Phase Unwarp - QPSK

Hello..

I want to do some QPSK facility to simple microcontroller. It's seems to
work in real life. I do the (not simplified) Goertzl algorithm and in the
end I just use lookup table to calculate arctan.

So for now I got phase signal in range pi/-pi (in my case -128 to 127 8 bit
output) and as I intend to use difference modulation I got just fine result
if I do BPSK. (ie just subtract last phase from previous one) It work's
well.

But after checking signal's I discover that phase error is so small that I
can do QPSK and double bitrate. Now I hit problem as simple difference of
last sample isn't good enougth. I think I need some phase unwarping but
can't find just right solution.

I wan't to do just phase difference as phase slowly rotate (no pll
synchronizer)

So I has PHASE (8 bit signed) and PreviousPHASE. How to calculate
difference? (0/90/180/270)?!?

eslavko wrote:
> Hello..
> 
> I want to do some QPSK facility to simple microcontroller. It's seems to
> work in real life. I do the (not simplified) Goertzl algorithm and in the
> end I just use lookup table to calculate arctan.
> 
> So for now I got phase signal in range pi/-pi (in my case -128 to 127 8 bit
> output) and as I intend to use difference modulation I got just fine result
> if I do BPSK. (ie just subtract last phase from previous one) It work's
> well.
> 
> But after checking signal's I discover that phase error is so small that I
> can do QPSK and double bitrate. Now I hit problem as simple difference of
> last sample isn't good enougth. I think I need some phase unwarping but
> can't find just right solution.
> 
> I wan't to do just phase difference as phase slowly rotate (no pll
> synchronizer)
> 
> So I has PHASE (8 bit signed) and PreviousPHASE. How to calculate
> difference? (0/90/180/270)?!?

Idqpsk = Iprev I + Qprev Q
Qdqpsk = Iprev Q - Qprev I


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

>So I has PHASE (8 bit signed) and PreviousPHASE. How to calculate
>difference? (0/90/180/270)?!?

temp = (PHASE - PreviousPHASE) + 00100000

temp(8,7) = 00 -> 0
temp(8,7) = 01 -> 90
temp(8,7) = 10 -> 180
temp(8,7) = 11 -> 270