Low frequency oscillator - Page 1
Forum section: electronics / beginners

author: epaelectronic
author_rank: Contributor
timestamp: May 28, 2021, 09:30:27 pm
content: Hello.What would be the best way to get a stable 7-8Khz tunable oscillator?Preferably a voltage controlled one.What I have in mind currently is using a 4060 to divide down a 4Mhz crystal that is being slightly tuned with a varactor diode.Would this kind of arrangement get me to the 1Khz frequency span I need?

author: Zero999
author_rank: Super Contributor
timestamp: May 28, 2021, 09:39:29 pm
content: The 4046 is the easiest way, I know of.https://hackaday.com/2015/08/07/logic-noise-4046-voltage-controlled-oscillator-part-one/http://www.learningaboutelectronics.com/Articles/4046-voltage-controlled-oscillator-VCO-circuit.php

author: BrianHG
author_rank: Super Contributor
timestamp: May 28, 2021, 09:52:12 pm
content: Quote from: Zero999 on May 28, 2021, 09:39:29 pmYup, that's the cheapest and easiest. Also, if you only need 5v or 3.3v, you can use the 74HC4046 if it is easier to get. Just one word of warning, configure the chip with some headroom. IE, if you need 7-8Khz, tune it for a range from 6Khz to 9Khz. The IC can drift a little over time and temperature. Also, the cap which you use to set the oscillator frequency should be an NPO variant so that the capacitor value will be immune to ambient temperature.If you need precision, it can be complicatedly done with a cheap 1$ MCU. Such a solution properly done would guarantee that with the voltage input at 0v, you will get dead on 7KHz, and with the voltage in at VCC, you will get exactly 8KHz. Practically 0 drift as a reference crystal source's drift would be measured in the parts-per-million.

author: Benta
author_rank: Super Contributor
timestamp: May 28, 2021, 11:19:02 pm
content: Sorry, but I'm trying to wrap my mind around the suggestions here. I've worked a lot with the 4046 (call me "4046-lover"), but for this application it's the last chip in the world I'd select.The 4046 VCO is not very precise nor stable. It relies on the feedbacl loop in a PLL.BrianHG introduced the idea of a $1 MCU.A much better idea. Find one with ADC, use the timer to divide down to what you need controlled by the ADC voltage.No continuous frequency sweep, but should do what you need.Otherwise you'll need to provide MUCH more information on your needed spec.

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2021, 09:00:59 am
content: Well, I did say the easiest, not the best.Experience with this kind of request tells me, they normally want to avoid an MCU.How about the LTC6990?https://www.mouser.co.uk/datasheet/2/609/LTC6990-1776026.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2021, 11:01:10 am
content: The trim range for a cystal oscillator is too small, more like 0.1% of the frequency. It would be more an LC oscillator with varactor diode at the 4060.

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2021, 05:40:20 pm
content: Quote from: Kleinstein on May 29, 2021, 11:01:10 amOne solution could be to use two HF oscillators, a fixed one, modulated by a VCO and the beat frequency demodulated to give the low frequency output. It's not the simplest solution though.

author: Benta
author_rank: Super Contributor
timestamp: May 29, 2021, 06:26:05 pm
content: The right device would be the Philips NE566, but it's obsolete for decades (if it's a one-off, you might find some NOS somewhere).Here's a link to the datasheet, it might give you an idea on how to roll your own with a couple of opamps.https://html.alldatasheet.com/html-pdf/17982/PHILIPS/NE566/244/1/NE566.html

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2021, 09:48:57 pm
content: How about the LM339?The circuit is on page 10, figure 17 of the ST data sheet.https://www.st.com/resource/en/datasheet/lm139.pdf

author: Benta
author_rank: Super Contributor
timestamp: May 29, 2021, 10:54:46 pm
content: Basically a copy of the NE566. Could work.

author: emece67
author_rank: Frequent Contributor
timestamp: May 30, 2021, 01:41:17 pm
content: .

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2021, 04:23:37 am
content: Quote from: epaelectronic on May 28, 2021, 09:30:27 pmDividing down a crystal oscillator source will not allow much tuning range. If you need crystal stability, then two crystal oscillator outputs could be mixed to produce a stable variable low frequency output.The more modern option is a crystal oscillator driving a DDS generator which can take many forms.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2021, 10:49:40 am
content: Quote from: Benta on May 29, 2021, 10:54:46 pmYes, I suppose it is similar to the circuitry in the NE566. It's a shame the IC was discontinued, but obvious why.