If you are modulating a bitstream what are you are really doing is 2-FSK in this situation, not traditional analog FM - so that may help you find the definitions you are looking for a bit easier.
If you want a modulation index of 2 and your rate is 1200K then you want the difference between the high and low frequencies to be 2400KHz. Take a look at the example FSK mod/demod flowgraph on the wiki<https://wiki.gnuradio.org/index.php/Simulation_example:_FSK> -> it explains the 'sensitivity' parameter. Kyle Logue Senior Engineering Specialist ⚝ The Aerospace Corporation ________________________________ From: Discuss-gnuradio <discuss-gnuradio-bounces+kyle.a.logue=aero....@gnu.org> on behalf of Anthony <anthonyld...@gmail.com> Sent: Monday, April 4, 2022 00:23 To: Discuss Gnuradio <discuss-gnuradio@gnu.org> Subject: FM Transmitter tuning issue (pre-emphais, modulation index..) Hello all, I am implementing a FM Transceiver following this prerequisites : Frequency modulation with a pre-emphasis of 6 dB/octave (phase modulation) with frequency-shift of the modulating sub-carrier : – frequency-shift between 1 300 and 2 100 Hz; the sub-carrier being at 1 700 Hz; – the frequency tolerance of the 1 300 and 2 100 Hz tones is 10 Hz; – the modulation rate is 1 200 (bit/s) * (1 ± 30 * 10−6); – the index of modulation is 2.0 10%. To implement the frequency modulation, I've followed the example based on the use of a VCO in GNU Radio : https://wiki.gnuradio.org/index.php/VCO with these parameters : - full-scale frequency of 3000 Hz -> VCO sensitivity of 2*pi*3000 (I chose 3000 to fit my 2 frequencies of interest). - I adapted the Multiply and Add Const block to set the space/mark frequency to 1300 and 2100 Hz. The input of the VCO is quite the same as in the example : I am sending bursts of bytes from a ZMQ socket that pass in a Repeat Block to be interpolated 40 times. I have managed the sample rate with a USRP B210 at the end, so that the input sample rate before the Repeat block is 1200 (1200x40 = 48 KS/s, 48 KS/s being the VCO's sample rate). I then interpolate the output signal of the VCO using a Rational Resampler to match the sampling rate used in the USRP block (960 kS/s). In this scheme, I don't know if I have the right approach to implementing this modulation. Especially : - How the pre-emphasis is added in this scenario, where my input data has a rather constant frequency (bitstream at 1.2 kHz) ? - How is the modulation index set (which variables can affect it ?). I've found a definition like this : m = deviation frequency / modulating frequency -> deviation frequency is +/- 400 Hz to me (around 1.7 kHz) -> modulating frequency is 1200Hz (as the symbol rate equals the bitrate here) So my modulation index is 1/3, quite far from 2. I surely misinterpret it, and my output signal in frequency is quite thinner than an expected signal. What parameters could I use in my implementation to set it to 2 ? I outputted the resulting signal, which is quite thinner but still I see pics at 1.3 and 2.1 kHz, and I'm able to demodulate it and recover my data on a RX path. I think I got some obvious things wrong in my FM transmitter. If you have some resources or advices to point me in the right direction, that'd be useful! :) I am using GNU Radio 3.8.5 (maint branch) and UHD 3.15. Anthony
