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 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 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 sine/square/triangle function generator in a single chip !.
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While it is possible to build a complete function generator 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 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 design consists of little more than three 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 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 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 t he absolute minimum, minimum, it was decided not to include i nclude 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 i t uses a simple relaxation r elaxation 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 capacitor and the input current at the Iin pin.
This internal triangle wave is fed into an internal comparator 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 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) (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 frequency range that the chip will operate operate within. The second and third factors are the Current Input "IIN" (pin 10) and the Frequency Adjust "FADJ" (pin 8) pins. Somewhat contrary contrary to it's name, the Frequency Adjust pin is not the best way to adjust the output frequency. The output frequency is actually actually directly proportional to the current current going into the IIN pin. The FADJ input is merely used for finer frequency adjustment (or for frequency modulation), 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 microamps to 700 microamps. microamps. The IIN pin acts as a virtual ground, and thus it is a simple matter matter of applying a positive voltage via a resistor resistor to IIN. This produces a current into IIN which can 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) IIN(æA)/Cf(pF) Where Cf is the value of the oscillator capacitor on pin 5. This formula assumes that 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 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 ranges respectively. For the highest range, 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. state. This is why the PCB has separate power tracks tracks running to pins 16 & 17, each with it's own decoupling capacitor. capacitor.
Output from the MAX038 is fixed at +/-1V regardless of the waveform 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 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 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, 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. components. With IC1 being more expensive than most of the other components 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 normal operation. However, if a higher voltage 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 for the pots is about 30mm in from from one side of the front panel. If the PCB is mounted to close close to the middle of the box then there there will be no room to mount the transformer. transformer. There are no front panel mounting mounting nuts used on the switches, so make make sure these holes are clean and and aligned properly. Now apply the front panel 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 recomm r ecommended ended to mount the components as shown 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 piece of earthed PCB between the transformer 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 blank PCB to the mains earth.
When wiring the fuse holder to the active pin of the IEC connector, make 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. replaced. Be sure to insulate all mains wiring, wiring, and make a solid mains mains earth connection to the transformer 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 ty pes of waveform are selectable, and that the MAIN and FINE frequency adjustment controls work on all three ranges selectable selectable by S1. The three frequency ranges ranges should overlap each other, 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. components. So setting the range to the "500Hz-100KHz" "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 l east 100MHz.
Two prototype units reached a frequency of around 25MHz, 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 i s not warranted in a low cost design such as this.
That's all there is to the new low cost 20MHz function generator. 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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