RTL-SDR with Raspberry Pi 'PiFm' project

Hi all...

       I was totally excited after receiving the RTL-SDR dongle, the poor man's Software Defined Radio Receiver. I hooked it up to see the beautiful spectrum of signals in SDR# software.   And it worked like awesome. The blue waterfall is just awesome.... I wanted to test the device as fast as i could...But since i live in a villlage with no Commercial FM coverage, it semed to difficult to get hold to any strong RF signals.. Still i could get some random energies in 945-960 Mhz GSM band...pretty noisy... digital data.. so not much to be heard to...

After some random browsing, i came across a beautiful project to make the Rasp Pi transmit in Wide Band FM mode. The project name is PiFm. It is a nice simple example of SDR transmitter. So without any hardware for FM modulation, a 300 lines C code modulates the DPLL output of Pi's ARM Processor with respect to the digital music data from a wave file. "Kudos to the guys who did it..". The output comes at GPIO pin 4 of R pi.

 So i thought to listen to PiFm transmitter using my new RTL-SDR.

So logged to R pi through ssh and downloaded the PiFm source code. Compiled and run it with default parameters

Tx freq : 103.3 Mhz

file       : sound.wave, sample came with PiFm project

command for running PiFm : sudo ./a.out sound.wav

Now i opened SDR# and tuned to 103.3Mhz at WFM mode.. Voila.. nothing was heard.....!!!!

So to make sure R pi was transmitting i tuned to this frequency from my VHF/UHF handy radio...which was success, i heard some good music playing...

But nothing through RTL-SDR....mm.. creepy.. rite..?

The blue spectrum shows no signal gain in this frequency.. i was confused a little.. but...

I simply hooked up a wire to the GPIO 4 pin... Really Voila.. this time...

The blue spectrum changing its color to yellow, red, and so on with the music..

I turned the volume up... and here the music comes....

So i understand that the antenna with RTL-SDR is real crap... R pi and RTL-SDR both connected to PC where sitting less than 20 cm away...

Any way the simple wire hooked at GPIO 4 just did a great job...

And being my first experiment with SDRs, i am satisfied...

Some observations..

Tuning to 103.3 Mhz on SDR gave noisy signal output. A shift of 100Khz was necessary to listen the clean

music, which i think is the tuning error of RTL-SDR dongle. SDR#  has option to correct if any shift present.

Also another observation is the delay while playing music from SDR. It is the obvious software processing delay i understand. The delay was noticed while both my VHF/UHF handy radio tuned to 103.3 Mhz and SDR# tuned to 103.4Mhz. A 0.25 sec delay was present

Next Steps...

Build an external antenna for accessing the VHF repeater at 144-146Mhz HAM band through SDR.

Another external antenna to monitor the ADS-B signals around at 1090 Mhz. We do have an International Airport within 120 kms. So airplanes must be present...

Some pics...