John, you gave me a few options and food for thought that has
led to a solution. Not an "ideal" one, but what is?
I want to build a cheap (less than $100) but rather special
type of EEG biofeedback unit designed to allow one to learn
how to produce 4.0 Hz theta, 7.0 Hz theta, 8.3 Hz alpha, and
20.00 Hz beta waves; and possibly two or three other
frequencies if the need arises. commercial units usually run
around a thousand bucks!
I started with analog filters but finding precise resistor
values and matching capacitors to be a major drag. I replaced
the analog filters with the tunable LTC-1164-8a bandpass
filter. It's not good for anything high frequency (read that
as greater than 5 kHz) but it should be perfect for this
application. However, the bandpass frequency is determined by
the clock frequency: 500 Hz in equals 5 Hz out in a 100:1
ratio.
I intended to use the VCO in a CD4046 as a clock, but it
wasn't stable; drifting about 1 or 2 Hz every few seconds. For
most other applications, a slight deviation in clock frequency
wouldn't matter, but here a one or two Hz drift would mean the
filter might cycle between 5Hz and 5.01 Hz sporadically. For
this biofeedback purpose, that wouldn't be acceptable in the
least. Hence the need for a stable square wave clock source.
And to further complicate things a bit, the LTC1164-8a
requires that square wave to have a 50% duty cycle.
There are a lot of oscillators out there, but so far I can't
find any that's highly stable without using crystals. I hear
he old tube-based Franklin oscillator was very frequency
stable but I don't know about the newer FET-based version.
Going with microprocessors would add to the complexity, time,
work, and cost— But at least crystals are a relatively cheap
sure thing.
Ron
And if I understand you right, John, an awful lot of counter
chips; at least fifteen, and more if I need to add additional
clock frequencies. That makes what starts out as a simple
project immensely complicated with, ironically, the clock for
the filter becoming far more complicated than the rest of the
project.
I think, while it's more expensive to purchase and use several
custom crystals, it would be easier to use crystals like 1.000
MHz, 1.400 MHz, 1.660 MHz, etc and divide those frequencies by
10/10/10/2— thus requiring overall only four chips. Three
chips (minus the oscillator), if I use double decade counters.
I believe in that basic engineering tenet, "keep it simple,
[stupid]." It started as a simple project and I really want to
keep it that way.
However, thanks the advice.
Ron
I did not make a judgment that this method was a good solution
to your
problem. I answered your question about how multiple
frequencies
could be derived from a single crystal oscillator. If the
technique
is some use to you, that is good.
I don't even know that you need the stability of crystal
oscillators
for your frequencies. For all I know, a few 555 timers might
be a
better solution to your problem.