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[SOLVED] Digital Function Generator: how does it work?

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rob_the_inductor

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Digital Function Generator : how does it work ?

Hi,

I'm looking for a better understanting of the principles of digital function generator.

Let's take for example specs from a known device (SDG5000)

-Up to 160MHz Output Frequency
-500MSa/S Sampling Rate
-14-bit Vertical Resolution
-Memory depths of 512kpts for CH2 and 16kpts for CH1

As far as I understood, this device can generate periodic signal up to 160MHz.
Sampling rate is related to the number of samples per period, for instance a 1MHz signal will be described 500 points in that case.The more the frequency increases the more the number of samples decreases

A sample is composed of 14 bits ans is stored in its relative memory (let's take the bigger one 512kpts).
I have trouble understanding when the frequency is as low as the number of samples is higher than the number of samples stored in the memory. For instance at 1Hz, you need to load 500M samples to respect the specs but only 512k are awailable.

Thank you for help
 

Re: Digital Function Generator : how does it work ?

The question can be reduced to "how are built-in and user defined arbitrary waveforms represented in memory and how can they be output at different frequencies"? Reviewing the user manual and programming guide clarifies that the waveform are tabulated as equidistant points which can be replayed at different speeds. In other words, the keyword is "interpolation". 500 Ms/s is the internal sampling rate, stored waveforms are sampled with this rate by a DDS generator.

- - - Updated - - -

Looking at the programming manual 4.1.5. arbitrary waveform example, it seems like no interpolation takes place, just resampling the tabulated waveform. Respectively waveforms with only few points are replayed as staircase rather than piecewise linear interpolation. Thus you need a large number of point for a smooth waveform.
 

Re: Digital Function Generator : how does it work ?

I didn't find the chapter you quoted but it is clearer to me now. Thanks
In the following sentences I use "low" and "high" standing for low/high against the sampling frequency.
So when the output frequency is "low" points can be either interpolated to fill the gaps (if the output resolution permits it); or not and this will result a staircase looking signal.
On the other hand, when the output frequency is "high", since the sampling frequency is constant, only few points will describe the signal (not the whole sample memory). In that case some samples are skipped.

Is there a rule of the thumb to choose the number of samples stored in memory ?
 

Re: Digital Function Generator : how does it work ?

I was referring to the recent programming guide downloadable from Siglent.

If data points are skipped with high signal frequency, different points are used in each period if the output frequency isn't an integer fraction of 500 MHz. I would use at least 2^8 to 2^10 data points if the waveform is generated programmatically.
 

Re: Digital Function Generator : how does it work ?

As far as I understood, this device can generate periodic signal up to 160MHz.

That will be true for a simple square wave.

You need to describe an arbitrary waveform in terms of points- each point must be 14 bits (virtual amplitude resolution) and use a max of 512k points to describe the waveform.

These values will be output at the max rate of 160MHz.

To do this reliably, the internal processing must be done at a somewhat higher frequency.

The two channels can be used like a mixer or modulator.
 

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