Design Ideas: August 1, 1996

Cell phones aren't the only devices to benefit from ever- smaller and more highly integrated communications-IC technology. The simple, three-IC superheterodyne radio in Figure 1 can receive stations in the 4.5- to 10-MHz range from around the world with only a 10-ft antenna. A superheterodyne radio works by mixing the incoming RF signal with a local-oscillator (LO) signal to produce an IF. We don't need to spell it out. The circuit then filters, amplifies, and diode-detects the IF signal to reproduce the audio signal contained in the RF input.

The transformer-capacitor circuit at the input provides impedance matching to the antenna; the T₂-C_2A tuned circuit provides rough preselection for the 4.4- to 10-MHz RF signal. IC₁, a Philips NE602, contains the required RF stages. IC₁ also contains an active Gilbert-cell mixer and a transistor configured to provide the LO function (pins 6 and 7). The LO uses a simple Colpitts configuration. The L₁-C_2B tank circuit determines the Colpitt configuration's frequency. The LO operates at 455 kHz above the incoming RF, thus producing a constant 455-kHz IF output at pin 5 of IC₁.

The Toko ceramic filter removes any out-of-band responses. The 4-kHz passband of the filter provides surprisingly good audio quality and adjacent band rejection. The workhorse of the receiver is IC₂, a Plessey ZN414, originally designed as a simple, one-chip AM radio. This IC provides more than 70 dB of IF amplification, an AGC, and a detector circuit in a TO-92 package. You can set the gain of the ZN414 by changing the bias on the device using the IF-gain trim pot.

When IC₂ amplifies, provides AGC, and detects the IF, it produces baseband audio. IC₂ can directly drive high-impedance headphones, but this design uses an LM386 audio amplifier to drive a 3-in. loudspeaker. Alignment of the receiver is simple. First, connect a suitable 5V power supply to the unit. The current drain is only 10 mA, so almost any supply can work. For example, a low-dropout regulator running from a 9V battery works well. You should verify that the LO is oscillating from approximately 5 to 10 MHz when you tune C₂. You can perform this verification by placing a 1× probe or antenna from a frequency counter on or near the NE602 chip. Don't connect the counter directly to the NE602 pins, because the added capacitance changes the oscillation frequency.

Next, set the IF-gain trim pot near the center of its range and add an antenna to the circuit. You could use 10 or 20 ft of wire randomly strung about the house, but a longer outdoor antenna substantially improves performance. Tune to the center of the receiver's range and carefully listen for a station. You can usually hear a Department of Defense fax transmission on 8080 kHz at night from anywhere in the United States. (The transmission sounds like a scratchy record.) Once you hear a station, carefully tune C₂ to peak the signal.

During the peaking adjustment, you may need to adjust the IF gain or volume for optimum reception. Now, tune to the upper portion of the band. You can hear WWV, the National Institutes of Standards and Technology standard-time-and-frequency station, at 10 MHz. The station transmits time information 24 hours a day. Repeat the adjustment of C₁ for optimum reception. You may need to adjust C₂ at the low, center, and high portions of the band for a best compromise. Now, tune to any moderately strong station and set the IF-gain trim pot for maximum signal with minimum audio distortion. A large increase in the output-noise level accompanies the onset of distortion. (DI #1905)