OK, OK. I'll wait, next time.
LOL - no worries. Actually, in that test mode, it's SUPPOSED to read 100MHz, so that part is working correctly. Perhaps it will come in from 99.9999 once it has cooked for a while - lord only knows when it had last been turned on.
I was initially going to us a Tek 545B that I acquired about a year ago in an old test gear haul and had never turned on. Unfortunately I wasn't recording when I powered it up, as it IMMEDIATELY expelled a nice cloud of smoke from its starboard side, and for the last few hours the pungent scent of eau de Allen Bradley has been slowly dissipating from the house (that or I'm acclimating to it!). I don't want to open the windows and lose all my conditioned air - it's humid as balls outside at present. The smoke cloud would have made for an amusing opening to a video - "we'll use this old vintage Tektronix scope to check the oscillator in the " <click> (flips mains switch) - - "...or not..." <excavates Tek 516> "...counter."
And yet another thing joins the repair queue...
Yup, you can't stop now. You're already two episodes into the repair. Gotta see what's causing it to read 100 MHz, too.
I should have known better and anticipated that was coming!
-Pat
the 99.999 is perfectly normal in an antique direct counter. it will eventually drift to 100.000. and then maybe to 100.001 (and back again)
its the 'ol lsd sampling error problem. see hp app note 200-4:
For a digital instrument like a counter, output readings are discrete (quantized) even though the inputs are continuous. Even for the case where the input quantity is perfectly stable, the counter’s readings may fluctuate. This fluctuation is due to quantization error (±1 count error).
The value of the LSD Displayed often is the same as the quantization error which represents the smallest non-zero change which may be observed on the display. Because of this, resolution and accuracy statements often specify quantization error as ± LSD Displayed.
Quantization error arises because the counter can’t count a fraction of a pulse — a pulse is either present during the gate time or it isn’t. The counter can’t detect that a pulse was present just after the gate closed. Additionally, since the opening of the counter’s main gate is not synchronized to the input signal, the quantization error may be in either direction. Consider a 2.5 Hz signal as shown in the figure. In case 1, the counter’s gate is open for 1 second and accumulates 2 counts — the display will show 2 Hz. In case 2, the same length gate accumulates 3 counts for a display of 3 Hz.
Although we say ±1 count, we do not mean that a particular measurement can vary by both + and – one count. The measurement can vary by one count at most. The reason that you have to say ±1 count is that from a single measurement, you don’t know a priori which way the next measurement will jump or if it will jump at all. So the specification has to include both possibilities.