Hi Marc,
welcome to GNU Radio!
The grey ports are "message passing" ports. You can, for example, use them with the Message Strobe block. For more info on message passing, I'd like to refer you to the guided tutorials,
I think somewhere in chapter 4 or 5. I'd recommend starting at chapter 1 and going linearly through the chapters – they build atop of each other, and since you've obviously already know a lot, they should be a fun and quick read!
The signal you describe is a linear chirp and you wouldn't produce it by changing the frequency of a signal source all the time. Instead, use the Phase Mod block! Go to its documentation tab. It says there:
output = complex(cos(in*sensitivity), sin(in*sensitivity))
Now, to make a constant-frequency tone, we'd connect a signal source to the Phase Mod's input (hint: connect a throttle to its output, and a QT Gui time sink to that), and set that to produce a Saw Tooth of amplitude 1 with the frequency we want. (read, process, yeah: that means that when the sawtooth does one period, the output of the phase mod is one full sine/cosine).
Now about the frequency:
From theory, we know that frequency is simply the derivate of phase over time (right, we showed that just now practically!),
, which implies that
So, if the frequency itself changes over time linearly, we know that
From which follows that
. So the phase needs to be a quadratic function, not a linear one like the one we're using now!
We arbitrarily demand that the starting phase should be 0, so
. Because I'm lazy, I'm also setting 
(and
leave you to figure out why), and hint at 
having very much to do with how fast the frequency changes (hint:
frequency is derivate of phase in time). So, how do we make phase a quadratic function of time?
Very simple: just add a multiply block between the sawtooth and the phase mod. And set the phase mod's sensitivity to a value that makes sense for you (see hint above). Your flow graph might look a bit like this:
A comment on your intended application:
I don't understand, what are you planning to do with that chirp as soon as you have it?
Best regards,
Marcus
Hello everyone,
I have noticed that the Signal Source block has an input which says "Freq" (in gray color), but I don't know how to use it properly (I'm quite new in GNUradio)... I wanted to use it to change the generated signal's frequency automatically, without having to use a slider in the GUI or something similar that requires user interaction.
My goal is to make a frequency scanner for a band of 45 MHz that I cannot look at once because of the sample rate limitation, and then try to detect the strongest signals in that band. The idea to do this was generating a saw tooth signal which output value would be the input of a Signal Source that generates a cosine wave (in order to modulate the center frequency gradually until the entire band is scanned).
I don't know if I have explained myself well... The question is basically if it is possible to automatically change the frequency of a sinusoid between certain predefined values.
If you need more information just ask!
Thank you very much for your time,
Marc.
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