Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

pulses of high frequency on a signal of lower frequency in a relaxation oscillator

Status
Not open for further replies.

bondadoso

Newbie level 4
Joined
Nov 11, 2010
Messages
6
Helped
2
Reputation
4
Reaction score
2
Trophy points
1,283
Activity points
1,317
Capture1.PNGCapture.PNG

i made the relaxation oscillator of the picture with a tank circuit L=100uh C=1uf an it is oscillating but with pulses of higher frequency betwen transition of the oscillating frequency the picture shows, i don't know if it is for bad power source or bad gain or what this came from , because i need to clean up the signal and get a clean frequency oscillation, please help
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,864
Helped
14,122
Reputation
28,501
Reaction score
12,804
Trophy points
1,393
Location
Bochum, Germany
Activity points
277,805
The reason is simple. The oscillator circuit which is wellknown from LC-meters doesn't work at this high resonance frequency. With higher L and C values you get continuous oscillation. A considerably faster comparator would be needed to work with the given dimensioning.
 
  • Like
Reactions: tpetar

    tpetar

    points: 2
    Helpful Answer Positive Rating

dick_freebird

Advanced Member level 5
Joined
Mar 4, 2008
Messages
7,037
Helped
2,055
Reputation
4,114
Reaction score
1,903
Trophy points
1,393
Location
USA
Activity points
56,410
I believe this is just comparator chatter. You can add enough
hysteresis to cover up the various node-bounces that are
kicked by the first comparator switching event, of you can
move to a two-comparators, SRFF control that "ratchets"
and won't reswitch.
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,864
Helped
14,122
Reputation
28,501
Reaction score
12,804
Trophy points
1,393
Location
Bochum, Germany
Activity points
277,805
I believe this is just comparator chatter. You can add enough
hysteresis to cover up the various node-bounces that are
kicked by the first comparator switching event, of you can
move to a two-comparators, SRFF control that "ratchets"
and won't reswitch.
I fear, adding hysteresis will stop the oscillator from intended operation. In reasonable operation range, the circuit is working as harmonic rather than relaxation oscillator, by the way.

There are previous threads discussing the oscillator topology:
https://www.edaboard.com/thread126119.html
https://www.edaboard.com/thread231205.html
 
  • Like
Reactions: tpetar

    tpetar

    points: 2
    Helpful Answer Positive Rating

crutschow

Advanced Member level 5
Joined
Feb 22, 2012
Messages
3,789
Helped
903
Reputation
1,804
Reaction score
888
Trophy points
1,393
Location
L.A. USA Zulu -8
Activity points
21,352
What period oscillation do you want?

With C1 = 0.1µF and L1 = 2mH my LTspice simulation gave a pulse period of about 90µs.
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,864
Helped
14,122
Reputation
28,501
Reaction score
12,804
Trophy points
1,393
Location
Bochum, Germany
Activity points
277,805
The circuit operates continuously over a wide frequency range. But above a certain freqquency, LM311 gain isn't sufficient to maintain continuous oscillations. The circuit falls into relaxation mode with each edge triggering a decaying oscillation at the LC resonance. It appears as a pulse train at the output, as shown in the initial post.
 
  • Like
Reactions: tpetar

    tpetar

    points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top