I want to make a LFM (chirp) signal generator which will generate 4 different frequency LFM pulses.
Any suggestions what hardware I can use to design it? I wanted to use MAX4725 DAC, but now I find out it can't work with high frequencies.
I guess I have to use VCO PLL circuit connected to Atmega with DAC but designing it will make things even more complicated.
Any recommendations?
Thanks in advance.
It would be immensely helpful if you were to specify the frequency range and sweep speed you're looking to generate. Also it might be helpful to know the application for context.
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My supervisor wants the frequency to be above 5-10 MHz at least, sweep speed is not important for now, any speed is acceptable for him. The whole system is a prototype of LFM-PC radar for further research.
What do you mean by "4 different frequencies"? The speed must be an issue depending on how the pulse compression is done surely? Also important is the waveform, does it need to be sinusoidal? Generating a 10MHz sinusoid implies a sampling rate of at least 20MHz so you are well out of Arduino territory and into DSPs.
What is that, a VCO or a PLL and if a PLL why is there a DAC? Getting a PLL to chirp at any useful rate could be a challenge.
A better approach may be a direct digital synthesiser device - see this AN from Analog Devices. All About Direct Digital Synthesis | Analog Devices You might supply this with control signals from an Arduino.
Also this: Generate Digital Chirp Signals With DDS | Microwaves & RF
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Look for AD9850 DDS Signal gen .
You get these modules build complete only need to add a Arduino board , also lots of sample code on the Internet.
Not sure if it is fast enough for your project.
Good luck.
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For example, the 1st pulse start frequency is 10 MHz, the 2nd one is 11 MHz, the 3rd is 12MHz, the 4th is 13 MHz, but all 4 pulses have the same bandwidth (or sweep speed) and "pause" between them.
Yes, it needs to be sinusoidal.
I am not sure if DDS can give sinusoidal chirp. To be precise I could not find any source with some code examples for DDS.
I though your chirp was linear but you are chirping a sine wave? What is the chirp bandwidth? And how long?
That ADI device can generate up to 40MHz so covers your requirement. Whether it can chirp fast enough you'll need to assess.
Sorry, you might have to write your own code...
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DDS can do sinusoids (or pretty much any waveform in a generalized implementation). In a microcontroller controlled DDS chip, one could periodically update the phase step (increase the frequency) at intervals to get a swept frequency.
If I were implementing this I'd take a hard look at FPGA based approaches. Something like the HackRF One (or a higher end SDR) might be a good starting point that will address the "getting to RF" part of the problem.
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Thank you all 
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