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Original question:

AFE7950: two tone generation from DAC

AFE7950: Signal generation DAC AFE7950EVM

WEI WANG
Prodigy 80 points
Part Number: AFE7950

Dear Engineer,

I want to use AFE7950 to generate square wave instead of sine wave. So in the “sine_64point_att.sv” file of TI JESD 204C IP(vcu118_64b66b_2), I'm using two recurring points to replace the original 64-point sine wave. Like this:

After settig the txNco to 9GHz, I got the modulated waveform(red waveform) in the scope which shows in the first following picture. For contrast, I obtained another modulated waveform(blue waveform) by mixing the square wave generated by the arbitrary wave generator(Tektronix AWG 5208) with the carrier generated by the signal generator(Keysight EB257D).

 see, the quality of waveform generated by AFE7950 is not  as good as the blue waveform. The difference is mainly reflected in excessive current signal jitter and harmonic effect. Besides, the quality of spectrum is also different. Is this because the blue waveform mixed through an analog mixer?

I would like to ask what causes this signal difference and what methods can be used to improve the existing waveform generated by AFE7950.

Thanks.

Best wishes,

WANGWEI

  • 0
    TI__Expert 4925 points

    Hi WANGWEI,

    We will set up to test this in the lab. We hope to have some results by next week. 

    Best,

    Camilo

  • 0 in reply to Camilo Garcia Trujillo
    Prodigy 80 points

    Dear camilo,

    Is there any new test result? Looking forward to your reply.

    Best,

    WANGWEI

  • 0 in reply to WEI WANG
    TI__Expert 4925 points

    Hi WANGWEI,

    To make a better square wave our suggestion would be to use the following sample vector:

    assign samples_vector = {                                                        
16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000,  
16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000,  
16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000,  
16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000, 16'h0000,  
16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF,  
16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF,  
16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF,  
16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF, 16'h7FFF };

    However, the square wave might still not look as good as your blue waveform. The reason for this can be seen in the following example:

    If your DAC Fs = 12GSPS and your interpolation factor is 16. Your interface rate will be Fint = 750MSPS. Meaning you will be able to transmit at most a signal with an IBW = 750*0.8 = 600MHz without it being distorted because of the 80% passband of the interpolation filters. A square wave has a very high IBW since it is made up of an infinite series of sine wave harmonics. So, when you pass that square wave through the interpolation filter the harmonics that are outside of the passband will be attenuated leading to a distorted signal.

    In theory, to make a square wave you would want as high of an interface rate as possible and a samples_vector as long as possible with only one transition.

    Best,

    Camilo

  • 0 in reply to Camilo Garcia Trujillo
    Prodigy 80 points

    Dear Camilo,

    Thanks for your reply. I have a chip purchase question to consult with you.

    I know that the DUC module cannot be bypassed in the AFE7950, so the input baseband frequency cannot be larger. But if I want to generate a 1 GHz square wave with good quality, just like the waveform generated by AWG with 5Gsps sampling rate. (Although the input method of the digital side is different, The AWG does not need to be limited by the interface rate.) The chip I want also needs to contain both ADC and DAC modules. Perhaps it needs more lanes and a higher sampling rate. So far, I have not found such a chip in your product library. I would like to ask if you have any recommended chips to buy. If not, are there any good solutions using existing chips? 

    Best,

    WANGWEI 

  • 0 in reply to WEI WANG
    TI__Expert 4925 points

    Hi WANGWEI,

    Our suggestion would be to set the AFE7950 into a mode with 3000MSPS interface rate, which is the maximum it can do for the TX channels. This is to have the widest BW. Then send a pattern as long as possible with only one transition to represent the square wave. With this you could evaluate if the Square wave is still not good enough for your application.

    Also, just so that we understand your application and are able to better assist you, why are you generating a square wave out of the TX channel? What do you plan to do with the square wave?

    Best,

    Camilo