Part Number: DAC38RF89
In our application, the flowing is DAC38RF89's configuration. When we output 1GHz signal, there is no spurs. But other frequencies, it can see spurs.
We found that it was caused by the inverse sinc filter. If disable the inverse sinc filter, there is no spurs.
The frequency of spurs is related to the signal frequency. For example, the spurs cannot be seen at 1GHz, but the output power has a few dB change; at 999.999MHz, the spurs will be at 999.989MHz and 1000.009MHz; at 999.998MHz, the spurs will appear At 999.978MHz and 1000.018MHz; that is, the spurious frequency is Fout±(1G-Fout)*10. When the inverse sinc filter is turned off, the spurs disappear.
However, if the signal is output through the NCO, whether or not the inverse sinc filter is enabled, there is no spurious.
Why does theinverse sinc filter cause this problem？spurs.rar
When using inverse sinc filter, you will need to reduce the signal power as the inverse sinc filter provides gain. If signal power is not backed off, then there will be digital saturation
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
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In reply to Kang Hsia:
If the signal is output through the NCO(the flowing is the configuration), the constant value is 0x7fff ,there is no power back off. But there is no spurious. What is the reason?
In reply to user6193156:
When I do this test, I see spurs added and the level of the NCO increases. Not sure why you are not seeing this. Please send screen shots of the DAC output in both cases like I have sent you.
In reply to jim s:
1, The flowing is use FPGA send data.The frequency is 999MHz, and the power is -8.7dBm. The spurs can be seen at 989MHz and 1009MHz.
2,The flowing is use NCO, and constant value is 0x7fff.The power is changed to -9.11dBm(why the power is decreased?). There is no spurs at 989MHz and 1009MHz.
The NCO/MIXER function has a -6 dB loss of power through the digital processing. If you set the mixer gain to 6dB, the two cases output power will match assuming your FPGA is sending full scale data.
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