Hi all,

I have recently built this clock generator using a counter hooked up as a frequency divider, and a flip flop to clean the output.



It works well but has a number of drawbacks:
- because I use the borrow output as a clock, I need to clean up the duty cycle with a flip flop, which divides the frequency by 2
- the maximum counting frequency of the 74HC193 is 20MHz, which means I'm limited to 10MHz output clock. My goal is to reach at least 14MHz
- going to higher speed parts like a 74AC161 means a division by 4, because the '161 carry output doesn't toggle when loading 1111. I would need a 56Mhz oscillator, which is a no go on a breadboard
- and finally, the steps are divisors, meaning the intervals are not regular (I go from 14Mhz to 7Mhz in one step).

So, in order to solve this, I've been considering a PLL clock generator based on something like the 74HC7046. I've never used one, and resources on the net are scarce besides general overviews of PLLs.

This would be a general idea: Using a 1MHz master clock and my divider circuit, I get to increase the frequency by 1Mhz increments, until 16Mhz. No large frequencies are needed on the breadboard and no high-speed parts either.



This is what I have pieced together from the datasheet. I'm probably missing things as it does not work. I've chosen R1 and C1 from Figure 13 to get a 1MHz center frequency. Not sure if that's the right thing to do.



Output I get with the divisor set to 2 is indeed 2Mhz, but the signal is pretty bad, and I loose the lock by hovering my hand... Also doesn't work with a higher divider.



Has anyone built such a thing before, or has a link to good resources on how to build such a circuit? Any help would be much appreciated!

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BB816 Computer YouTube series

From your schematic it looks like you're missing the loop filter. If you look at page 7 of the data sheet here:

https://pdf1.alldatasheet.com/datasheet ... 7046A.html

you can see that you shouldn't just connect the phase detector output (pin 13) to the VCO control input (pin 9). Instead you should have a resistor from 13 to 9, and a series RC combination from pin 9 to ground (R3, R4 and C2 in the data sheet schematic).

Duh, after reading the datasheet again today that is indeed obvious. Now I need to figure out how to select those components.
The datasheet you linked seems to have a good step-by-step in the "PLL design example" section, so that should also help! The TI one I was using didn't have that.

I'll experiment more and report back.

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BB816 Computer YouTube series