C14 and P1 are to cancel residual carrier before it passes to the two ceramic IF filters then IF amplifier IC3 which also does the envelope demodulation.
Receiver circuit is from Elektor 7/2010 UK edition, AM receiver with quadrature mixer.
I think is a mistake in the Elektor article. The Image Noise in a superhetrodyne receiver can degrade the sensitivity with maximum 3dB, and not 6dB as is mentioned in the article.
Its the image of the IF component that is cancelled not the modulation.
But If I did not use an IF and alter the schematic so that the quadrature mixer downconverts directly to audio instead of 455KHz (DCR), then this quadrature mixer should cancel the opposite sideband of the signal. Ie, the "image" signal would be just the opposite sideband, and the carrier would be also suppressed, am I right?
I guess "DCR" stands for Direct Conversion Reciver (right?).
In that case, the 90 degrees filter is at baseband and must have a very wide relative (percentual) bandwidth. At FI is very simple because it covers a narrow relative bandwidth.
Such a filter (a Hilbert filter) has ideally 0 transfer at dc, so carrier is suppressed if the LO has exactly the frequency of the carrier.