This project came about as an unexpected aside to another project.
The other project is an MCU based function generator using a MAX038. That project uses a VCA for reasons that are explained on the function generator page.
The VCA can easily be converted into a simple low level AM Transmitter.
How it works.
The carrier is generated from a 4MHz crystal, and divided down to 1MHz. Any digital tuner will lock into the signal on 999kHz. IC2a and IC2b form a 4 stage Butterworth filter so we get a nice clean 1MHz sine wave. Q1,2,and 3 do the modulation of the carrier. Q1 simply alters the gain of the matched Q2-Q3 pair. The signal is then fed into a differential amplifier with a gain of 5, and then a final stage with a gain of 5.
IC3 is simply wired up as a summing amplifier. R23 sets the initial level of the carrier. R24 is used to fine tune the gain. R25 sets the symmetry of the matched transistors.
Without an RF amplifier, the transmitter goes about 10m.