1. Circuit from http://www.high-voltage-lab.com/78/am-receiver 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

2. * 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 c2 2330 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=1.259 + 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 1000 10k 100x.tran 1n 10u.options post.end cc fb b2 b3 c2 c3 ant Rfb Cfb Cblock Q3 Q2 Q1 ****** operating point information tnom= 25.000 temp= 25.000 ***** ***** operating point status is all simulation time is 0. node =voltage node =voltage node =voltage +0:ant = 1.3861 0:b2 = 692.8320m 0:b3 = 657.2999m +0:c2 = 1.4840 0:c3 = 534.8725m 0:cc = 9.0000 +0:e1 = 789.1450m 0:fb = 1.3862 0:lint = 1.3861 subckt element 0:q1 0:q2 0:q3 model 0:npn3904 0:npn3904 0:npn3904 ib 4.8926u 96.3131u 32.0873u ic 91.4205u 3.2208m 846.5128u vbe 596.9967m 692.8320m 657.2999m vce 8.2109 1.4840 534.8725m vbc -7.6139 -791.2099m 122.4274m vs -8.9999 -1.4808 -534.0260m power 753.5613u 4.8466m 473.8674u betad 18.6856 33.4414 26.3815 gm 3.5528m 119.8599m 32.5292m rpi 6.5422k 328.7536 991.5584 rx 10.0000 10.0000 10.0000 ro 893.0581k 23.2296k 87.3074k

3. 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.