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Low Cost 20MHz Function Generator with MAX038

By David L. Jones

The High Speed Function Generator was published in the professional electronics section of

the Aug 1996 issue of Electronics Australia, and has proven to be extremely popular. The kit

is no longer available from any of the kit suppliers. The project is capable of generating

20MHz or greater Sine, Square, Triangle, and TTL waveforms.

I thought that I had a relatively well equipped work bench up until a while ago when I wanted to test

some op-amp circuits for a new design. They needed to work at several megahertz, and it would

have been nice to test them at over 10MHz. However, it all soon came to a halt when I remembered

that both of my function generators were only capable of a little over 1MHz. I did have access to

some expensive commercial digitally synthesised function generators capable of over 10MHz, but I

wanted a simple and low cost one for my own bench !.

Want being the mother of invention, I put my current design aside for a while (as usual!) and

decided that I needed to build a high speed function generator.

The first thing I did was to look for any previous function generator projects that meet my

requirements. The only one that catered for frequencies in the MHz region was the digitally

synthesised designed published in the April 93 edition of EA. Whilst it did handle the frequency I

needed, I wanted something with real "KNOB'S" that could be easily adjusted.

The design was starting to look terribly complicated when along came the MAX038 chip from Maxim

in the United States. A complete 20MHz sine/square/triangle function generator in a single chip !.

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While it is possible to build a complete function generator using just the MAX038 and a few

capacitors and resistors, I found that it needed just a little bit more circuitry to make it into a fully

fledged function generator design.

The final design described here is capable of producing sine, square, triangle, and TTL level

waveforms at up to and beyond 20MHz. See the accompanying table for full specifications.

Normally, an analog function generator capable of this frequency would be very expensive and very

complex. But as you can see, this entire design consists of little more than three IC's !.

Please note, this project has been designed for the highest possible frequency coverage, and NOT for

the lowest possible distortion level. With a sine wave harmonic distortion of greater than 1%, it is

not suited for precision audio applications. But it should be fine for general audio use however.

The MAX038 was originally quite hard to obtain in Australia, but it is now stocked by RS Components

and is quite readily obtainable. The performance of the MAX038 does not come cheaply at almost

$50 for the chip, however considering the specifications of the device, it is the lowest cost way to get

a function generator in this frequency range.

To keep the cost of the unit to the absolute minimum, it was decided not to include a frequency

display. It is assumed that the unit will be used in conjunction with a frequency counter or an

oscilloscope, the new EA frequency counter design published in February 93 would be an ideal low

cost solution for this. Other features such as duty cycle adjustment and modulation inputs have also

been omitted for the same reason.

How The MAX038 Works

Surprisingly, the MAX038 does not use any fancy or patented circuit techniques to produce

frequencies up to and beyond 20MHz. Instead it uses a simple relaxation type oscillator that

operates by charging and discharging a capacitor (on pin 5) using a constant current. It is basically a

dual slope integrator that produces a triangle wave the frequency of which is determined by the

external oscillator capacitor and the input current at the Iin pin.

This internal triangle wave is fed into an internal comparator to produce the square wave function.

The sine wave function is produced by feeding the triangle wave into a sine wave shaping circuit that

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automatically corrects for the desired frequency, and produces a reasonably low distortion sine

wave at a constant amplitude.

The sine, square, and triangle waves are then fed into a multiplexer that selects which function to

output to the low impedance output buffer.

Circuit Details The MAX038 does all of the work in generating the actual waveform, so all we have to do is to tell it

which frequency to operate at, and which type of waveform (sine/square/triangle) to produce.

The output frequency is determined by three factors. The first one is the value of the oscillator

capacitor on pin 5, which sets the frequency range that the chip will operate within. The second and

third factors are the Current Input "IIN" (pin 10) and the Frequency Adjust "FADJ" (pin 8) pins.

Somewhat contrary to it's name, the Frequency Adjust pin is not the best way to adjust the output

frequency. The output frequency is actually directly proportional to the current going into the IIN

pin. The FADJ input is merely used for finer frequency adjustment (or for frequency modulation), as

it only has about 70% of the adjustment range of IIN.

The current into IIN used to adjust the output frequency can range from approximately 2 microamps

to 700 microamps. The IIN pin acts as a virtual ground, and thus it is a simple matter of applying a

positive voltage via a resistor to IIN. This produces a current into IIN which can be simply worked

out using Ohm's law. In fact, the output frequency can be related by the simple formula : Fo (MHz)=

IIN(æA)/Cf(pF) Where Cf is the value of the oscillator capacitor on pin 5. This formula assumes that

FADJ is at zero volts.

The buffer circuit formed by IC2a is used to provide a stable voltage reference to drive the current

input pin IIN. This also allows use of a low value pot (VR1) for the main frequency adjustment. An on

chip 2.5V bandgap voltage reference is fed into the voltage divider arrangement of R5, VR1, and R6.

Stopper resistors R5 and R6 stop the value of IIN from going outside of it's allowable linear range.

Fine frequency adjustment is achieved by varying the voltage on the FADJ pin. Unlike the IIN pin, the

FADJ pin works by varying the input voltage up to plus or minus two volts. In this circuit, the range is

set to +/-1.6V by the voltage divider action of R3, R4, and VR3. IC2b then buffers the voltage which

goes to the FADJ pin via the low pass filter of R2 and C7.

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The three frequency ranges are selected by range switch S1. This simply parallels C2 or C3 across C4

to give the low and medium ranges respectively. For the highest range, only C4 is connected in

circuit.

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The Sine, Square, and Triangle functions are selected by S2. A0 (pin 3) and A1 (pin 4) are digital

select inputs which select via an internal mutiplexer, either the Sine, Square, or triangle waveform to

be fed to the output.

The digital TTL output is taken directly from the SYNC output on pin 14. This output changes from

low to high when the analog output crosses zero volts going positive. This can be used to

synchronise the analog output to some circuit under test, however it's main use is just as a

standalone TTL level output.

The SYNC output has it's own supply pin (pin 16) that has to be decoupled from the main analog

power supply on pin 17, otherwise a small "spike" will appear on the analog output when the digital

sync output changes state. This is why the PCB has separate power tracks running to pins 16 & 17,

each with it's own decoupling capacitor.

Output from the MAX038 is fixed at +/-1V regardless of the waveform type selected. This output

level is referenced to the chip's internal 2.5 bandgap voltage reference. If a larger output voltage is

required, then an external amplifier will have to be added.

The MAX038 contains it's own low impedance output buffer, but this is of no use if the final output

level needs to be varied, in this case by VR2. The wiper of VR2 is fed to the final stage buffer chip

IC3, an EL2001 manufactured by Elantec. The EL2001 is a 70MHz fixed unity gain buffer capable of

driving 50ê coaxial lines. R17 provides close to a standard 50Ω output impedance.

The power supply circuit comprising REG1 and REG2 is a standard +/- 5V regulated supply fed by a

half wave rectified AC input.

CONSTRUCTION

As can be seen from the photos, the unit is housed in a standard UB1 jiffy box measuring 90 x 150 x

50mm. This allows just enough room for both the main PCB and a small 7V AC mains transformer

with an IEC mains input connector and fuse.

All of the circuitry for the unit is contained on a single sided PCB measuring 115 x 40mm, which is

attached via the POT's to the front panel. The only internal wiring is to and from the mains

transformer, and BNC output connectors. All of the POT's and switches are mounted at right angles

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along one side of the PCB. This not only makes assembly easier, but also reduces noise from internal

wiring and provides a support for the PCB.

PCB Mount Version

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Regular PCB BVersion

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Start assembly with the low profile components such as the resistors and two diodes, followed by

the bypass capacitors and IC's, and then the rest of the components. With IC1 being more expensive

than most of the other components combined, some constructors may wish to mount it in a socket.

However, for best performance it is recommended that it be soldered directly onto the PCB. If using

a socket, then ensure that it is a low profile machine pin type.

Using a 7VAC transformer for the supply, the regulators will only become warm and should not

require a heatsink for normal operation. However, if a higher voltage AC supply is used, the

regulators will most probably require a heatsink for any sort of continuous operation. If adding a

heatsink, be sure not to short the tag's of both regulators together, as one is the supply input and

the other is ground !.

The next step is to drill the required holes in the front panel. Use a photocopy of the drilling

template provided. Make sure that the centre hole for the pots is about 30mm in from one side of

the front panel. If the PCB is mounted to close to the middle of the box then there will be no room

to mount the transformer. There are no front panel mounting nuts used on the switches, so make

sure these holes are clean and aligned properly. Now apply the front panel label ensuring that it is

aligned with the controls.

Now turn your attention to the holes required for the IEC mains connector, fuse holder, and

transformer. The alignment of these holes is not critical, however it is recommended to mount the

components as shown in the inside shot of the unit.

The mains wiring and transformer should be kept as far away from the PCB as possible, to minimise

hum and risk of shorting. You can add a blank piece of earthed PCB between the transformer and

PCB to divide the box into two halves, one for the PCB and the other for the transformer and mains

wiring. If you do this, be sure to connect the blank PCB to the mains earth.

When wiring the fuse holder to the active pin of the IEC connector, make sure it is connected to the

END tag of the fuse holder and not the side tag. This is to ensure that the fuse holder does not

become live when the fuse is replaced. Be sure to insulate all mains wiring, and make a solid mains

earth connection to the transformer chassis.

The transformer is a Dick Smith type M-2824, which is a 7V centre-tap with an extra 7.5V winding.

Only the 7V winding is used, so the centre-tap and extra windings can be cut off. Alternatively,

insulate the ends and tie them up neatly within the case.

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Before connecting power, double check all of the mains wiring and measure the transformer primary

at the IEC input pins, you should get a value of around 2KΩ.

Before installing IC1 in it's socket, apply power and make sure that you get +5V and -5V on pins 17

and 20 respectively.

Once you have finished construction and the power on checks, the unit should be ready for use.

IN OPERATION

As there are no alignment or adjustment operations to perform prior to operation, the unit should

work first go as described.

Connect the output to an oscilloscope and apply power. With the level control set to maximum, you

should be able to get a waveform of the type selected by S2. Ensure that the three types of

waveform are selectable, and that the MAIN and FINE frequency adjustment controls work on all

three ranges selectable by S1. The three frequency ranges should overlap each other, but if there is

a gap between any of the ranges, C2, C3, or C4 might not have the correct tolerance, however this

would be unlikely.

The three frequency ranges on the front panel are to be used as a guide only, as the actual range

values are determined by the tolerance of the oscillator components. So setting the range to the

"500Hz-100KHz" position and turning the frequency controls fully in one direction will most probably

NOT give the indicated value.

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The front panel frequency adjustment controls do not include a calibrated scale, as the very wide

frequency coverage of each range would make the scale almost meaningless. This unit therefore,

should be used in conjunction with a frequency meter. A "T" piece BNC connector can be used to

allow simultaneous use of a frequency meter and the circuit to be tested.

Using an oscilloscope, there should be little or no apparent distortion of the sine wave up to 20MHz.

However, the square wave function begins to approach a sine wave at frequencies above about

10MHz. This is due to a bandwidth limitation of the MAX038's output buffer. The triangle wave also

suffers from this bandwidth limitation, but to a much lesser degree, and is still usable approaching

20MHz. To get any sort of accurate picture of the output waveform at 20MHz and above, will require

the use of an oscilloscope with a bandwidth of at least 100MHz.

Two prototype units reached a frequency of around 25MHz, with the sine wave still being

reasonably clean. The final upper frequency all depends on the actual value of C4. C4 may be able to

be tweaked to reach an upper frequency of over 30MHz. Higher frequencies and lower distortion are

possible, but require the use of a properly laid out double sided PCB which is not warranted in a low

cost design such as this.

That's all there is to the new low cost 20MHz function generator. I'm sure you'll find the unit as

invaluable as I have. Happy generating !.

Notes & Errata There is a problem with the TTL output when the generator is used on the LOW and MEDIUM

frequency ranges AND the MAIN frequency adjust control is set to the lower 15% of it's range. Any

significant loading on the TTL output will cause it to osscillate on the positive and negative edges.

This is apparently an inherent problem with the MAX038 chip !. The only solution is to buffer the

SYNC output of the MAX038 with a 74HC14 schmitt inverter. This can be mounted on a small piece

of vero-board along with R10 and a bypass capacitor. Use the other 5 inverters in the package in

parallel to provide a high current buffered output. Be sure to connect the supply pins of the 74HC14

directly to pins 15 and 16 of the MAX038.

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