The Hewlett-Packard Model 606A is a general-purpose signal generator with a frequency range of 50 kHz to 65 MHz. The instrument has a direct reading frequency dial calibrated to an accuracy of 1%. Output is held constant within ± 1 dB and is continuously adjustable from.01 microvolt to 3 volts into a 50 ohm resistive load. An internal crystal calibrator provides check points at 100 kHz and 1 MHz intervals with an error of less than 0.01%. A front-panel meter accurately indicates the percent amplitude modulation for frequencies within the modulation bandwidth of the signal generator. The Model 606A has a highly refined amplitude modulation system which allows modulation up to 90% with low distortion and incidental FM. This feature makes possible precision distortion checks of receivers from antenna to output. The instrument can be internally modulated at 400- or 1000 Hz. It can be externally modulated from dc to 20kHz or more, depending on RF frequency in use. Complex waveforms, square waves, and dc voltages may be used to modulate the Model 606A for testing and evaluating filters, networks, amplifiers, and receivers. DIFFERENCES BETWEEN INSTRUMENTS. The Model 606A carries a five-digit serial number with a three-digit prefix (000-00000). The prefix changes only when a change is made in the instrument. The prefix, then, is an identifier, and it appears on the title page of this manual to indicate to which instrument this manual directly applies. A supplement may be included with this manual to indicate the necessary changes to be made in the manual to make it apply directly to instruments which carry a different serial number prefix.