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Questions about AD9957 configuration for QPSK generation

Hello, I am an RF and MW engineer trying to figure out how to configure AD9957 to obtain a 1 MHz bandwidth QPSK signal at fc = 70 MHz. My conclusions up to now are that I should use the following values:

Fsysclk = 176 MHz

R = 44

FTW = 1,708,225,629 (not exactly 70 MHz of output frequency, but near enough)

Is that correct?

I also have some doubts on how to generate a QPSK. I mean in that kind of modulation I transmit 2 bits per symbol, but I should provide 18 bits per path (I or Q) to AD9957 parallel data input. How do I translate my 2 bits to all those bits I should put at data input?

Probably all those questions are quite simple to someone who is used to work with baseband, but it is a bit out of my scope, so I would appreciate any clarification. It also would help any advice or reminder of important facts that I should keep in mind to generate my signals.

Thanks for your support!

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  • The value you propose for the FTW is spot on. Be aware, however, that Fsysclk=176MHz necessitates an external 176MHz source. That is, you cannot use the internal PLL as its output frequency is limited by the VCO band selections. The lowest VCO band bottoms out near 350MHz.

    The R=44 value establishes the rate at which the device accepts 18b I/Q pairs at its parallel data port. In your specific case, the device takes in an I/Q pair every micro-second. Said another way, the "vector rate" is 1MHz. Of course, the actual clock rate at the input is twice the vector rate because I and Q are time interleaved (this was done to reduce the device pin count -- otherwise another 18 pins would have been necessary).

    Here's where it gets a little muddled...

    The vector rate and the symbol rate are usually not the same. In practice, one typically oversamples the individual symbols running the oversampled symbols through a transmit filter (e.g., raised cosine) and feeding the oversampled result to the AD9957. Hence, the vector rate is typically an integer multiple (the oversample factor) of the symbol rate. That said, if your symbol rate is 1MHz and you oversample the symbols by 4 (for example), then a 4MHz vector rate is necessary (i.e., R=11). The bandwidth at the output will be ~1MHz (the actual BW depends on the specifics of the baseband filtering).

    You may find ADI application note AN-922 helpful on this subject. It describes the process of baseband filtering and the appendix describes symbol encoding for 16-QAM. The idea is the same for QPSK (which is essentially 4-QAM). Table 1 in the appendix gives I & Q values in terms of the peak value of the symbol pulses. You can build a similar table for QPSK (it will only have 4 rows instead of 16). In the end, you will need to scale the I & Q values (whether or not you go through the filtering exercise) so that they span an 18b range prior to sending them to the AD9957.

    Hopefully, you're now on your way to QPSK.


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  • The value you propose for the FTW is spot on. Be aware, however, that Fsysclk=176MHz necessitates an external 176MHz source. That is, you cannot use the internal PLL as its output frequency is limited by the VCO band selections. The lowest VCO band bottoms out near 350MHz.

    The R=44 value establishes the rate at which the device accepts 18b I/Q pairs at its parallel data port. In your specific case, the device takes in an I/Q pair every micro-second. Said another way, the "vector rate" is 1MHz. Of course, the actual clock rate at the input is twice the vector rate because I and Q are time interleaved (this was done to reduce the device pin count -- otherwise another 18 pins would have been necessary).

    Here's where it gets a little muddled...

    The vector rate and the symbol rate are usually not the same. In practice, one typically oversamples the individual symbols running the oversampled symbols through a transmit filter (e.g., raised cosine) and feeding the oversampled result to the AD9957. Hence, the vector rate is typically an integer multiple (the oversample factor) of the symbol rate. That said, if your symbol rate is 1MHz and you oversample the symbols by 4 (for example), then a 4MHz vector rate is necessary (i.e., R=11). The bandwidth at the output will be ~1MHz (the actual BW depends on the specifics of the baseband filtering).

    You may find ADI application note AN-922 helpful on this subject. It describes the process of baseband filtering and the appendix describes symbol encoding for 16-QAM. The idea is the same for QPSK (which is essentially 4-QAM). Table 1 in the appendix gives I & Q values in terms of the peak value of the symbol pulses. You can build a similar table for QPSK (it will only have 4 rows instead of 16). In the end, you will need to scale the I & Q values (whether or not you go through the filtering exercise) so that they span an 18b range prior to sending them to the AD9957.

    Hopefully, you're now on your way to QPSK.


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