Harmonics or Intermodulation?

[Vishwas_acc]

Dear all, I am new to this forum.
Here I have one question. We have designed a board which constitutes a FPGA clocked by 10 MHz, followed by a 3 DAC's which is followed by transformer, LPF and Amplifier.
What I am expecting is a 20 MHz pure tone at outputs of 3 amplifiers of 3 DAC's.
But there is a presence of small tones in around 28 , 36 , 44 MHz so on with decreasing amplitudes.
What can be the reason?
Also the presence of extra tone is not visible in DAC1 where as it is found in DAC2 and 3.

 

[raiya_23]

Are you using any filters?

 

[FvM]

A "few" informations missing from your post:
- DAC output rate
- signal characteristic
- LPF specification
- level of observed spurs

I presume the output data rate is hígher than 10 MHz, so which other clock frequencies are involved in the design?

 

[Vishwas_acc]

Well , the FPGA is operating at 160 MHz with 10 MHz clock input.
It is connected to 16 bit DAC.
DAC(AD9726) clock is given by FPGA clocked at 160 MHz.

LPF used is 50 MHz filter.
Expected output is a BPSK modulated tone with bandwidth 20 MHz.
For test case we used only carrier where we observed a tone at 20 MHz. At 28 MHz around 10dB less than the one at 20 MHz.

 

[FvM]

According to the information you gave, the strong spurs can be neither harmocics nor intermodulation products, because no respective fundamental or intermodulating signal seems to exist. Apparently something in the setup is seriously wrong, so you should check all involved signals and hardware components.

 

[Vishwas_acc]

According to the information you gave, the strong spurs can be neither harmocics nor intermodulation products, because no respective fundamental or intermodulating signal seems to exist. Apparently something in the setup is seriously wrong, so you should check all involved signals and hardware components.

Yea sure. Will check that. Thank you.

 

[rohitkhanna]

Look for 8Mhz and 16Mhz anomalies. All of your numbers are related to this...