AD9959 frequency linear sweep down

Question

Hi guys, I'm trying to use the linear sweep mode of AD9959 to sweep the frequency down. It seems that it can sweep from s0 to e0 but the e0 frequency must be larger. I tried to let p0 always 1 (I'm using channel 0) such that it starts at e0 but obviously it still has to finish the sweep up. Correct me if I'm wrong since I'm really interested in achieving this sweeping-down function!

ADIApproved · Accepted Answer

Yes E0 must be greater than S0. 
 That said, you can accomplish the desired down sweep by taking advantage of the fact that the DDS is a sampled system. Normally, we think of operating only in the 1st Nyquist zone (0 to 1/2 the system sample rate). Therefore, we typically think of the useful range of FTWs as being from FTW = 0 to FTW = 1/2 of the maximum value. However, the DDS will accept FTWs in the 2nd Nyquist zone. That is, FTWs from greater that 1/2 the maximum value up to the maximum value. 
 When you program a frequency in the 2nd Nyquist zone, you will see its image in the 1st Nyquist zone. Specifically, the 2nd Nyquist zone frequency and 1st Nyquist zone frequency appear as two frequencies "reflected" about the Nyquist frequency (1/2 the system clock sample rate). 
 For example, let FTW0 represent a frequency in the 1st Nyquist zone and FTW1 represent the corresponding image frequency in the 2nd Nyquist zone. The FTWs relate as follows: 
 FTW1 = FTWmax - FTW0, and 
 FTW0 = FTWmax - FTW1 
 Where FTWmax is the maximum FTW value (that is, all bits = 1). 
 The clue is the negative sign imposed on the FTW on righthand side of the equations due to the subtraction operation. 
 This means you can fool the DDS into doing a down sweep in the 1st Nyquist zone by programming an up sweep in the 2nd Nyquist zone. This will satisfy the E0 > S0 requirement, but still allow for a down sweep (in 1st Nyquist zone) by executing an up sweep (in 2nd Nyquist zone).)

ADIApproved · Answer

Yes, you would program the specific channels by setting the corresponding CSR bit for a particular channel. Once they are programmed, you can use to P0-P3 pins to control each channel independently.

Jules.Nikko · Answer

Hi zyli , 
 I understand you want to sweep from 100MHz to 90MHZ. 
 1. In linear sweep mode, S0 is loaded into the Channel Word 0 register (S0 is represented by one of three registers: Register 0x04 is for the frequency type of sweep) and E0 is always loaded into Channel Word 1 (Register 0x0A). If E0 is configured for frequency sweep, the resolution is 32 bits. The profile pins are used to trigger and control the direction of the linear sweep for frequency, phase, and amplitude. All channels can be programmed separately for a linear sweep. In linear sweep mode, Profile Pin P0 is dedicated to Channel 0. Profile Pin P1 is dedicated to Channel 1, and so on. 
 2. With your CFR address, you activated the following: 
 [15:8] 0x43 = Enabled Linear Sweep and DAC Full-Scale Current Control 
 [0:7] 0x80 = Digital Power Down 
 But I think you are also missing this line : 
 The AFP select bits determine the type of linear sweep to be performed. To enable linear sweep mode for a particular channel, the AFP select bits (CFR[23:22]), the modulation level bits (FR1[9:8]), and the linear sweep enable bit (CFR[14]) are programmed. So you need to program 
 [23:16] = 80 to enable frequency sweep. 
 
 3. Your E0 should be in Channel Word 1 (Register 0x0A) of 90MHZ in hex. 
 4. In linear sweep, each channel is assigned a rising delta word (RDW, Register 0x08) and a rising sweep ramp rate word (RSRR, Register 0x07). These settings apply when sweeping up toward E0. The falling delta word (FDW, Register 0x09) and falling sweep ramp rate (FSRR, Register 0x07) apply when sweeping down toward S0.

Just let me know if this helped you. 
 All the best, 
 Jules

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AD9959 frequency linear sweep down