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Need DDS module, 2nd round

J

Joerg

Jan 1, 1970
0
Hi Folks,

Long story short, none of the DDS modules I've found so far has the
ability to be sweeped or FM-modulated. This one has a signal input
terminal block "SIN" but I received a response from the manufacturer
that it connects to nowhere, it has no function:

http://imall.iteadstudio.com/tools-and-equipment/im120723003.html

What I need in my case is DC-100Hz of sweep or FM bandwidth. Operating
frequency under 10kHz and sweep range up to 2kHz. Of course I can whip
up an analog solution but it won't be as precise and most of all not
really programmable.

Is there anything better? Cost is not a big issue but should be somewhat
small and a display would be nice.
 
N

Nico Coesel

Jan 1, 1970
0
Joerg said:
Hi Folks,

Long story short, none of the DDS modules I've found so far has the
ability to be sweeped or FM-modulated. This one has a signal input
terminal block "SIN" but I received a response from the manufacturer
that it connects to nowhere, it has no function:

http://imall.iteadstudio.com/tools-and-equipment/im120723003.html

What I need in my case is DC-100Hz of sweep or FM bandwidth. Operating
frequency under 10kHz and sweep range up to 2kHz. Of course I can whip
up an analog solution but it won't be as precise and most of all not
really programmable.

Is there anything better? Cost is not a big issue but should be somewhat
small and a display would be nice.

If by 'operating frequency under 10kHz' you mean the output frequency
doesn't exceed 10kHz a fast microcontroller and a DAC (codec) should
be more than sufficient. It doesn't sound like its rocket science to
me...
 
J

Joerg

Jan 1, 1970
0
Nico said:
If by 'operating frequency under 10kHz' you mean the output frequency
doesn't exceed 10kHz a fast microcontroller and a DAC (codec) should
be more than sufficient. It doesn't sound like its rocket science to
me...

But it is to me, I am an analog guy who knows barely enough C and
assembler to be dangerous. Plus there ain't much time, I need some
module that I can simply plop in and connect. The one in the link would
be ideal if the modulator input weren't be dead.
 
N

Nico Coesel

Jan 1, 1970
0
Joerg said:
But it is to me, I am an analog guy who knows barely enough C and
assembler to be dangerous. Plus there ain't much time, I need some
module that I can simply plop in and connect. The one in the link would
be ideal if the modulator input weren't be dead.

I guess they thought they could re-program the DDS fast enough / on
the fly to get a glitch-free output. How much time do you have?
 
B

Bill Sloman

Jan 1, 1970
0
I guess they thought they could re-program the DDS fast enough / on
the fly to get a glitch-free output. How much time do you have?

DDS outputs are intrinsicly glitch-free - what you change is the
number added to the phase accumulator on each step, and the stair-case
approximation to the sine wave remains continuous, though the cycle -
you are just changing the spacing of the treads..

Joerg should be able to sweep his sine wave pretty smoothly by
incrementing/decrementing the last digit of his frequency-setting
number frequently. Whether he can do it frequently enough is another
question. He might have to program a PAL to do it for him.
 
I guess they thought they could re-program the DDS fast enough / on
the fly to get a glitch-free output. How much time do you have?

it should be possible, I just looked at the code it is just a single
file (arduino Sketch)
and a library for the lcd. The SPI to the DDS is bit banged a 16MHz
should
be able to do that quite fast

The schematic shows JP4 connected to ADC0 ac coupled with 4.7u cap and
2x10k

so everything needed is there I think, all that is need is a timer
interrupt to
read the adc and update the DDS

-Lasse




-Lasse
 
N

Nico Coesel

Jan 1, 1970
0
Bill Sloman said:
DDS outputs are intrinsicly glitch-free - what you change is the
number added to the phase accumulator on each step, and the stair-case
approximation to the sine wave remains continuous, though the cycle -
you are just changing the spacing of the treads..

Joerg should be able to sweep his sine wave pretty smoothly by
incrementing/decrementing the last digit of his frequency-setting
number frequently. Whether he can do it frequently enough is another
question. He might have to program a PAL to do it for him.

That depends on whether the DDS is reset or not when the frequency
setting is updated. There has to be a reason the designers at
iteadstudio didn't implement the modulation. If it where simple they
would have implemented it.
 
J

Joerg

Jan 1, 1970
0
Nico said:
I guess they thought they could re-program the DDS fast enough / on
the fly to get a glitch-free output. How much time do you have?

As I said, about 100Hz is the max sweep cycle frequency. So a few kHz
update rate should do. Normally that isn't a problem with a DDS chip.
 
J

Joerg

Jan 1, 1970
0
it should be possible, I just looked at the code it is just a single
file (arduino Sketch)
and a library for the lcd. The SPI to the DDS is bit banged a 16MHz
should
be able to do that quite fast

The schematic shows JP4 connected to ADC0 ac coupled with 4.7u cap and
2x10k

so everything needed is there I think, all that is need is a timer
interrupt to
read the adc and update the DDS

If necessary I could do it that way. But I am not really a good
programmer. Ideall would be a module that has FM modulation or sweep. It
doesn't have to be DDS, just stable and not too large. Ripping it out of
a function generator is ok as well, as long as it's not much more than
two cigarette packs or so in size.
 
J

Joerg

Jan 1, 1970
0
Tim said:
Have you by any chance looked at National Instruments for signal
generators, or for similar instruments or vendors?

Haven't found anything useful for this application.

I suppose you're looking for something stand-alone, though, not something
that needs an umbilicus to a computer in order to function.

Yup. Something that can be plopped into my RF board and control two
frequencies (so two of those modules are needed). SPI or RS232, power
supply, output, modulation input.
 
J

Joerg

Jan 1, 1970
0
John said:
This will do it, but it's overkill, roughly 100x your probable budget.

http://www.highlandtechnology.com/DSS/T346DS.shtml

Yep, that sure would be overkill :)

If you wanted to roll your own, it would take a small FPGA and a DAC.

That's what I want to avoid, I've got enough on my plate as it is. If I
really can't find a module I'll buy function generators and hack them
apart on arrival, prying their main boards out. That would make my
circuit board the size of a large family pizza, but man's gotta do what
man's gotta do.
 
If necessary I could do it that way. But I am not really a good
programmer. Ideall would be a module that has FM modulation or sweep. It
doesn't have to be DDS, just stable and not too large. Ripping it out of
a function generator is ok as well, as long as it's not much more than
two cigarette packs or so in size.

if you don't need external input it is even easier, if I had the HW
I'd give it a try

do you really need the display and the controls or could you do with
few preprogrammed
waveforms/sweeps/frequencies on an easy to get dev-board with a few
buttons added?


-Lasse



-Lasse
 
Actually, there will be a netbook involved and I'll probably program the
SCADA stuff on that myself. I believe they don't make the Samsung NC-10
anymore, but it'll be something like that.

use the a sound card, possible external usb?

-Lasse
 
J

Joerg

Jan 1, 1970
0
if you don't need external input it is even easier, if I had the HW
I'd give it a try

I need the input because it'll be used in a loop. It will actually
become a glorified dipmeter. The phase response at the input of an LC
lowpass filter will be measured and then this becomes the error signal
in a PID loop. The PID loops adjusts the frequency of the DDS (or
whatever other sine wave generator) so the phase remains constant even
when the lowpass components change their values.

do you really need the display and the controls or could you do with
few preprogrammed
waveforms/sweeps/frequencies on an easy to get dev-board with a few
buttons added?

A display would be nice but I could also roach on a frequency counter.
The output has to be sine wave, can't easily filter that because the
thing has to work in frequency ranges I won't know up front. Being able
to set the base frequency locally would be very helpful.
 
J

Joerg

Jan 1, 1970
0
use the a sound card, possible external usb?

Theoretically, yes. But since this whole thing will sit inside a PID
loop I'd then have to process the error signal (a phase information
measured in my box) in the PC. Meaning lots of code writing and that's
what I am trying to avoid as much as possible.
 
Haven't found anything useful for this application.


Yup. Something that can be plopped into my RF board and control two
frequencies (so two of those modules are needed). SPI or RS232, power
supply, output, modulation input.

if you want it to be standalone what would spi/rs232 be for?


-Lasse
 
J

Jamie

Jan 1, 1970
0
Joerg said:
If necessary I could do it that way. But I am not really a good
programmer. Ideall would be a module that has FM modulation or sweep. It
doesn't have to be DDS, just stable and not too large. Ripping it out of
a function generator is ok as well, as long as it's not much more than
two cigarette packs or so in size.

The AVr's handle jobs like that very easy and they are easy to code.

You can get one witha ADC input and code a SPI output.

Jamie
 
J

Joerg

Jan 1, 1970
0
if you want it to be standalone what would spi/rs232 be for?

To set the base frequency. Otherwise it won't know where to center the
FM signal upon start-up.
 
J

Joerg

Jan 1, 1970
0
Joerg said:
Hi Folks,

Long story short, none of the DDS modules I've found so far has the
ability to be sweeped or FM-modulated. This one has a signal input
terminal block "SIN" but I received a response from the manufacturer
that it connects to nowhere, it has no function:

http://imall.iteadstudio.com/tools-and-equipment/im120723003.html

What I need in my case is DC-100Hz of sweep or FM bandwidth. Operating
frequency under 10kHz and sweep range up to 2kHz. Of course I can whip
up an analog solution but it won't be as precise and most of all not
really programmable.

Is there anything better? Cost is not a big issue but should be somewhat
small and a display would be nice.

In case someone else is in a similar situation, I've just discovered two
interesting alternatives if no suitable DDS module can be found. The 1st
is ye olde XR2206 which I thought had gone unobtanium:

http://electronics-diy.com/product_details.php?pid=537&name=1Hz - 2MHz XR2206 Function Generator Kit

The 2nd solution is rather ugly. I could take these DDS modules and
replace the oscillator with a LTC resistor-set silicon oscillator. Those
can be current steered. Of course then the digital frequency display of
the module would become fairly meaningless but could still sort of
signal if my PID loop has locked or not.
 
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