Last night I had a bit more of a play with my osciallator. I wanted to look at a few things:
- The RF on the supply rail;
- The use of an Rd capacitor in the oscillator circuit; and
For RF on the supply rail I was reminded by both Dale (VK1DSH) and Dimitris (VK1SV) about the use of a bypass capacitor on the Vcc pins for ICs. I say reminded as I remember reading about these when I was playing with microcontrollers briefly, but had completely forgotten.
Being a best practice, I decided to follow suit and add them. So the slighly revised schematic is now – note C6 and C7:
On the use of Rd in oscillator circuit, maybe it’d help if I first show what I’m talking about. In the below is a red box where often an Rd resistor is added:
In the datasheet for the 74HC4060N they suggest trying a 2k2 resistor, but for my crystal this stopped oscillation. Indeed, testing last night showed 1k5 stopped oscillation and only once I went to 1k did it kick off. However, I found at 1k the waveform was a distorted sine wave and only once I got it down around 100R did things resume a reasonable sine wave. So in the end, I decided I’d do what many others before me have, and continue to leave it out.
But, an interesting exercise none the less, and first real good opportunity to play with my new resistance wheel.
Finally, I also wanted to see if this thing was stable. Not an overly detailed test, just something indicative. So, I hooked it up to the scope and ran it for just shy of two hours on the bench. In that time, the frequency counter on my ‘scope did not budge from 137.500kHz. So, at least to an accuracy of 1Hz I can say it seems pretty damn stable. 😉