Spectral Copies: The digital-to-analog converter on the DDS is unfiltered, which means that there is no anti-imaging filter to remove the spectral replicas. To see this, plug the output of the DDS board directly into the vector signal analyzer (VSA), and observe the spectrum. Use 104.9 MHz as the center frequency, and set the span wide enough so that you can see the spectra of the replicas to the left and right of the 104.9 MHz signal. Use the marker to find the peaks of the other replicas, and record their frequencies. Once you've done that, reattach the antenna to the DDS output, and tune the receiver to the frequencies you just recorded. You should be able to hear your audio on each of the other frequencies.

The FM code you just ran (also listed here) is fairly straightforward. The program first calls the radioinit subroutine. This routine sets the DDS to single-tone mode and turns off an inverse-sinc filter to conserve power. Following radioinit, the setcarrier subroutine is called. This routine sets the frequency of the DDS output by writing to the two most significant 8-bit frequency registers of the 48-bit frequency-tuning word on the DDS1. Although the frequency-tuning word on the DDS has 48 bits of resolution, the upper 16 bits provide us with enough resolution for the purposes of this lab, and so we will only be writing to the two most significant registers. See page 26 in the DDS data sheet for a layout of the frequency-tuning word.

To set the carrier frequency, we first need to determine what frequency word has to be written to the frequency registers on the DDS. This can be done using Equation 1:

Writing to the frequency registers is accomplished using the portw instruction. To write to the frequency or phase registers on the DDS, the second operand of the portw instruction must be 10xxxxxx, where the lower six bits are the address of the specific register to be written to. The address of the most significant frequency register on the DDS is 04h, and the address of the second most significant frequency register on the DDS is 05h (see page 26 in the data sheet). It is important to note that the way our DDS boards were built, you will not be allowed to make two consecutive writes. To solve this problem, a subroutine called nullop is called to waste some CPU time between writes. nullop does this by simply repeating the nop instruction 128 times.

After the program returns from the setcarrier subroutine, it enters an infinite loop in which it waits for a serial interrupt to occur. The serial interrupt occurs every time a new sample is acquired from one of the two input channels and is transmitted to the DSP via the serial port. When the interrupt occurs, an interrupt service routine called ANALOG_IFC (see core_mod.asm executes and calls the handle_sample subroutine. The handle_sample subroutine reads in the acquired sample from the serial port and scales that sample so that it can be "mapped" to a frequency in the range of ±75 kHz2. The scaled sample therefore determines the frequency deviation and is added to 6Dh. The last step is to write the result to the second most significant frequency register so that the frequency of the DDS output can be updated.

The process for changing the phase of the DDS output is the same as it was for changing the frequency of the DDS output. To change the phase, you need to write a phase word to a phase-adjust register on the DDS. The phase-adjust register is 14 bits wide and is split up into two smaller registers that you can write to (see page 26 in the data sheet). The upper 6 bits have address 00h, and the lower 8 bits have address 01h. The phase word can be calculated using Equation 2:

The DDS also allows you to program the amplitude, but this functionality is not addressed in this lab. You will be able to implement a digital communication system in ECE320 without having to program the amplitude. Interested readers are referred to the data sheet.

Note that the corresponding baseband frequencies for 120.005 MHz and 120.011 MHz are 5 kHz, and 11 kHz, respectively. Since these baseband frequencies lie within the 22.05 kHz bandwidth of the DSP, you will be able to view your FSK signal in real time on the oscilloscope without the contribution from the spectral replicas. Just feed the output of the DDS into a second DSP (the anti-aliasing filter on the DSP will get rid of the spectral replicas), and pass it through to the output and the scope. You should be able to verify that there is continuous phase between frequency symbols, and that your symbol length is approximately 725μs. You should also view the spectrum of your DDS output on the VSA to verify that your symbols have the correct frequencies.