circuito detector am

:D Hi! I need to construct an AM detector to be used in the 915MHz range. I can always get access to the carrier (transmitter's output) frequency. The big problem is that the input signal (915MHz from the receiving antenna) can vary from -40dBm to +20dBm and the AM modulation deep is very (really very) small. Some -90dBm sensitivity would be required to demodulate the signal. That means that conventional diode/rectifier demodulators are impracticable. I'm looking for a circuit that can down convert the signal to the baseband using the known LO transmitter's carrier frequency (something like a zero freq. IF mixer). I have already tested a circuit like the one shown in h**p://www.anotherurl.com/library/2.4GHz_AM_RX.htm but it seems to be impracticable at 915MHz an suffers from poor selectivity (however such a circuit developed for 915MHz would be welcome). Could someone help, please?
Thank you all.

Can you increase the modulation level? One further thing would be to have two receivers, one with some RF gain in front of the diode. Examine the two outputs and accept the information from the one that appears more valid.

:? Hi ! The modulation level can't be increased. It is in fact the backscattering signal coming from an RFID system placed 20feet apart. The transmitting and receiving antennas are placed side by side, that's why the input signal can reach a +20dBm level. The big advantage is that I can have access to the carrier frequqncy. I know of a circuit that uses a mixer (marked MCL JSIQ-911D-1, couldn't find any spec. from it) that works in this application.
Big thanx.

With all AM detectors you need to have AGC, the larger the dynamic range the better. I used a Philips device that was actually meant for TV applications in one of my designs some years ago. I will look up the number and if it is still available post it here. You will need to mix down the freq to fall within the chip's max operating range +/- 38MHz
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It was a TDA8341 but it looks like it is no longer available. :(

http://www.semiconductors.philips.co...40_1_CNV_2.pdf

You might find something as a replacement in a search

I attach the circuit diagram of where I used it in one of our spectrum analyzer design's demod function for FM/AM at the time if it is of any help

With ZIF configuration, use a DC block cap to take out the LO then amplifire the AM signal.