5 V oscillator output to GCLK

Just to belabor a point: When the output side of a transmission line is parallel-terminated, there is no requirement to also properly terminate the driving side. Using a "wrong" series resistor to adjust the amplitude is ok, since there is no signal coming back to the driver, and thus no need for proper termination at that end. Peter Alfke

essentially

No. It is a transmission line with an impedance matched driver. You can get any frequency accross that setup that you like.

The setup will not change the rise time of the signal. The slew rate will be halved, because the voltage swing is halved, but that's it. The receiver will see the same waveform as the source with halve the voltage.

But other posters where correct with noting that the oscillator needs to be capable to drive a 100R transmission line, which might not be the case.

Kolja Sulimma

If matched with the characteristic impedance it works like a transmission line, if not it looks like a capacitor or inductor depending on the termination.

The phone system (US, anyway) seems to use 600 ohm termination on about 100 ohm UTP cable. The result is that the cable looks like a capacitor and, for long lines, results in a high frequency drop off.

The telephone solution is loading coils that increase the series inductance (on average) resulting in a flatter response to the required 4kHz, and a sharp drop after that.

The effect also shows up in antenna design when the frequency doesn't match the appropriate length for the antenna elements.

As previously stated, though, it is only necessary to match one end. If the sink impedance is matched the voltage will be reduced appropriately by the series source resistor and the cable impedance as a voltage divider.

-- glen