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Radio Spectrum Monitor.

LCD Display.


    This Radio Spectrum Monitor by a Japanese engineer looks and works great, there are full construction details and code available on his site. You have to watch the video, it’s very impressive.
Video after the jump.

“This is an experimental work to monitor a spectrum pattern in radio band, and is a continuous project from Audio Spectrum Monitor. To analyze the spectrum of an input signal, I chose an Atmel AVR microcontroller that used in the Audio Spectrum Monitor to process FFT. When think it easy, it can be thought that sample an input RF signal directly and analyze it will do. However, you will able to recoginize that there are some techinical difficulties from following reasons.

    1. The acquisition unit must have a sufficient speed and accuracy that covering over the radio frequency range. As for AM radio band like this project, fast 12 bit ADC and specific controller will able to cover this range. However there is no proper ADC for UHF band.

    2. Number of samples to meet required frequency resolution, fSAMP/fFUND samples, becomes too large. When monitor an AM radio band around 1 MHz in frequency resolution of 500 Hz, over 4000 samples will be required at least. And when monitor a VHF band in same frequency resolution, how many samples will be required...

To solve this difficulties, there is a generic method called Frequency Conversion. In brief, down-convert the RF signal to lower frequency with a mixer (multiplyer) before sampling stage. When process the signals as complex signal for the frequency conversion, it can handle negative frequency, and the center frequency can be moved to zero hertz without interference by image signals. This means that the sampling frequency higher than span frequency range will do. When monitor a 100 MHz band in span of ±1 MHz, it will be converted down to 0±1 MHz and sampled it in only 2 Msps. You may able to understand easy when explain it as `cut and paste` on the frequency domain. Actually, this kind of radio spectrum monitors are being supplied from some radio equipment vendors.


Frequency Conversion.


In this project, an intermediate frequency signal from mixer output of an AM radio (fC=455kHz) is used as an input signal. It is converted again down to zero hertz in complex signal, so that the signal path, mixer and local oscillator, must be composed for complex signal processing. The complex signal is expressed in two wire IQ signal, the real part corresponds to I signal and the imaginary part corresponds to Q signal. The arithmetic circuits for IQ signal are realized in method of complex arithmetic. For example, a mixing circuit for IQ signals requires four multiplyers and two adders from the formula: (a1+jb1)(a2+jb2) = (a1a2-b1b2)+j(a1b2+a2b1)”


Block diagram.


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