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

[Bob kb8tq]

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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