FSK receiver using sa606 - Need help
I am doing my master's degree thesis and i neet to build a transceiver for human body communication.
For the communication i am using FSK modulation.
I'm having some trouble to understand the sa606 IC and how to use it to demodulate the fsk signal.
I will use the f0 and f1 frequencies arround f1=(10 MHZ + delta) and f0=(10MHZ - delta).
My teacher gave me this IC and said to use it, but i need some help.
If you know something about this subject, please help me.
The integrated circuit is this : http://www.nxp.com/documents/data_sheet/SA606.pdf
Thank you
Humberto Castanheira
Its an FM receiver.
Pins 1 & 2 are the input to the chip. it is a diferential input, and since your antenna is probably single ended (like a monopole antenna) you need to convert single ended to differential (that is what C1, C2, L1, and C5 are for).
There is half of an oscillator inside of the chip--the active part. A transistor is set up with feedback to form an oscillation. Pins 3 and 4 have a frequency determining network that forces the oscillation to be at only one frequency. The X1 is a crystal resonator that very precisely sets the frequency, and C6 & C8 trim the phase shifts so the oscillation starts up repeatably. C7 & L2 are there to stop the crystal resonator oscillator from oscillating at the wrong frequency (i.e. kills other resonant modes in the crystal resonator from being able to oscillate).
Once the oscillator is running and stable, it is used as a local oscillator in the mixer. The local oscillator downconverts the input frequency to a much lower frequency.
FL1 eliminates other unwanted signals from the antenna, as well as LO leakage, from getting into the IF amplifier. A second filter FL2 filters some more. The output of FL2 is sent to a limiting amplifier (probably up to 60 dB of amplifier gain). The actual IF sine wave gets clipped in this amplifier into a square wave like waveform. That signal is sent to a FSK demodulator.
The FSK demodulator appears to be a mixer set up as a phase detector. A phase lag/lead circuit is attached to pin 10. So if the frequency is above the center, the phase circuit lags, if the frequency is below, the phase circuit leads. The phase detector senses this phase change and outputs a DC voltage proportional to the changing phase. In other works, an FM modulation is turned into a phase modulation, which is then phase detected and turned into a video voltage.
That output voltage goes into a simple op amp, and the output is pin 8.
The discriminator circuit connected to pin 10 probably has a limited bandwidth, so you need to tailor your transmitter FM deviation to be within the frequency limits of the discriminator.
Also, of course, the transmitter FM deviation has to be small enough to get thru bandpass filters FL1 and FL2.
Note, the transmitter has to be stable enough in frequency so that transmissions are centered around where the pin 10 discriminator circuit is tuned to.
Thank you very much for your answer biff44.
If i have any more doubts can i PM you?
Thank you for your time.
Best regards
Humberto Castanheira