**Next message:**Steven L. Farmer: "Re: 20 or 30?"**Previous message:**James Speer: "20 or 30?"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

*>
*

*> Hmmm, nobody caught the obvious error(s) in my otherwise good post or
*

*> perhaps you are all such nice guys that you thought you wouldn't mention it :-)
*

*>
*

*> A square wave (with 1:1 mark:space ratio) has only odd harmonics. This
*

*> means the basic ideas presented are OK (and the basic conclusion is
*

*> unchanged -- you need a LPF on the output) but the numbers are wrong.
*

*>
*

*> The amplitudes of the harmonics of a square wave goes as 1/n**2 (did I
*

*> get this wrong too?)
*

*>
*

*> The first spurious emission is the third harmonic which will be 1/9 of
*

*> the fundamental in amplitude and 1/81of the power of the fundamental
*

*> (power goes as the voltage squared). This implies the spurious
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*> emissions are about -19dBc. The FCC wants -40dBc or better.
*

*>
*

*> Anyone got a spectrum analyzer and a square wave oscillator to check
*

*> this revised figure?
*

I caught it, but was a nice guy :-) (too busy to reply :-) )

Still think you're wrong - I beleive *power* goes as 1/n**2, as I can

barely see -20db distortion on a scope. I have an analyzer, and when

I looked at it I got approx -6ish for the 3rd. Will double-check on

Friday with the comm analyzer and a 20M oscillator.

Also, FCC regs want -30dbc if you're power out is <5 watts, if over 5

they want -40dbc *or* some small level, whichever is less.

-->jjw

ps if you're the one building a DDS rig, Analog Devices has been a real

bear to work with - AD9955 lead time is 22 weeks for samples, they won't

sell partial trays, the price for <100 pieces is *very* high, they don't

let *ordinary* people buy the eval kits and so forth. I've had much

better luck with Qualcomm...

**Next message:**Steven L. Farmer: "Re: 20 or 30?"**Previous message:**James Speer: "20 or 30?"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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