=13
> > [2] https://cdn.sparkfun.com/assets/6/c/2/5/b/SparkFun_GNSS_Timing-ZED-F9T_Schematic.pdf
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