[time-nuts] Re: [OT] MB506 pre-scaler module ??

[ed breya]

It's a bit tricky to use prescalers for arbitrary input signals. As 
others have mentioned, they self-oscillate when there isn't a valid 
input signal. This is not only a possible, but nearly certain 
characteristic due to the (typically) ECL input amplifier being biased 
right at the logic threshold, followed by lots of gain, then divided by 
flip-flops. These were typically used in PLL synthesizers over limited 
frequency ranges, with known, adequate power levels.

If you are counting strong signals at well defined DC levels, you can 
use ECL directly with proper biasing. For a general purpose counter, you 
want lots of sensitivity and wide frequency range, so the usual 
prescaler with self-biasing and AC coupling is a simple way to go. You 
just have to be aware of the limitations. You can eliminate the 
oscillation tendency by giving up some sensitivity, but it's easiest to 
just ignore it unless it causes trouble.

I'm quite familiar with the MB series and other prescalers, and have 
used them in many projects. All the prescalers I have were salvaged from 
old equipment, and I believe that most are long obsolete, since their 
function has been mostly absorbed into the modern PLL chips.

Nowadays you can get 100-series ECL parts that can toggle to several 
GHz, and in various gates, FFs, and counter types. You could build say, 
a divide by 10 or other values with a programmable counter, I believe up 
to a couple GHz. I don't recall all the parts and specs, but they're out 
there. If you want to make it for arbitrary inputs, you'd first buffer 
it with an AC coupled, self-biased line receiver cascade before 
counting, and you'd be right back to a self-oscillating - but much more 
versatile - prescaler.

So, if you can get this fully assembled, connectorized, ready to go 
MB506 module for seven pounds, just do it - maybe get several - if the 
specs are suitable for your needs. It beats the hell out a DIY one, 
except for not having a nice convenient decade divide ratio. You should 
look up the MB506 datasheet to see the actual capabilities - you can 
usually extend the low end, for instance, by adding more input coupling 
capacitance. Anyway, I don't know if the MB506 is even made anymore, but 
if it is, I'd guess it alone would cost more than this module.

There used to be all sorts of prescaler ICs with various ratios, 
including 10, so you might find NOS parts, but with the grief of 
building something. There's also an old trick in doing counters with 
binary prescalers, which is to change the counter clock frequency so the 
net overall divide ratio is a nice decade value, but making the clock 
and working the decimal points gets tricky. Also, if the binary divide 
is big, the counter is correspondingly very slow.

Of course, if you want to get fancier, you can make a PLL/VCO based 
converter that replicates the unknown input frequency at say 1/10th. But 
you'll still likely have the same sort of prescaler deal involved, 
inside of a PLL chip.

Ed