by using a comparator and the sawtooth or triangle signal. Most function generators also contain a non-linear diode shaping circuit that can convert the triangle wave into a reasonably accurate sine wave by rounding off the corners of the triangle wave in a process similar to clipping in audio systems. A walking ring counter, also called a Johnson counter, and a (linear) resistor-only shaping circuit is an alternative way to produce an approximation of a sine wave. This is perhaps the simplest numerically-controlled oscillator. Two such walking ring counters are perhaps the simplest way to generate the continuous-phase frequency-shift keying used in dual-tone multi-frequency signaling and early modem tones. A typical function generator can provide frequencies up to 20 MHz. RF generators for higher frequencies are not function generators in the strict sense since they typically produce pure or modulated sine signals only. Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively "sweep" the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit. Some function generators can also generate white or pink noise. More advanced function generators are called arbitrary waveform generators (AWG). They use direct digital synthesis (DDS) techniques to generate any waveform that can be described by a table of amplitudes and time steps. Specifications Typical specifications for a general-purpose function generator are: Produces sine, square, triangular, sawtooth (ramp), and pulse output. Arbitrary waveform generators can produce waves of any shape. It can generate a wide range of frequencies. For example, the Tektronix FG 502 (ca 1974) covers 0.1 Hz to 11 MHz. Frequency stability of 0.1 percent per hour for analog generators or for a digital generator. Maximum sinewave distortion of about 1% (accuracy of diode shaping network) for analog generators. Arbitrary waveform generators may have distortion less than below and less than above Some function generators can be phase locked to an external signal source, which may be a frequency reference or another function generator. Amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) may be supported. Output amplitude up to peak-to-peak. Amplitude can be modified, usually by a calibrated attenuator with decade steps and continuous adjustment within each decade. Some generators provide a DC offset