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Technical Topics => The RF Workbench => Topic started by: ThaDood on May 28, 2020, 1907 UTC

Title: Ya broke? Need a decent OSC? Look at trashed computers!
Post by: ThaDood on May 28, 2020, 1907 UTC

My bro-in-law 2 years ago wanted to trash his Y2K Dell desktop computer. So, I did rip & tear for the components and came across the 25MHz, SG-615P, CPU clock OSC. So, around Christmas 2019, I've built a PS circuit around it using a 9VDC wallwart, filter CAP's, an LM7805 +5VDC REG, another filter CAP, a 250uH choke, and by the info on-line, added a .1uF CAP before that IC. On the RF OUT I've built a homebrew adjustable Butterfly Filter and terminated that with just a 1k-Ohm resistor. And??? Nice clean sinewave on the 'scope, and the FREQ starts up right at 25.00000MHz, then sags to 24.999986MHz. Not bad, but I want better tolerance. However, I'd put that OSC project to the side for other 'Must fix' projects. Then, went back to it, after the scrubbed SpaceX manned launch. That OSC project still sags down to 24.999986MHz. So, I then looked at the AC ripple on the output of the 7805 REG  = .058V. Not bad DC, but did a 'What if' and added another 2,200uF CAP on the output of that REG. Now AC ripple is .001V. NICE!!!! And RF = 24.99997MHz. I then loaded to a 50 Ohm dummy load and now get 25.00000MHz. Huh.... Now that looks better, and useful for receiver aligning. So, if folks trash computers, especially by the roadside, and you're super cheap, (Like me!), there's some clock OSC possibilities there for use. And, the price is right! (Minus the Bob Barker.) 

Title: Re: Ya broke? Need a decent OSC? Look at trashed computers!
Post by: Stretchyman on May 30, 2020, 0917 UTC

$2.75 from DK!

+ I've got a programmer for them.

Str.