Circuit from Model for RF power input Note rectification of the signal: low-pass filter this (which happens mechanically in the microphone) and you get the amplitude of the RF signal. Presto! AM demodulation! 20mV 4mV 100mV 8V

* receiver from high-voltage-lab.com/projects/78/bigs/2sradio.jpg Vcc cc 0 9 Q1 cc ant e1 npn3904 Q2 c2 b2 0 npn3904 Q3 c3 b3 0 npn3904 Cfb fb 0 0.1u Cblock c2 b3 100n Rfb c2 fb 20k R1 e1 b2 1k R2 cc c R3 cc c3 10k Rb3 cc b3 260k * LC tank with series resistor and current source to model RF input power irf ant fb 0 ac 1 sin (0 1u 1.52x) L1 ant Lint 100u RL1 Lint fb 10 C1 ant fb 100p.model npn3904 NPN (Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne= Ise=6.734 Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=3.638p + Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75 Tr=239.5n + Tf=301.2p Itf=.4 Vtf=4 Xtf=2 Rb=10).op.ac dec k 100x.tran 1n 10u.options post.end cc fb b2 b3 c2 c3 ant Rfb Cfb Cblock Q3 Q2 Q1 ****** operating point information tnom= temp= ***** ***** operating point status is all simulation time is 0. node =voltage node =voltage node =voltage +0:ant = :b2 = m 0:b3 = m +0:c2 = :c3 = m 0:cc = :e1 = m 0:fb = :lint = subckt element 0:q1 0:q2 0:q3 model 0:npn3904 0:npn3904 0:npn3904 ib u u u ic u m u vbe m m m vce m vbc m m vs m power u m u betad gm m m m rpi k rx ro k k k

The AC sweep shows the resonant frequency of the tank. Make sure that you match the resonant frequencies of your transmitter and receiver! V(c3) top trace. V(ant) bottom trace.