On a sunny day (Sat, 27 Jun 2020 16:38:07 +0100) it happened David Higton
 wrote in :

>In message 
>          Jan Panteltje  wrote:
>
>> 30 MHz has a wavelength of 10 meter.
>> for distances on an eurocard (100x160 mm) board the transmission line issue
>> is not very significant.
>
>A common misconception is to relate these issues to the repetition rate.
>It can be one cycle per fortnight, but if the transition times are short
>(roughly a nanosecond or less),

OP was discusing 74LS

Transitions are slower and it does not matter much if those are distorted.
Measured that effect.

>what arrives at the destination may have
>more edges in it than were sent, unless the line is appropriately
>terminated.

74LS inputs are not terminated.

There is a lot of crap going around about RF.
Here is a 1.something GHz board connected to raspi 1 GPIO:
 http://panteltje.com/panteltje/raspberry_pi_dvb-s_transmitter/

scroll down for picture of the board backside.
Where it is about 1 GHz (~ 30 cm wave length!!!) connections are short and
direct,
not wired.
Wire length becomes important, 1/4 wavelength wires can do funny things.
For the rest it is all simple.

Some interfaces for example between VCO an mixer and QAM modulator chip are 50
Ohm.

GPIO edges are FAST, yet the flat cable causes no problems and is in no way
'terminated'.

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