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AnsweredHot!Maximum Digital Signal Modulator Speed

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iRun26.2
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2019/07/16 09:58:38 (permalink)
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Maximum Digital Signal Modulator Speed

I am doing some experiments with the DSM on a PIC16LF1823.  I am sending in a square waveform with an adjustable frequency to a carrier input on the PIC16LF1823 and I have disabled the 'slew rate limiting'.
 
As I get the frequency over 20MHz the output pulses start shrinking in amplitude (the DSM seems to not work above 20MHz).
 
Is there a specification in the data sheet that defines this explicitly?  
 
I can't find anything.  I was hoping to use the DSM at 30MHz.
 
Thanks in advance for anyone who can help!
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iRun26.2
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Re: Maximum Digital Signal Modulator Speed 2019/07/16 10:21:01 (permalink)
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After thinking about it more...
 
The voltage that I was trying to run the system on (1.8V) is too low.  However, it still is not about to work at 30MHz when I power the chip with 2.5V.  I am very disappointed because I don't think that I will be to use the DSM in my application.
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Nikolay_Po
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Re: Maximum Digital Signal Modulator Speed 2019/07/18 15:03:29 (permalink) ☼ Best Answerby iRun26.2 2019/07/19 05:27:23
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Had took a look into the datasheet. Nothing special. The DSM doesn't depend on system clock because can operate in sleep mode. So none timing related to CLKOUT parameters can be applied. One thing relevant to your question is http://ww1.microchip.com/downloads/en/DeviceDoc/40001413E.pdf TABLE 30-4: CLKOUT AND I/O TIMING PARAMETERS, OS18 and OS19 parameters. Data signal modulator output is an IO pin, so I think OS18 and OS19 are applicable. They are 72ns and 55ns maximum and 40ns and 28ns typical. This means that at the frequency of 1/(40ns+28ns)=14MHz you may got a saw tooth signal and it will be in specs for 1.8V Vdd. Moreover, in worst case (worst chip at worst temperature) you can get saw teeth at 7.8MHz and it will be still in specifications. Sad but true.
-=-
Your observations are quite good. That is why the prototyping is needed. To check real suitability of the chip to your application.
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iRun26.2
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Re: Maximum Digital Signal Modulator Speed 2019/07/19 05:37:06 (permalink)
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Thanks, Nikolay_Po,
 
I  am moving to modulating my digital signal outside of my PIC16LF1823 using a small external logic gate.  They are capable of operating at a higher speed that the DSM of the PIC16LF1823.  It is, unfortunately an addition part, but it seems necessary.
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