Computer speedometers

[Tony Duell]

> > Well sure, why not?  If you had an all static-CMOS MPU, you could even take
> > one of those big rheostats used for stage lighting and hook it up to a VFO
> > to provide a variable clock to your CPU.  "Captain, if I turn it up any
> > higher, it'll blow for sure!".
> 
> A PDP that'll go all the way up to 11? :-)

Actually, the PDP11/45 has 2 clock circuits. One is the crystal clock 
(33MHz) for normal running. The other is an RC clock, with a preset pot 
to set the frequency. You can select the latter by sticking a KM11 
maintenance board in and flipping the appropriate swtich on said board. 
The RC clock will go faster than 33MHz, the idea being that you ensure 
that the machine will run at the higher speed, if so, then it will 
certainly work at 33MHz.

The HP9825 (and the 9831) use a dual VCO chip for the CPU clock. One of 
the osciallators runs at fixed frequency, the output of that is turned 
into an approximately triangular wave, and used to freqeuncy-modulate the 
other one. The reason, of course, is to spread out the spectrum of the RF 
noise coming from the machine. This is mentioned in passing in the 
board-swapper service manual, and is fairly clear from the schematics.

-tony