AD9911 - General questions

Question

Hi, 
 We recently bought an eval board for the AD9911 to experiment a bit with DDS and discover how it works, but I had some trouble understanding a few things about both the evaluation software and the chip itself. 
 
 Linear Sweep : 
 
 First is about the linear sweep. I got it working and everything, but one thing that troubled me was the need to press "P1" to apply the channel output config. Same for a linear sweep on CH0, I need to press P0 to get my linear sweep working. Does it mean that the channel output config is only applied on channel X if pin X is activated ? (Just making sure I understood correctly here) 
 In the linear sweep configuration window, there is a falling rate and falling step. I don't understand what these do. My guess is, the rising step is for sweeps from f1 to f2 (or phase 1 to phase 2) where f1 < f2, but I can't understand the correct config to get a falling sweep. Or maybe it's something else entierely ?

AM Modulation : 
 
 As soon as I activate CH0 and "Test tone modulation" I can't control the amplitude of CH1 anymore, but I can control the amplitude of CH0. Is this normal ? Here's my setup : 
 
 Also, as you can see, I don't need to select "Amplitude" in the "Modulation Output Type". I'm not sure what this window is used for aside from linear sweep, and because of that I don't understand why I can set 15 different tuning words for each Channel. I tried changing values but it doesn't do anything, and I guess it's because it's used for something that needs the correct configuration but I have no idea what these 15 boxes are used for. 
 
 Phase modulation : 
 
 I tried using phase modulation by setting up my PM CH2. I can't synchronize my oscilloscope because it's a cheap one (or maybe I can but I haven't found how to do it yet) , so my output is very unstable: 
 
 Here is my setup :

General questions: 
 
 I had a few questions about the chip and some functionnalities that I didn't quite understand from reading the datasheet. 
 What is the Ramp up/Ramp Down used for ? 
 What are PPC bits on the software ? 
 Why are there 15 auxiliary channels tuning words, and what's the use for them ? I understand channel 1 being used for the end point of sweeps, but I don't understand the other ones. 
 I don't quite understand the "2/4/8/16-level modulation" option on the chip. I know this is mainly due to my lack of knowledge on the topic, so if you have any external source that could help me get a better feeling of these I would be very grateful.

I apologize about some of these questions that should be clear after reading the datasheet, but I've just graduated and had some trouble understanding some of the basic functions of this chip. Thank you for your help.

Jules.Nikko · Accepted Answer

Hi RFnewbie , 
 1. Linear Sweep Question: 
 These profile pins mentioned are used to configure to control modulation, linear sweep, or RU/RD operations. To perform your desired process (modulation, linear sweep, or RU/RD) you just have to activate the profile/pin associated with the operation. When pressed, they are set to Logic 1. These data puns are used for modulation (FSK, PSK, ASK), start/stop for the sweep accumulator, and ramping up/down the output amplitude. More of these configurations are in the AD9911 data sheet page 22 and 23. ' 
 You're right about the rising step size mechanics. They entire the same concept falling step size, it is just that if you're going to sweep from a bigger number to a smaller number (f1>f2) so you sweep how much step or interval to sweep it. 
 2. AM Modulation: 
 The channel output config tab has channels tab. These tabs have different roles. You can use these tabs to select between fundamental channel and the spur reduction channel. CH1 (Fundamental) is used for the fundamental channel output. SR CH0, SR CH2, and SR CH3 are the spur reduction channels. 
 Also, these tabs become the tabs to select the channel used in the test tone modulation mode. CH1 is still the fundamental channel. AM CH0 is used for amplitude modulation. 
 
 To conclude, yes, what you're experiencing controlling CH0 is normal. 
 3. Phase Modulation 
 I suspect it might be from your setup. Check the wires are tightly closed. If not then it might be from the instrument you are using, 
 
 4. General Questions: 
 a. What is Ramp up/ Ramp Down used for? - AD9911 has the ability to ramp up or ramp down the output amplitude before, during, or after a modulation sequence. It is simply ramping the frequency (whether up or down) from one point to another, 
 b. What are PPC bits on the software? - It supports the RU/RD flexibility. These are FR1<14:12>. Each level modulation corresponds to their PPC bits as discussed in page 22 and 23 of the datasheet. 
 c. Why are there 15 auxiliary channels tuning words, and what's the use for them? - When not using the software, storing the values to your tuning words to these registers are convenient in a way that you don't have to manually input each value for easy access and calling the registers. That's how I see it. 
 d. 2/4/8/16-level modulation - 
 It depends on the of frequency/phase or amplitude to modulate. As an example, if your goal is to switch between 15 different frequencies, use the 16-Level Modulation -- No RU/RD mode (not the Linear Sweep (LS) mode). You must still select the modulated parameter (frequency, phase, amplitude), This mode allows you to program up to 16 frequencies in Channels 0 - 15. 
 
 All the best, 
 Jules)

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AD9911 - General questions