The NE605 is ideal for a simple AM receiver. It contains a mixer good to 500 MHz, an onboard local-oscillator (LO) transistor, 80 dB of stable IF gain, and an FM-quadrature detector. This application does not use the FM detector; instead, the received signal-strength indicator (RSSI) serves to demodulate the AM signal's sidebands. The RSSI output is intended to provide an indication of the received signal's strength, but if an amplitude-modulated signal is tuned to the upper or lower sideband (slightly off center of the carrier), the RSSI acts like an AM demodulator.

In this circuit, the LO is based on a simple Colpitts-oscillator circuit. An MV209 varactor diode tunes the LO to the proper frequency. The LO feeds the internal mixer that directly gets its RF from a short antenna. The circuit uses no preselection at the input to the radio, because aircraft communications at all airports are sufficiently spread across the spectrum so that intermodulation arising from closely adjacent signals is usually not a problem. The intermodulation becomes even less of a problem thanks to the use of a small antenna. However, because of the lack of RF preselection, you hear each station at two spots on the radio tuning dial. You hear the signal when the LO is tuned 455 kHz below the RF signal and again when the LO is tuned 455 kHz above the desired channel's frequency.

The IF is 455 kHz, and the IF filtering comes from two low-cost ceramic filters. The filters are 12 kHz wide to make the tuning less sensitive (again, strong, adjacent channels are not usually a problem around an airport). The IF section consists of 80 dB of gain; each gain stage feeds a small current to the RSSI pin. When all these RSSI currents add up and the LO is tuned to one of the AM sidebands, the RSSI provides a kind of AM demodulation. To get the output audio-signal level high enough to drive a small set of Walkman-type headphones (usually 8 or 16 ohms), the circuit includes a simple audio amplifier. The amplifier consists of a 2N4416A- or an MPF-102-type n-channel JFET that drives a simple 1-kiloohm to 8 ohm audio transformer.

The battery drain is about 10 mA, so you can expect approximately 40 hours of operation from a standard 9V alkaline battery. You can use this basic circuit for FM communications on other VHF bands by using the built-in FM detector, or you can use it in AM mode from nearly dc to VHF by simply changing the LO frequency.