[time-nuts] Transmission line question

Alan Melia alan.melia at btinternet.com
Wed Apr 6 22:43:43 UTC 2011


Hi Don excuse me I was thinking RF :-))  That should be OK  Though it
depends what you are trying to do bus termination is usually slightly
different to RF. At least you should not get nasty reflections from the ends
like that. There may come a problem if you send a pulse from a 100
termination into the network. There will be a little diggle and reflection
as it sees the 50 ohm pin as a small mis-match. But if driving out only from
the middle I see no problem. (but then I might just be wrong :-))  )

Alan

----- Original Message ----- 
From: "Don Otknow" <donald.otknow at gmail.com>
To: <time-nuts at febo.com>
Sent: Wednesday, April 06, 2011 11:05 PM
Subject: Re: [time-nuts] Transmission line question


> Just for the record I meant pin as in the metal contact of a device, not a
> PIN diode. So I need the impedance the pin "see's" (that of the
transmission
> line(s)) to be the same as the output impedance of the pin.
>
> Here's an extrapolation on the original question. If this is a high speed
> digital signal, with frequency elements ranging from say 133 MHz to 1GHz,
> then what I am really worried about, given the trace lengths (maybe 2 cm
> each way) is the high frequency components getting reflected or not sent
> down the line properly. If my source impedance from the the device is 50
> ohms, the lines are each 100 ohms, and the terminations at each end are
100
> ohms (probably AC terminated, but let's just say 100 ohms for simplicity),
> am I setting myself up for the best possible signal integrity?
>
> Here's a primitive diagram of the setup
>
>                                  100 ohm line      100 ohm line
> 100 ohm term. to gnd _________________________100 ohm termination to gnd
>                                                        |
>                                                        |
>                                                        | 50 ohm source
>                                                        |
>
>
> Donald
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