Experiments with Detector Diodes

When building crystal radios or other simple receivers, the experimenter often wonders about the relative performance of the different diodes in the junk box. Here are the results of several experiments using the typical types available to the hobbyist. The source is a low impedance and the load is a fairly high impedance. A particular diode will behave differently with different impedance levels but for low received signal levels these measurements are fairly predictive of the relative performance in most circuits. The diode types include germanium, silicon, Schottky, and even a light-emitting diode! The test setup uses an accurate RF synthesizer, a homemade AM pin diode modulator driven by an audio generator, a simple test fixture, a DC power supply for adding bias current, and a sensitive audio voltmeter. The setup shown below was used to test the diodes at several frequencies with a low modulation index (about 20%) and the near optimum bias current was determined by varying the DC supply.



The Schottky diodes are the all-around winners if bias is used but they do not perform as well as a germanium diode without bias. Other small-signal Schottky diodes gave nearly the same results as the 1N5711. Other germanium diodes were tried but the results were nearly identical to those shown. (The 1N60 was not available for testing.) The 1N4454 is similar to other ordinary silicon diodes and the results were poor, as expected. Several LEDs were tried and a bright type red led gave fairly good results as shown. Zeners were tried with dismal results in both directions. Large rectifiers like the 1N4001 were similarly poor.