High Frequency Function Generator With the Maxim MAX038

We needed a function generator in PCBheaven and instead of buying one we decided to build one from scratch. The whole idea is based in maxim's MAX038 chip. This chip is rather expensive, yet has many capabilities. It can provide 3 different types of waveforms, triangular, rectangular and sine wave, as well as it provides a TTL output with 50% duty cycle for synchronization reasons. The frequency can be adjusted from 0.1 Hz up to 20 MHz.

The frequency range can be selected by placing a different capacitor to the COSC input. A 12 positions rotary switch with 12 different capacitors is perfect for this reason. The output frequency can be selected from within the frequency range by a potentiometer and another pots is responsible for the fine tuning

The DADJ input of the chip is the control for the duty cycle of the waveform. It has a switch to either disable it (50% fixed duty cycle), or enable the duty cycle adjustment. If enabled, the duty cycle can be adjusted by a potentiometer.

The output waveform type can be selected from a "FUNCTION" switch. This is a 3-position rotary switch. The common of the switch is driven HIGH, while the two inputs of the chip, A0 and A1 are pulled LOW with 10K resistors. When both inputs are '0', the output is square wave. When A0 is '1', the output is triangular waveform, and when A1 is '1' and A0 is '0', the output is sine wave.

The output is buffered with an OP37Z OP-AMP. The peak to peak amplitude is also controlled a potentiometer. By default, the output waveform is symmetrical to the ground. A DC offset can be added to the output if the switch "OFFSET ENABLED" is closed. The DC offset is also controlled by a potensiometer.

Step 1: Materials

Here is a rough list of materials used for this project. Since completion some changes have been made but you can read them in the worklog.

R1 10K        R13 75 Ohm   C1 0,1uF     C16 22uF   D1    1N4001
R2 10K        R14 330 Ohm   C2 0,1uF     C17 100uF    D2    1N4001
R3 47 Ohm   R15 6.8k            C3 0,1uF     C18     IC1    MAX038
R4 10K        R16 820 Ohm   C4 0,1uF     C19 0,1uF      IC3    OP372
R5 1K pots   R17 100K           C5 0,1uF     C20 10nF       IC4 A    TL072P
R6 1K pots   R18 100K           C6 22pF      C21 0,1uF      IC4 B    TL072P
R7 1K pots   R19 100K           C7 82pF      C22 0,1uF      IC5    LM741P
R8 5K pots   R20 100K           C8 330pF   C23 0,1uF       IC6    7805T
R9 10K        R21 10K             C9 1nF        C24 0,1uF       IC7    7805T
R10 1K       R22 10K pots    C10 4,7nF   C25 1000uF    S2    12 pos. rotary switch
R11 1K     R23 500 Ohm   C11 32nF    C26 0,1uF
R12 10K       R24 500 Ohm   C12 68nF    C27 1000uF
                                                      C13 330nF C28 0,1uF
                                                      C14 1uF    C29 1000uF
                                                      C15 4,7uF   C30 1000uF

Apart from the electronics, 2 acrylic glass plates 16x10cm used as face plate for the controls along with some colored buttons to give a more pro look.

Step 2: PCB

PCB and finished result.
Keen eye can observe some slight modifications form original schematic. Since this is going to be used in our labs near the main power supply, the 7805 and 7905 chips have been removed. Instead, the +- 5 volts come directly from power supply. Also the PCB was created with the IC2 (TL72P) to control the frequency select and fine tuning. But during the tests, didn't behave as expected and therefore was replaced with two 10K resistors. Updated schematic can be found here.
The etching came out perfect with our home made etching bath but that is another instructable to come.
The soldering of the parts had to be done very carefully, as the grounding layer had just .1'' insulation.

Step 3: Range Selector

The range selector is a 12-position single pole rotary switch. Each switch position corresponds to a different capacitor. So, either you solder the capacitors to the PCB and draw 12 (+1 for common) wires to the selector, or do this:

First, make a ring with a thick single core wire (2.5 mm²). The diameter should be about 10mm larger than the diameter of the rotary switch. Cut and solder the two edges of this wire to make a closed ring. Then, solder one terminal of a capacitor to one position of the rotary switch. The other terminal of the capacitor is free, outside the switch's diameter. On this terminal, solder the ring. Then, on the symmetrical pin of the switch, solder likewise another capacitor. Do that for all 12 capacitors. When finished, all 12 capacitors connected directly on the selector switch, and only 2 wires would now be required from the PCB, the 0V wire which is connected on the ring, and the COSC input for the chip.

Step 4: Artwork and Face Plate

The need of a casing is not in the plans since the whole project will be put in a big closet that houses the mains supply and other staff. The only thing needed is a plate to mount the pots and switches and a nice face plate so you know what that knob is doing, the design is kept to the bare essentials.
As mounting plate we took a 10x16 cm 10mm thick white acrylic. When artwork was done we made a cut-out mask for the holes and the wood that will hold all. After finishing the holes and wood cuts the whole thing was in place and another 10x16 transparent acrylic plate was put in front of all to give that nice proffesional look.