DDS module
I have a few of those $5 AD9850 DDS modules from China. I'm going to use
one to replace a crystal in a transmitter. I attached it to a TI MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is. Using
cellophane tape, I put a few pieces of foam around the oscillator can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over 24 hours
now. So far, the frequency has held to within 0.01-0.08 of the programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little module works
and how stable it is. There is an Amateur in town who has been using
several of these modules to drive WSPR and QRSS transmitters. He has done
up a poor man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
Hi Joe,
It is good that they are that stable, but what is the phase noise like? If a
ham across town is using them for WSPR and QRSS they are most probably OK in
that regard, but not all DDS modules are.
Best 73's
Nic
VK2KXN / VK5ZAT
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Joseph Gray
Sent: Sunday, 21 July 2013 2:04 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] DDS module
I have a few of those $5 AD9850 DDS modules from China. I'm going to use one
to replace a crystal in a transmitter. I attached it to a TI MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is. Using
cellophane tape, I put a few pieces of foam around the oscillator can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over 24 hours
now. So far, the frequency has held to within 0.01-0.08 of the programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little module works
and how stable it is. There is an Amateur in town who has been using several
of these modules to drive WSPR and QRSS transmitters. He has done up a poor
man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
For the AD9850 (as well as the higher-tech more-bit-resolution modules)
there are programming words/freqs that are awful with a lot of close-in
spurs, and others that are comparatively clean (well, maybe just as many
spurs by some measure but they are far far away).
If you go to a DDS with higher base frequency and more bits resolution
(e.g. comparing my AD9954 DDS with my AD9850 synth) these "bad freq words"
become less common/less severe.
These frequency-dependent close-in spurs show up in a receiver, as sudden
appearance of raspies at certain receiving freqs while others just a
fraction of a Hz away sound clean.
Several ham designs for the AD9850 follow it with a simple one-to-one
tracking PLL for some cleanup.
There are some AD app notes that hint the patterns of the words with lots
of close in spurs but I've never found an easy programmatic way to skip
over them for the better words.
http://www.analog.com/static/imported-files/application_notes/131351807AN_927.pdf
especially
section titled "Predicting and Exploiting Spur "Sweet Spots" in a DDS'
Tuning Range".
Analog.com website has some tools to explore spurs vs tuning word:
http://designtools.analog.com/dtDDSWeb/dtDDSMain.aspx
Tim N3QE
On Sun, Jul 21, 2013 at 4:21 AM, Nic McLean mcleannb@bigpond.com wrote:
Hi Joe,
It is good that they are that stable, but what is the phase noise like? If
a
ham across town is using them for WSPR and QRSS they are most probably OK
in
that regard, but not all DDS modules are.
Best 73's
Nic
VK2KXN / VK5ZAT
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Joseph Gray
Sent: Sunday, 21 July 2013 2:04 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] DDS module
I have a few of those $5 AD9850 DDS modules from China. I'm going to use
one
to replace a crystal in a transmitter. I attached it to a TI MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is. Using
cellophane tape, I put a few pieces of foam around the oscillator can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over 24 hours
now. So far, the frequency has held to within 0.01-0.08 of the programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little module works
and how stable it is. There is an Amateur in town who has been using
several
of these modules to drive WSPR and QRSS transmitters. He has done up a poor
man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Good morning Joe (and all),
I have been playing around with a few of these DDS modules as well as a
couple of the lower end DDS chips (i.e. AD9834, AD9850, AD9851) and they
do work well but they are not perfect.
I have a Fox Delta Antenna SWR analyzer which has an AD9850 and a 125Mhz
surface mount oscillator and although I have not done any extensive
testing of the devices stability it is relatively stable to a N3ZI
design DDS VFO which uses a AD9834 and a 80Mhz dip can (ie. not surface
mount) oscillator.
The AD9834/80Mhz dip can oscillator DDS wanders quite a bit and when I
listen to it on a receiver it sounds "raspy". I have simple heater
attached to the dip can - it is just as bad with as without. There has
been some discussion on the Knights QRSS reflector on the subject. If
you are not yet a member, you can join here:
http://mail.cnts.be/mailman/listinfo/knightsqrss_cnts.be
I am guessing "the amateur in town" is Dave WA5DJJ. He and I have
exchanged a few emails on this subject (and others). There is a fellow
in Australia as well which I have exchanged a few ideas.
One common method of taming some of wandering habits of the inexpensive
DDS modules is to put a small coin on the dip can oscillator. I noted
this behavior as well some time ago. I can't quite put my finger on why
it helps. Some postulate that it is adding just enough thermal mass to
keep drift down when there is moving air around the can oscillator.
What I have noticed is that the DDS with the surface mount oscillator is
more stable (remember, no extensive and exhaustive testing at this
point, just simple observation). I have also noticed that if I have one
of the DDS modules on my bench that when I monitor the output of said
device that tapping on the desk also seems to cause some wandering of
the synthesized frequency indicating that the the dip can oscillator is
possibly microphonic in some way.
It seems at this point as though the SMD oscillators are a better choice
between these two low cost options. Of course, TCXO or OCXO would be a
better choice.
I stumbled across an Analog Devices application note AN-419 title "A
Discrete Low Phase Noise 125Mhz Crystal Oscillator for the AD9850
Complete Direct Digital Synthesizer" which has some interesting tid bits
of information. I am using this as a starting point for some
experimenting with reference clock sources for DDS's. One bit of
information that no doubt is in the DDS chip's data sheet is that the
reference clock needs to be at least 3 Vp-p and centered on 1/2 VDD.
Seems so obvious but makes me wonder if adding a small coin to top of
the DIP can oscillator is not only helping thermal wise but is somehow
helping improve phase noise.
I think we have tended to think of a DDS as being "it". That is, we can
generate something stable and precise digitally and it will be good no
matter what. What we (I) forgot when first exploring the devices was
that what we get out the synthesized end is (relatively speaking) no
better than what we put in (i.e. the reference clock), another example
of GIGO.
cheers, Graham ve3gtc
On 13-07-21 04:03 AM, Joseph Gray wrote:
I have a few of those $5 AD9850 DDS modules from China. I'm going to use
one to replace a crystal in a transmitter. I attached it to a TI MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is. Using
cellophane tape, I put a few pieces of foam around the oscillator can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over 24 hours
now. So far, the frequency has held to within 0.01-0.08 of the programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little module works
and how stable it is. There is an Amateur in town who has been using
several of these modules to drive WSPR and QRSS transmitters. He has done
up a poor man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
Hi
If the DDS is acting pretty much as a divider you don't get a lot of spurs. The rest of the time there are spurs somewhere in the output spectrum. Put another way, there are thousands of "bad" tuning words for every good one. The good ones are evenly spaced over the range at a spacing determined by the DAC width and the phase truncation process.
Bob
On Jul 21, 2013, at 7:07 AM, Tim Shoppa tshoppa@gmail.com wrote:
For the AD9850 (as well as the higher-tech more-bit-resolution modules)
there are programming words/freqs that are awful with a lot of close-in
spurs, and others that are comparatively clean (well, maybe just as many
spurs by some measure but they are far far away).
If you go to a DDS with higher base frequency and more bits resolution
(e.g. comparing my AD9954 DDS with my AD9850 synth) these "bad freq words"
become less common/less severe.
These frequency-dependent close-in spurs show up in a receiver, as sudden
appearance of raspies at certain receiving freqs while others just a
fraction of a Hz away sound clean.
Several ham designs for the AD9850 follow it with a simple one-to-one
tracking PLL for some cleanup.
There are some AD app notes that hint the patterns of the words with lots
of close in spurs but I've never found an easy programmatic way to skip
over them for the better words.
http://www.analog.com/static/imported-files/application_notes/131351807AN_927.pdf
especially
section titled "Predicting and Exploiting Spur "Sweet Spots" in a DDS'
Tuning Range".
Analog.com website has some tools to explore spurs vs tuning word:
http://designtools.analog.com/dtDDSWeb/dtDDSMain.aspx
Tim N3QE
On Sun, Jul 21, 2013 at 4:21 AM, Nic McLean mcleannb@bigpond.com wrote:
Hi Joe,
It is good that they are that stable, but what is the phase noise like? If
a
ham across town is using them for WSPR and QRSS they are most probably OK
in
that regard, but not all DDS modules are.
Best 73's
Nic
VK2KXN / VK5ZAT
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Joseph Gray
Sent: Sunday, 21 July 2013 2:04 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] DDS module
I have a few of those $5 AD9850 DDS modules from China. I'm going to use
one
to replace a crystal in a transmitter. I attached it to a TI MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is. Using
cellophane tape, I put a few pieces of foam around the oscillator can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over 24 hours
now. So far, the frequency has held to within 0.01-0.08 of the programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little module works
and how stable it is. There is an Amateur in town who has been using
several
of these modules to drive WSPR and QRSS transmitters. He has done up a poor
man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Some of the small surface mount canned oscillators are actually pll and not very good at that. I have a page about it somewhere on my web site.
Didier
KO4BB.com
Graham planophore@aei.ca wrote:
Good morning Joe (and all),
I have been playing around with a few of these DDS modules as well as a
couple of the lower end DDS chips (i.e. AD9834, AD9850, AD9851) and
they
do work well but they are not perfect.
I have a Fox Delta Antenna SWR analyzer which has an AD9850 and a
125Mhz
surface mount oscillator and although I have not done any extensive
testing of the devices stability it is relatively stable to a N3ZI
design DDS VFO which uses a AD9834 and a 80Mhz dip can (ie. not surface
mount) oscillator.
The AD9834/80Mhz dip can oscillator DDS wanders quite a bit and when I
listen to it on a receiver it sounds "raspy". I have simple heater
attached to the dip can - it is just as bad with as without. There has
been some discussion on the Knights QRSS reflector on the subject. If
you are not yet a member, you can join here:
http://mail.cnts.be/mailman/listinfo/knightsqrss_cnts.be
I am guessing "the amateur in town" is Dave WA5DJJ. He and I have
exchanged a few emails on this subject (and others). There is a fellow
in Australia as well which I have exchanged a few ideas.
One common method of taming some of wandering habits of the inexpensive
DDS modules is to put a small coin on the dip can oscillator. I noted
this behavior as well some time ago. I can't quite put my finger on why
it helps. Some postulate that it is adding just enough thermal mass to
keep drift down when there is moving air around the can oscillator.
What I have noticed is that the DDS with the surface mount oscillator
is
more stable (remember, no extensive and exhaustive testing at this
point, just simple observation). I have also noticed that if I have one
of the DDS modules on my bench that when I monitor the output of said
device that tapping on the desk also seems to cause some wandering of
the synthesized frequency indicating that the the dip can oscillator is
possibly microphonic in some way.
It seems at this point as though the SMD oscillators are a better
choice
between these two low cost options. Of course, TCXO or OCXO would be a
better choice.
I stumbled across an Analog Devices application note AN-419 title "A
Discrete Low Phase Noise 125Mhz Crystal Oscillator for the AD9850
Complete Direct Digital Synthesizer" which has some interesting tid
bits
of information. I am using this as a starting point for some
experimenting with reference clock sources for DDS's. One bit of
information that no doubt is in the DDS chip's data sheet is that the
reference clock needs to be at least 3 Vp-p and centered on 1/2 VDD.
Seems so obvious but makes me wonder if adding a small coin to top of
the DIP can oscillator is not only helping thermal wise but is somehow
helping improve phase noise.
I think we have tended to think of a DDS as being "it". That is, we can
generate something stable and precise digitally and it will be good no
matter what. What we (I) forgot when first exploring the devices was
that what we get out the synthesized end is (relatively speaking) no
better than what we put in (i.e. the reference clock), another example
of GIGO.
cheers, Graham ve3gtc
On 13-07-21 04:03 AM, Joseph Gray wrote:
I have a few of those $5 AD9850 DDS modules from China. I'm going to
use
one to replace a crystal in a transmitter. I attached it to a TI
MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is.
Using
cellophane tape, I put a few pieces of foam around the oscillator
can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over 24
hours
now. So far, the frequency has held to within 0.01-0.08 of the
programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat
warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little module
works
and how stable it is. There is an Amateur in town who has been using
several of these modules to drive WSPR and QRSS transmitters. He has
done
up a poor man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Sent from my Motorola Droid Razr 4G LTE wireless tracker while I do other things.
You probably meant "as an integer divider", you don't get a lot of spurs.
Didier
Bob Camp lists@rtty.us wrote:
Hi
If the DDS is acting pretty much as a divider you don't get a lot of
spurs. The rest of the time there are spurs somewhere in the output
spectrum. Put another way, there are thousands of "bad" tuning words
for every good one. The good ones are evenly spaced over the range at a
spacing determined by the DAC width and the phase truncation process.
Bob
On Jul 21, 2013, at 7:07 AM, Tim Shoppa tshoppa@gmail.com wrote:
For the AD9850 (as well as the higher-tech more-bit-resolution
modules)
there are programming words/freqs that are awful with a lot of
close-in
spurs, and others that are comparatively clean (well, maybe just as
many
spurs by some measure but they are far far away).
If you go to a DDS with higher base frequency and more bits
resolution
(e.g. comparing my AD9954 DDS with my AD9850 synth) these "bad freq
words"
become less common/less severe.
These frequency-dependent close-in spurs show up in a receiver, as
sudden
appearance of raspies at certain receiving freqs while others just a
fraction of a Hz away sound clean.
Several ham designs for the AD9850 follow it with a simple one-to-one
tracking PLL for some cleanup.
There are some AD app notes that hint the patterns of the words with
lots
of close in spurs but I've never found an easy programmatic way to
skip
over them for the better words.
especially
section titled "Predicting and Exploiting Spur "Sweet Spots" in a
DDS'
Tuning Range".
Analog.com website has some tools to explore spurs vs tuning word:
http://designtools.analog.com/dtDDSWeb/dtDDSMain.aspx
Tim N3QE
On Sun, Jul 21, 2013 at 4:21 AM, Nic McLean mcleannb@bigpond.com
wrote:
Hi Joe,
It is good that they are that stable, but what is the phase noise
like? If
a
ham across town is using them for WSPR and QRSS they are most
probably OK
in
that regard, but not all DDS modules are.
Best 73's
Nic
VK2KXN / VK5ZAT
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]
On
Behalf Of Joseph Gray
Sent: Sunday, 21 July 2013 2:04 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] DDS module
I have a few of those $5 AD9850 DDS modules from China. I'm going to
use
one
to replace a crystal in a transmitter. I attached it to a TI MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is.
Using
cellophane tape, I put a few pieces of foam around the oscillator
can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over
24 hours
now. So far, the frequency has held to within 0.01-0.08 of the
programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat
warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little
module works
and how stable it is. There is an Amateur in town who has been using
several
of these modules to drive WSPR and QRSS transmitters. He has done up
a poor
man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Sent from my Motorola Droid Razr 4G LTE wireless tracker while I do other things.
Hi
It's a bit more complex than an integer divide, but yes that's the right idea. You want something that will fit both in the truncation word and inside the range of the DAC.
If you have a 12 bit DAC and the truncation word ( sine table input , what ever you call it) is more than that, then it's going to be your high frequency clock divided by 2^12
Bob
On Jul 21, 2013, at 6:26 PM, Didier Juges shalimr9@gmail.com wrote:
You probably meant "as an integer divider", you don't get a lot of spurs.
Didier
Bob Camp lists@rtty.us wrote:
Hi
If the DDS is acting pretty much as a divider you don't get a lot of
spurs. The rest of the time there are spurs somewhere in the output
spectrum. Put another way, there are thousands of "bad" tuning words
for every good one. The good ones are evenly spaced over the range at a
spacing determined by the DAC width and the phase truncation process.
Bob
On Jul 21, 2013, at 7:07 AM, Tim Shoppa tshoppa@gmail.com wrote:
For the AD9850 (as well as the higher-tech more-bit-resolution
modules)
there are programming words/freqs that are awful with a lot of
close-in
spurs, and others that are comparatively clean (well, maybe just as
many
spurs by some measure but they are far far away).
If you go to a DDS with higher base frequency and more bits
resolution
(e.g. comparing my AD9954 DDS with my AD9850 synth) these "bad freq
words"
become less common/less severe.
These frequency-dependent close-in spurs show up in a receiver, as
sudden
appearance of raspies at certain receiving freqs while others just a
fraction of a Hz away sound clean.
Several ham designs for the AD9850 follow it with a simple one-to-one
tracking PLL for some cleanup.
There are some AD app notes that hint the patterns of the words with
lots
of close in spurs but I've never found an easy programmatic way to
skip
over them for the better words.
especially
section titled "Predicting and Exploiting Spur "Sweet Spots" in a
DDS'
Tuning Range".
Analog.com website has some tools to explore spurs vs tuning word:
http://designtools.analog.com/dtDDSWeb/dtDDSMain.aspx
Tim N3QE
On Sun, Jul 21, 2013 at 4:21 AM, Nic McLean mcleannb@bigpond.com
wrote:
Hi Joe,
It is good that they are that stable, but what is the phase noise
like? If
a
ham across town is using them for WSPR and QRSS they are most
probably OK
in
that regard, but not all DDS modules are.
Best 73's
Nic
VK2KXN / VK5ZAT
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com]
On
Behalf Of Joseph Gray
Sent: Sunday, 21 July 2013 2:04 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] DDS module
I have a few of those $5 AD9850 DDS modules from China. I'm going to
use
one
to replace a crystal in a transmitter. I attached it to a TI MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is.
Using
cellophane tape, I put a few pieces of foam around the oscillator
can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over
24 hours
now. So far, the frequency has held to within 0.01-0.08 of the
programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat
warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little
module works
and how stable it is. There is an Amateur in town who has been using
several
of these modules to drive WSPR and QRSS transmitters. He has done up
a poor
man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Sent from my Motorola Droid Razr 4G LTE wireless tracker while I do other things.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
A number of them are sigma delta PLL's and have some really nasty jitter and spur issues. Anything that shows up listed as "user programmable" - watch out.
Bob
On Jul 21, 2013, at 5:17 PM, Didier Juges shalimr9@gmail.com wrote:
Some of the small surface mount canned oscillators are actually pll and not very good at that. I have a page about it somewhere on my web site.
Didier
KO4BB.com
Graham planophore@aei.ca wrote:
Good morning Joe (and all),
I have been playing around with a few of these DDS modules as well as a
couple of the lower end DDS chips (i.e. AD9834, AD9850, AD9851) and
they
do work well but they are not perfect.
I have a Fox Delta Antenna SWR analyzer which has an AD9850 and a
125Mhz
surface mount oscillator and although I have not done any extensive
testing of the devices stability it is relatively stable to a N3ZI
design DDS VFO which uses a AD9834 and a 80Mhz dip can (ie. not surface
mount) oscillator.
The AD9834/80Mhz dip can oscillator DDS wanders quite a bit and when I
listen to it on a receiver it sounds "raspy". I have simple heater
attached to the dip can - it is just as bad with as without. There has
been some discussion on the Knights QRSS reflector on the subject. If
you are not yet a member, you can join here:
http://mail.cnts.be/mailman/listinfo/knightsqrss_cnts.be
I am guessing "the amateur in town" is Dave WA5DJJ. He and I have
exchanged a few emails on this subject (and others). There is a fellow
in Australia as well which I have exchanged a few ideas.
One common method of taming some of wandering habits of the inexpensive
DDS modules is to put a small coin on the dip can oscillator. I noted
this behavior as well some time ago. I can't quite put my finger on why
it helps. Some postulate that it is adding just enough thermal mass to
keep drift down when there is moving air around the can oscillator.
What I have noticed is that the DDS with the surface mount oscillator
is
more stable (remember, no extensive and exhaustive testing at this
point, just simple observation). I have also noticed that if I have one
of the DDS modules on my bench that when I monitor the output of said
device that tapping on the desk also seems to cause some wandering of
the synthesized frequency indicating that the the dip can oscillator is
possibly microphonic in some way.
It seems at this point as though the SMD oscillators are a better
choice
between these two low cost options. Of course, TCXO or OCXO would be a
better choice.
I stumbled across an Analog Devices application note AN-419 title "A
Discrete Low Phase Noise 125Mhz Crystal Oscillator for the AD9850
Complete Direct Digital Synthesizer" which has some interesting tid
bits
of information. I am using this as a starting point for some
experimenting with reference clock sources for DDS's. One bit of
information that no doubt is in the DDS chip's data sheet is that the
reference clock needs to be at least 3 Vp-p and centered on 1/2 VDD.
Seems so obvious but makes me wonder if adding a small coin to top of
the DIP can oscillator is not only helping thermal wise but is somehow
helping improve phase noise.
I think we have tended to think of a DDS as being "it". That is, we can
generate something stable and precise digitally and it will be good no
matter what. What we (I) forgot when first exploring the devices was
that what we get out the synthesized end is (relatively speaking) no
better than what we put in (i.e. the reference clock), another example
of GIGO.
cheers, Graham ve3gtc
On 13-07-21 04:03 AM, Joseph Gray wrote:
I have a few of those $5 AD9850 DDS modules from China. I'm going to
use
one to replace a crystal in a transmitter. I attached it to a TI
MSP430
Launchpad and programmed a fixed frequency.
Just out of curiosity, I wanted to see how stable this thing is.
Using
cellophane tape, I put a few pieces of foam around the oscillator
can. Then
I hooked it up to my HP counter, which is locked to my GPSDO.
I'm not logging, but I have kept an eye on the setup for well over 24
hours
now. So far, the frequency has held to within 0.01-0.08 of the
programmed
value.
The house thermostat is set at 75 F, but this room gets somewhat
warmer due
to the equipment in it. Also, this room gets the morning sun.
Considering the cost, I am impressed with how well this little module
works
and how stable it is. There is an Amateur in town who has been using
several of these modules to drive WSPR and QRSS transmitters. He has
done
up a poor man's oven to keep his transmitters on frequency.
Joe Gray
W5JG
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