Peak to average and QPSK modulation
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This forum is run by the BATC (British Amateur Television Club), it is service made freely available to all interested parties, please do not abuse this privilege.
Thank you
Re: Peak to average and QPSK modulation
If we were to carry the I and Q signals in separate RF amplifier chains and only combine them after their final amplifiers, could we relax somewhat the requirement for such good linearity? Obviously we would need to balance the two chains for amplitude and phase, but that should be not be much more of a problem than the adjustments required when we set up a normal I and Q modulator.
Robin.
Robin.
Re: Peak to average and QPSK modulation
This reply is for the local ham that has difficulty understanding there is a significant “peak to average power” ratio (PAPR) for transmitted QPSK signals. The chart below shows that most digital modulation technologies (except GMSK modulation) used by DATV have significant power peaks when compared to the true average power of the signal.
The way I look at this in my head is that…because there are power peaks, you have to reduce the RF drive so that the peaks go no higher than the P-1 dB compression point in the RF amplifier. If you drive the RF amp harder, the peaks will “flat top” in the RF amp and increase the distortion on your signal. You may not easily see the distortion on your receiver, but a spectrum analyzer wave form will begin to show increasingly wide signal (ie. interference to neighboring signals)...and the MER value at the receiver will begin to degrade.
On the RF Amplifier web site from Alberto (DGØVE) you can read (in German): “All amplifiers can also be used for DVB-S and DVB-T with reduced average power. You will notice that in the DVB-S mode only about 20% to 25% of the maximal power (P-1dB) can be used.”
I hope that this can help you with the concept.
73…de Ken W6HHC
The way I look at this in my head is that…because there are power peaks, you have to reduce the RF drive so that the peaks go no higher than the P-1 dB compression point in the RF amplifier. If you drive the RF amp harder, the peaks will “flat top” in the RF amp and increase the distortion on your signal. You may not easily see the distortion on your receiver, but a spectrum analyzer wave form will begin to show increasingly wide signal (ie. interference to neighboring signals)...and the MER value at the receiver will begin to degrade.
On the RF Amplifier web site from Alberto (DGØVE) you can read (in German): “All amplifiers can also be used for DVB-S and DVB-T with reduced average power. You will notice that in the DVB-S mode only about 20% to 25% of the maximal power (P-1dB) can be used.”
I hope that this can help you with the concept.
73…de Ken W6HHC
Last edited by KenW6HHC on Sun Sep 24, 2017 2:14 pm, edited 1 time in total.
Re: Peak to average and QPSK modulation
Maybe this will help - it explains the effects with fairly simple maths http://www.springer.com/cda/content/doc ... p174124973
Mike
Mike