Re: negative hex
- From: Jan Panteltje <pNaonStpealmtje@xxxxxxxxx>
- Date: Mon, 30 Jun 2008 19:50:01 GMT
On a sunny day (Mon, 30 Jun 2008 12:43:23 -0700) it happened John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
<6gdi64l8h7sd3vj3e0ou4frjth1oqbcta2@xxxxxxx>:
On Mon, 30 Jun 2008 19:08:23 GMT, Jan Panteltje
<pNaonStpealmtje@xxxxxxxxx> wrote:
On a sunny day (Mon, 30 Jun 2008 11:55:27 -0700) it happened John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in
<ipai64p0s2jpim0mn97673p038cc5i4jhg@xxxxxxx>:
I want "-0x40000000" to program a generator to make -16.000 MHz.
Apart from connecting the '-' sign to a '-' sign in the frequency display,
I have no idea what -16 MHz would be.
Or shift 16 MHz down?
Or 180 degrees phase flipped?
What is a negative frequency?
In a DDS, negative frequency corresponds to putting a negative number
into the phase accumulator. That walks a waveform table backwards.
Waveforms then come out "inverted" in time. And if I create a
polyphase waveform set that spins a motor clockwise, when I program
the frequency to be negative, the motor will spin CCW. There are all
sorts of uses for this.
It's equivalent to asking for a frequency past Nyquist, and using the
resulting alias.
John
OK, got it (I think) never too old to learn :-)
.
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