Posted on Nov 28, 2012 7536
Maxim's MAX038 function generator chip is capable of producing nearly constant amplitude sine, square, and triangle waves with a low output impedance, from a very low frequency to more than 20 MHz. Basically, a function generator on a chip. Here is a function/sweep generator that uses the MAX038 chip. The design described on these pages are not so much instructions for duplicating the design as they are a collection of ideas to stimulate your own thoughts. The symmetry of the waveform, or duty cycle in the case of the square wave output, is variable. The performance of the generator is highly dependent upon physical layout. There is one frequency marker circuit and two selectable controls. This, in conjunction with the ability to switch the oscillator's frequency control to the selected marker pot allows accurate setting or location of the frequency marker.
To set up the generator to sweep a given frequency range, first select the band that covers the frequency of interest. Press the FLOW button and then adjust the FLOW pot to set the lowest frequency in the sweep range, for example 400 kHz. Then press the FHIGH button and adjust the FHIGH pot to the highest frequency in the sweep range, for example. 500 kHz. Then press the SWEEP button and the output signal will repetitively sweep from frequency set with the FLOW up to the frequency set with FHIGH pot, in this case, from 400 kHz to 500 kHz, then the frequency will rapidly return to the FLOW. The large envelope in the middle of the waveform in the photograph above shows the bandpass of a 455 kHz ceramic filter, The two smaller envelopes at the sides correspond to when the generator rapidly sweeps through that frequency again as it returns the MAX038 to FLOW in preparation for another sweep. FLOW must be set to a frequency lower than that to which FHIGH is set in order for the circuit to sweep. The Much of the circuitry is dedicated to controlling the voltage that controls the MAX038's oscillation frequency. Multiplexers allow the selection of the Low Frequency Set Pot, High Frequency Set Pot, either of the two Marker Set Pots, or when the inputs to the multiplexer are properly set, the circuit toggles between the Low Frequency Pot and High Frequency Set Pot as the MAX038 output frequency is swept between the corresponding...