Hi DDS experts

We bought one of your DDS eval-board (AD9957/PCBZ). Can it do, Binary phase shift keying (BPSK). Hence, Will nulling/setting one of the inputs say "I" (in-phase component) to zero (0) effectively do the job.

If it can. Please can you clarify how I load the data file with a BPSK signal generated from baseband sequence into it.

e.g. A = [ 1 -1 1 1 1 -1 1 -1 1-1]?

A prompt reply is greatly appreciated.

Thanks and Regards,

Jackychan

There are two ways to implement BPSK with the AD9957: 1) via user-programmable Profiles, 2) via quadrature (I/Q) modulation.

1) Profiles

This is the simplest method. For example, let a baseband sequence of mark/space values (1/-1) be represented by logic levels 1/0. Then let the 1/0 logic levels represent +90/-90 degree phase shifts, respectively, for example. Now assign Profile 1 as a mark and Profile 0 as a space. That is, program both profiles with the same carrier frequency, but program Profile 1 with the phase offset for a mark and Profile 0 with the phase offset for a space. Note, however, the phase offset must be programmed as a positive value, which means the mark/space range of +90/-90 must be shifted to +180/0. Now let the logic levels associated with the baseband sequence control profile pin P0 (but be sure to force profile pins P1 and P2 to logic 0). This will cause the AD9957 to switch between Profile 1 and Profile 0 (representing carrier phase shifts of 180 and 0 degrees, respectively) in sympathy with the baseband sequence. Be aware, however, that the profile pins are internally synchronized with the rising edge of SYNC_CLK. For low baseband rates this is usually of no concern, but relatively high baseband rates require synchronization of the baseband sequence with SYNC_CLK.

2) Quadrature Modulation

Phase shifts are readily accomplished via quadrature modulation (see Equation 5 in ADI application note AN-924). The amount of phase shift is given by arctan(Q/I). For example, given a unit circle centered at the origin, the fraction, Q/I, defines the magnitude of the phase shift (up to pi/2 radian) with respect to the x-axis, while the sign of both I and Q determine the quadrant. Furthermore, the peak-to-peak modulated output level depends on I and Q, as well, via their vector sum. To accomplish a phase shift of 45 degrees requires that I and Q have the same positive value. To get -45 degrees, simply change the sign of Q (note that changing the sign of I instead of Q yields +135 degrees instead of -45 degrees). To get a phase shift of 90 degrees requires I to be zero. Then the sign of Q determines whether the phase shift is +90 or -90 degrees.

Implementing BPSK via quadrature modulation means choosing I and Q values that yield the appropriate phase shifts and feeding these I/Q values to the baseband input of the AD9957 in sympathy with the baseband sequence. For example, let a baseband sequence of mark/space values (1/-1) represent the desired mark/space phase shifts, respectively. Then, assuming a +45/-45 degree mark/space phase shift relationship, let K be a value equivalent to SQRT(2)/2 of the POSITIVE full-scale 18-bit baseband input of the AD9957. Then, +45 degrees is I = K and Q = K and -45 degrees is I = K and Q = -K. As a second example, consider a +90/-90 degree mark/space phase shift relationship. To accomplish this let K be the POSITIVE full-scale 18-bit baseband input of the AD9957. Then, +90 degrees is I = 0 and Q = K and -90 degrees is I = 0 and Q = -K. In the case of either example, simply apply the I/Q values to the AD9957 baseband input in sympathy with the baseband sequence. Note the baseband sequence rate must relate to the I/Q sample rate of the AD9957 as defined by its PDCLK output.