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Recently I got familiar with DDS family of ICs. They are fine and nice to work with but there is a problem with them: The output made by DAC has harmonics that according to the datasheet they need to be filtered by a low pass filter. That's not a complicated procedure but I need a very very stable output amplitude for a medical device. As anyone knows by filtering, the output drops by -20dB/decade. It can be managed by setting the threshold frequency higher but has the drawback of harmonic remnants. My question: Is there any way to stabilize the output amplitude? (by the IC itself or additional output components as an amplifier)? The output before filtering is like this: enter image description here

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  • \$\begingroup\$ What frequencies are we talking about? Where is your signal? Where are the harmonics? Also, filter output is very "stable", but it might not have the amplitude you want it to have. \$\endgroup\$ Sep 1, 2013 at 13:01
  • \$\begingroup\$ frequency is around 50-100 MHz. As seen in the picture, this is not a clean sine wave. \$\endgroup\$
    – Aug
    Sep 1, 2013 at 14:11
  • \$\begingroup\$ The signals generated should generally be below fclck/2. If this isn't the case, you will have a very hard time removing the harmonics. Very specifically, what is your clock frequency, what frequency range are you trying to generate, and what is the frequency of the observed harmonics? \$\endgroup\$ Sep 1, 2013 at 14:26
  • \$\begingroup\$ Also, the 20 dB/decade applies to one pole filters only. More poles means steeper dropoff, which you likely want, but more phase distortion, which you probably don't \$\endgroup\$ Sep 1, 2013 at 14:28
  • \$\begingroup\$ If you have a spectrum analyzer available, you may find it useful. But I would guess that your 'harmonics' are probably related more to the fundamental of the DDS. \$\endgroup\$
    – mng
    Sep 1, 2013 at 17:57

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At the outset, use a crystal of at least 8 times the desired maximum output frequency, to provide a greater gap between desired cut-off and DDS-generated harmonics:

Using a spectrum analyzer will show you that the first big noise harmonic is at crystal frequency / 2. A minor harmonic is also showing up for me at 2x output frequency, but its amplitude is pretty close to the noise floor.

By increasing the crystal frequency to beyond 4x desired output, the first big harmonic which remains at crystal frequency / 2 provides a lot more headroom for the low-pass attenuation curve.

Thus, for a 10 MHz maximum output frequency, I would use either a 80 MHz or preferably a 160 MHz crystal.

After this is done, there are several ways of dealing with the specific requirement:

  • Use LC filters instead of RC filters for an increase in filter attenuation (filter order) for each filter stage.
  • Use a 4th or 8th order active low-pass filter consisting of several stages, to increase attenuation per decade. See this paper for some insights: "10 MHz Butterworth Filter Using the Operational Amplifier THS4001"
  • Use a tightly tracking AGC (Automatic Gain Control) stage after the output, to keep the output frequency at a specific level regardless of filter stage attenuation - within meaningful limits, of course.

With a wideband Voltage Controlled Amplifier such as the Analog Devices AD600 / 602, two stages of AGC, both with a roll-off at 35 MHz (inherent to the IC), can be integrated, thus providing a stable output signal voltage for frequencies up to 35 MHz, while attenuating the 40 MHz or higher clock noise even further, beyond the preceding low pass filter stages. See figure 37 of the datasheet for an accurate AGC from DC to frequency range of interest:

Schematic

Multi-stage active filtering coupled with an AGC seems to be the simplest way to achieve flat response up to the desired frequency. If you keep the filter stage corner frequencies sufficiently higher than the maximum desired output frequency, phase shift will also be minimal.

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  • \$\begingroup\$ Nice answer! I couldn't understand what that AD590 doing there? As I know it is the thermo-sensor! \$\endgroup\$
    – Aug
    Sep 10, 2013 at 19:23

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