How is my signal recovered from carrier frequency ?

Hi All ?

Please could somebody explain this to me.

I have a 4GHz (c-band) received carried signal, this is 36MHz bandwidth modulated via QPSK.

How is this converted back to base band at the receiver ? Is the process of converting to an intermediate frequency still used with QPSK ? If so, what happens the signal after its been converted to IF?

Thanks in advance,

J

Yes, still downconverted to a more usable and transportable signal. A downmixer will retain the QPSK intact but drop the carrier to a lower frequency.

From there, an oscillator with I and Q outputs are fed to mixers to recover the data.

Brian.

Hi Brian,

So once the IF frequency has been reached (around 1150MHz), the same process of mixing with with local oscillators is applied again to further reduced the signal to the required frequency ?

Thanks again,

J

1150MHz is possible but even that is still high, something in the 100Mhz region or lower is more practical. I'm guessing you are dealing with a C band satellite downlink, the ~4GHz is too high to carry any distance using conventional cables so it is usually downmixed to ~1GHz to allow co-axial cable to carry it longer distances. It is then downmixed again, usually using a tunable local oscillator to set the required receive frequency, filtered then finally demodulated using conventional I&Q mixers.

If my assumption is correct, the bandwidth after the first downmix is wide enough that several QPSK carriers can pass down the cable simultaneously and the desired one selected by the LO and filters.

Brian.

Let me classical satellite reception scheme.
Downconvert this signal to few hundred MHz IF frequency to easily amplify then obtain IQ signals by using IQ Demodulator.
IF frequency can be anything around few MHz ( for instance 479.5MHz is common for old satellite broadcasting satellite IF frequency ) in order to pass a SAW filter ( or similar LC Bandpass ).
There are many solution as you see.