Electronic – Strange noise on DDS

ddsfilterfpganoise

I have built an FPGA based DDS (fc=200MHz. out: 0-50 or 0-80 MHz) . I have a decent DAC output wave and now trying to build an appropriate filter for it ( Fig-1 ). The filter works rather fine but there are 3-4 strange noises there ( I have another question here that I think the solution for each one may help in solving the other one) .

I say "strange" because according to Nyquist diagram , the image wave should be at a higher frequency but my noises are in lower parts of the band. For example at 20 MHz, we should see the image frequencies at fc-f = 200MHz-20MHz=180 MHz. This noise is easily removed by a low pass filter but I have these noises also:

1- Fig-2 50-60 Hz (this may be due to main line but this is 7 volts pk-pk !!!) .

All the upper frequencies drive on it

2- Fig-3 130-140 Hz . I can't guess where this has come.

All the upper frequencies drive on it.

3- Fig-4 Around 50 KHz: this one has steps between cycles and I have never seen anything like that before .

All the upper frequencies drive on it

The main frequency ( in this example 20 MHz) drives on to of all of them ( 4- Fig-5 ). This causes that the main frequency jumps up and down on the oscope screen

Settings:

1- These noises are just seen in higher frequencies ( > 5 MHz)

2- When putting the oscope probe on 10x, their amplitude is reduced ( not too much. just a bit) but the output shows step distortions.

3- Probe's barrel clasp is connected to the ground ( with many decoupling capacitors ).

4- tried the same configuration with dedicated DDSs ( AD9850 , AD9833) and the same noises arise at freq > 5MHz.

Edition: results with an analog Oscillosciope added at the end ( Fig 7-9 )

Fig-1 The filter:

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Fig-2: 50-60 Hz:

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Fig-3: 130-140 Hz

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Fig-4 50KHz

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Fig-5 Main frequency

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Fig-6 Up jumping main frequency:

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Fig 7- analog oscilloscope: low frequency:

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Fig -8 analog oscilloscope: main frequency:

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Fig-9 analog oscilloscope: Jitter like behavior seen at fx10 setting:

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Best Answer

  1. Your phantom signals are digital measurement errors from undersampling and a low alias frequency is the result.

Comments

  1. Your analog scope shows significant DC , low frequency components, which you may want to reject with a HPF.

  2. The filter design must be driven from a current source to work as designed with 6dB loss in the passband, since impedance ratio is 1/2. If this is my false assumption,then R1 should be in series with a voltage source.

  3. The values shown are for 80MHz LPF may has 2dB ripple in response in pass band but stopband begins at 100MHz, @ -66 dB. If you don't want the ripple, use, Chebyshev Type 2. The steep skirt as a large ripple penalty in your Cauer design or change the ripple factor.

Suggestion. Learn how to match or isolate output and load impedance so that your test measurement impedance does not introduce a false circuit load. e.g. dont load filter with a 20pF probe. Use 150 + 50 Ohm scope or Spectrum Analyzer or use a 1pF FET probe.