DDS based function generators

[MeggerMan]

Mark,
I look forward to seeing the pictures of it, although I will be interested to see how you drive the REF_CLK.
20-30MHz xtal is one way.

Jason,
All my bits have arrived, including a low jitter 125MHz 3.3V SAW oscillator, made by Epson Toyocom (I think) as well as an etch tank for making PCBs.
I have all the bits now to make my own AD9959 function generator, just need to put it all together.
With regards to the AD9959 chip and the SAW oscillator, seeing as these parts are fairly exepensive and difficult(impossible) to remove once soldered in I will be mounting these on surf boards that will sit on a main board. The surf board will contain the standard de-coupling caps and any parts that are needed by default.
These surf boards will make conections to the main board via small "whisker" wires to keep down stray capacitance and held in place by two small nylon bolts.

Regards
Rob

[Jdo300]

Hey Rob,

Sounds great! I'll be very glad to see how the output waveforms look, especially at high frequencies.

By the way, I'm stuck on the last couple of parts for my control circuit. Here's are links to the posts I made below. They concern the voltage detection circuit and the HV biasing circuit:

http://www.physicsforums.com/showthread.php?t=191440
http://www.physicsforums.com/showthread.php?t=191045

God Bless,
Jason O

[MeggerMan]

Hi Jason,
I read it, interesting stuff on the RMS to DC chip, not a cheap device either.
So these devices output an average voltage over a given time span.
So I guess your uC needs to be able to match the time spans to measure the voltage.  If you fix the load with a carbon resistor you should ba able accurately calculate the power.
You need to be careful not to get bogged down with a circuit that does everything - I think measuring output can be added later.
The main thing is working out the codes you need to send to the AD9959 to get it to operate.
I still need to do some more work on the assembler program to complete method for talking to the AD9959.
One option I may try is sending some preset codes to it using the output from the simulator on the Analog website.

Regards
Rob

[Jdo300]

Hi Rob,

You're right about that. Thankfully, I did figure out how to do both the HV throttling circuit and the voltage measurement circuit. The only reason that I'm even bothering with this is to give the controller the capability to run autonomously to self-regulate the coil; particularly in the event that we can't directly control it while the coil is running. Plus, the measurement capability will help us to do other experiments like frequency and bias sweeps to help tune the coils and automatically find the sweet spots.

But I can say that the design I ended up with the the voltage measurement circuit is greatly simplified now. I'm not concerned about measuring the current and the power but only the voltage. (plus we can calculate the current and power knowing the load on the coil anyway). The simple idea is to just go with a resistor divider with a cap in parallel to help smooth out some of the hash (and short out any RE that might work into the circuit). Then I'll just use an ADC to sample the voltage and send it to the microcontroller.

As for the biasing circuit, I'm keeping that simple too. I decided to go with the Op-amp that I mentioned in the post. After talking to the tech support people at Linear, they confirmed that the op-amp I spaced out would work fine to directly throttle the HV supply IC. Now I'm going to be working on drawing the circuit schematics to tie everything together.

Finally starting to see the light at the end of the tunnel .

God Bless,
Jason O

[MeggerMan]

Work on the case for the DDS 20 is progressing slowly.

Several pics showing the progress so far, as this is the first attempt  I have found problems that will be fixed in the next case. Progress through evolution.

Top view, LCD window not cut out yet.


End view shows where I had to cut into the casing to allow the shield to fit up against the front panel.


Regards
Rob