[Discuss-gnuradio] LFM Problems

[Michael Hill]

I'm using GRC v 3.6.0 and looking at what can be done with the standard library.


Although I've been able to apply FM to a wave, I'm still having issues understanding how to manipulate the variables.
There is a Frequency Modulation block.. but i'm a bit confused about what line 55
http://gnuradio.org/redmine/projects/gnuradio/repository/revisions/5d69a524f81f234b3fbc41d49ba18d6f6886baba/entry/gnuradio-core/src/lib/general/gr_frequency_modulator_fc.cc

From what I understand, to get Linear Frequency Modulation, I need a parabolic change in phase.

d_phase = d_phase + d _sensitivity*in[i]   

where (i believe)
d_sensitivity is set constant
and
in[i] is an input source of samples

From what I gather from an earlier discussion thread if I put add a sawtooth input then I should get a Linear Frequency response?
http://old.nabble.com/Generating-chrip-signals-td3985122.html

I appear to be getting a LFM, when i try feeding a Source [Sample Rate of 25M, Sawtooth wave,  Freq 1k, 
Ampl 1]  into a Frequency Modulation Block [Sensitivity of 0.002]
but I'm unsure how to manipulate the inputs.

E.g. how would I set my values for d_sensitivity and amplitude to get my LFM to shift from a frequency of 1000k to say 2000k?
Also, i'm a bit concerned that the sawtooth output doesn't appear to be linear :/

Is there a better way of doing this using existing blocks?
Also what is the maths behind it.. i've just been beating my head against the wall on this one and I appear to be just missing something

Please help!