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

[John Sloan]

> 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?

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).

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J. L. Sloan             Digital Aggregates Corporation
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