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Measurements >> Other Measurements >> Skirt at DAC output
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Message started by ywguo on Dec 19^th, 2007, 4:06am

Title: Skirt at DAC output
Post by ywguo on Dec 19^th, 2007, 4:06am

Hi Guys,

The DAC output spectrum looks like a skirt when I measure using a spectrum analyzer. The device under test is a 10 bit current-steering DAC, which has not any on-chip voltage buffer. An LM7372 amplifier on board drives the 50 Ohm load at the input of the spectrum analyzer. The clock is fed from Hewlett Packard 33120A 15MHz Function / Arbitrary Waveform Generator. The data vector is fed from a DDFS, which is implemented in an Altera FPGA. The HP 33120A clocks the DDFS, too. The conversion rate is 15MS/s. The DAC output is 1.21 MHz. The spectrum is shown below. The skirt becomes low when the DAC output frequency decreases, and becomes high when the DAC output frequency increases.

It looks like spectrum leakage. However, I don't know the operational principle of the spectrum. Is it a sampling system with DSP? Can it window the input?

Look forward to your comments.

Yawei

Title: Re: Skirt at DAC output
Post by ywguo on Dec 26^th, 2007, 10:26pm

Hi,

There is an introduction to spectrum analyzer, http://www.tutorialsweb.com/rf-measurements/spectrum-analyzer.htm. It is a wide band, very sensitve recreiver. So the spectrum leakage is impossible.

I replaced the clock source with some crystal oscillators. The skirt disappeared. :) Please see the following picture.

Yawei