PLL with 74S124 - Page 1
Forum section: electronics / projects

author: Dukov Ahzrukhal
author_rank: Regular Contributor
timestamp: August 21, 2020, 08:58:36 pm
content: Hello everyone, I am trying to design a PLL with the 74S124 VCO. The datasheet I found for this chip doesn't give any information on how the chip works, and I'm having trouble with one section affecting the other (there are two oscillators in each chip) even when one oscillator is disabled. Does anyone know how the chip works or have the internal schematics? I want to know what happens if I force the frequency control inputs to ground (the datasheet says the minimum voltage on these pins is 1v) or what happens if I short the timing capacitor pins on one side. The enable inputs simply disable the output of the oscillators, but it doesn't really stop them which is what I want to do. I would experiment to find out myself but I only have a few chips and I suspect forcing the pins to ground would damage the chip. Please feel free to suggest other methods and chips for my PLL. I am trying to synthesize a 10.7MHz FM signal to tune the IF section of FM radio receivers. I tried simply tuning the oscillators to 10.7MHz without a PLL circuit but the results were not good at all.

author: Benta
author_rank: Super Contributor
timestamp: August 21, 2020, 09:26:23 pm
content: Quote from: Dukov Ahzrukhal on August 21, 2020, 08:58:36 pmYou can't just stop and start a VCO. It requires quite some time to start oscillating, get up to speed and be stable. I can't be any more specific, as I've no idea what you're trying to achieve.But dinosaur technology (74S) is perhaps not the way forward.

author: BrianHG
author_rank: Super Contributor
timestamp: August 21, 2020, 10:15:54 pm
content: Why not just buy a ttl/cmos output 10.7 Mhz oscillator.Run it's output through an RCL filter as well as one of those cheap orange 3 pin 10.7MHz ceramic filters from an old FM radio to change that square wave output into a nice sine wave.Bingo, a crystal tuned 10.7MHz reference oscillator.

author: Dukov Ahzrukhal
author_rank: Regular Contributor
timestamp: August 21, 2020, 11:20:20 pm
content: There are two oscillators on the chip, and I want one of them to be permanently stopped. This is so it does not affect the other oscillator on the chip.I can use a crystal oscillator as a reference, but since I'm trying to synthesize an FM signal the crystal oscillator cannot be the only oscillator as I have no way of modulating its frequency.

author: Bud
author_rank: Super Contributor
timestamp: August 22, 2020, 01:09:41 am
content: You can ground the frequency control input, nothing bad will happen to it. The datasheet has an equivalent schematic of the input.

author: Dukov Ahzrukhal
author_rank: Regular Contributor
timestamp: August 22, 2020, 01:50:46 am
content: Thanks for that suggestion, I tried grounding the frequency control input for one oscillator and the chip was not damaged. Unfortunately the oscillator did not stop or even slow down significantly. Would it be safe to replace the timing capacitor with a short?Edit: I have attached oscilloscope screenshots showing the voltages on the timing capacitor pins.

author: BrianHG
author_rank: Super Contributor
timestamp: August 22, 2020, 02:20:12 am
content: How about something like this?http://circuit-zone.com/index.php?electronic_project=665Just filter the output with a second 10.7MHz filter is you want a sine wave instead of a square wave and get rid of the 96.3Mhz 9th overtone.It would probably work with a 74HC04, or 74HC14 as well.

author: Dukov Ahzrukhal
author_rank: Regular Contributor
timestamp: September 13, 2020, 11:59:32 pm
content: I decided to risk my chip and did some experiments to see if I could safely stop one of the oscillators. It turned out to be pretty easy, just connecting a 220 ohm resistor instead of the timing capacitor is enough to stop the oscillator. The current consumption of the chip does not seem to change at all and the oscillator still works if I remove the resistor. Stopping one of the oscillators does improve the stability of the other one.