If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
I'm making a DDS-based function generator and I'd need to make an output stage for the DDS-chip (AD9833) which outputs around 200mV p-p signal. The frequency range the DDS-board generates is from about 150Hz to 8MHz
I'd want two outputs, one for sine and one for 50% duty square wave. I know the DDS-chip can generate square wave by itself but I cant really use that feature due to how the LC-filter after the DDS-chip is built plus I dont have a programmer to program the pic with. So I need to generate the square wave from the sine.
The sinewave is pretty easily amplified with a noninverting opamp amplifier (I was looking at EL2044 as the opamp) and AC-coupled to yield a nice 5V p-p signal.
The real problem is how to make the square wave? The problem lies in that the signal from the DDS-chip varies a bit with frequency, the amplitude gets smaller in both of the extreme ends of the frequency range. How can I create a fairly stable 50% duty square wave from this signal?
Using a comparator for doing it is clear but how do I make the comparator to trigger at halfway of the sinewave cycles so it would consistently keep 50% duty even when the input signal varies a bit in amplitude? I'd need to somehow change the comparator reference voltage so it would change according to input signal amplitude but I cant think of a good and easy way to do this. Any suggestions?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
You might want to consider using the AD9834 DDS as this has an internal self-biasing comparator if I remember correctly and is also a more modern part.
As you correctly said, the squarewave should be generated using a comparator acting on the analogue sinewave _AFTER_ the lowpass filter. Any digital squarewave generated directly by the DDS has period jitter which becomes more significant at high frequencies, and you certainly don't want to put the squarewave through the lowpass filter!
If your eliptic reconstruction filter is designed properly then the period jitter on a squarewave generated the proper way with a fast comparator should be very low up to F_clock / 3.
Registered Member #538
Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181
I also thought about biasing the sinewave around 0v, then if the sinewave amplitude changes the sinewave should still be in the same position in relation to ground and you could make the comparator trigger at 0v. Another idea was to generate the reference voltage with a RC-filter but I dont think this can be made to work on such a wide frequency range.
I cant really change the DDS-part because its made from a kit, the board is meant as a VFO for a radioreceiver/transmitter (check out )
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
How to create a suitable reference voltage for a comparator?
Even if you don't use it, you can read the AD9850 datasheet to see how its built-in comparator works.