RE: Is this correct?

[Macy]

Actually, an excellent description. it's, as you said, like 'multiplying' the 
frequency because the starting point keeps 'scooting' forward. Dryer rack 
allegory eludes me, but the rung [starting point] moving forward caught. ;)

Not sure but from memory frequency is a 'convenient interim term' and is 
defined as dphase/dt, so always go back to that basic. Then, it becomes obvious 
that the t.^2 term ends up adding 2 times multiplier. 

CONCLUSION is "don't do ANYTHING by inspection." and, of course, verify, 
verify, verify.

Now I have to go reconcile what happens when the frequency stops at the end:
sig=sin(2*pi()*1e6*(300+100*[0:N/2-1]/N));
and by that erroneous inspection the 'frequency' looks like it ramps up to 
350MHz and then stays there, but as shown earlier the frequency ramps up to 
400MHz, then what? snaps back to 350MHz for the last half of the time packet. 
Arrggg! feel like Calvin and Hobbes, ...brain will explode.

At least out of all this, I've explored some new 'windowing' functions. like 
splitting the hanning window:
N=100000;
filt=hanning(10000)';
nuwindow=[filt(1:5000),ones(1,90000),filt(5001:end)];
% normalize to area = 1
nuwindow=nuwindow*N/sum(nuwindow);
sigfilt=sig.*nuwindow;

makes for an impressive fft display.
barely distorts the spectrum.



--- address@hidden wrote:

From: <address@hidden>
To: <address@hidden>, <address@hidden>
Subject: RE: Is this correct?
Date: Fri, 25 Apr 2014 18:50:58 +0000

> -----Original Message-----
> From: address@hidden
> 
> When I changed the dt=1/100/fc to get finer resolution it became
> obvious that going up using my original version, the final
> 'frequency was indeed 500MHz and going down the final frequency was
> indeed 200MHz. so the fft's were both correct, it was my 'time'
> waveform checks that were wrong!
> 
> I had checked that time looked right, but the resolution was so
> poor, didn't notice that the frequency did go up to 500MHz! Thanks!
> 
> To correct for this, I had originally thrown in the divide by two,
> but did not understand the 'why'

The "why" I think is that you are changing the frequency starting at
the beginning of the sweep.  This sounds confusing...what I mean is
for every frequency step you start with zero phase at zero time, so
you have a multiplier effect by the time you get to wherever you are
in the sweep.  So effectively the sweep rate speeds up with time.
If you saved the phase angle every step and started at that phase
angle for the next step you would be OK.  It's like a folding drying
rack where if you move the first rung a bit the end zooms out.
Hope this is helpful but I doubt it :-)

Allen