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Direct Digital Synthesis (DDS) makes it very easy to design a low-cost, high-performance function generator – one of the handiest pieces of test gear you can have. This one can be set to any specific frequency between 1Hz and 10MHz and offers both sine and square wave output. T he 20MHz Low Cost Function Generator I described in the August 96 issue of “Electronics Australia” has proven very popular, with thousands having been built. The low cost, simple construction and wide bandwidth made it a very attractive project. However, there were two problems with that design – the lack of a frequency display and being able to set the frequency to exactly what you wanted. To overcome these, one kit supplier bundled the function generator with another frequency counter kit. So it is not surprising that many people have asked for an updated design with a built-in frequency display. While a frequency display would be relatively easy to add, it would add significantly to the cost and complexity of the project. In addition, the analog 32  Silicon Chip nature of the original design meant that temperature drift would also be an issue. This new design overcomes these problems by adding a frequency display and digital frequency selection, while still maintaining the low-cost approach. The design The design is based on an Analog Devices AD9835­, a complete 50MHz by David L Jones* (clock) Direct Digital Synthesis (DDS) sinewave generator on a single chip. The MAX038 used in the previous design is analog in nature and setting the exact output frequency is difficult unless you have many ranges with fine adjustment. Going digital with the DDS chip is the obvious way to go: it allows you to set the output frequency exactly from 1Hz to 10MHz in 1Hz steps and there is effectively no drift with temperature or time as the output frequency is crystal-locked. As the AD9835 is clocked at 50MHz, in theory it is capable of generating a sinewave up to 25MHz. At this frequency, the output waveform quality is more difficult to control and amplify, so 10MHz was taken as the arbitrary upper limit. This results in a good quality, low distortion, large output signal level over a 0-10MHz range. While 10MHz is not as high as the 20MHz+ in the original design, it is still sufficient for most applications and is way beyond the 2MHz or so of most commercial analog bench generators. www.siliconchip.com.au Low Cost 1HZ –10MHz DDS Function Generator It’s a tiny, tiny chip! It’s nice – but by no means essential. Our frequency display is now oneTo set and display the frequency tenth the cost of an LCD and is easier Unfortunately, the AD9835 DDS to assemble. chip is only available in a 16-pin you really only need to display one To also help reduce the cost, the PC TSSOP (Thin Shrink Small Outline digit at any one time, in which case a single 7-segment LED display can board has been designed to fit into a Package) surface-mount package, be used. standard UB3 Jiffy box, with no wirwhich makes it challenging(!) to solder ing required – everything mounts on You do need some other indication to by hand. A TSSOP package has half the pin tell you what digit you are setting – but the PC board. RCA output connectors were chopitch of your typical SOIC surface a simple LED can do that. sen in preference to BNC connectors, mount IC package, a mere 25-thou as the PC board mounting (0.635mm). Compare it with Specifications: RCA connectors are about the 74HC14 SO14 package one-tenth the cost of the also used in this design. BNC type. Waveform Generation: 32-bit DDS, 10-bit DAC More will be mentioned In fact, you can buy a Distortion: <1% at 1kHz about how to solder this PC board mounting RCA Frequency Range: 1Hz to 10MHz in 1Hz steps device later. connector plus an RCA-toFrequency Display: “Sliding Window” 7 digit There are other DDS chips in BNC convertor for less than (EEPROM frequency retention) the Analog Devices DDS prodthe cost of a BNC PC board User Input: Three push buttons uct range with easier-to-handle mount connector. Sinewave Output: 0-5Vp-p adjustable, 50Ω packages but they are much Squarewave Output: CMOS/TTL compatible, 50% duty cycle bigger, more expensive and Main controller Output Connections: RCA don’t have a nice serial interPower: 9VAC 100mA plugpack A PIC16F628 8-bit miface like the AD9835. cro- controller was chosen My first prototype for this as the main controller. It design used an LCD panel for has 2KB of internal FLASH The end result is a single 7-segment the frequency readout but this would have significantly increased the cost LED display with a row of 3mm LEDs program memory, 224 bytes of RAM of the project. So I thought about it to indicate which digit of the output and 128 bytes of EEPROM. This is for a while and came to the realisation frequency is being displayed and set. adequate for our control program and So the row of LEDs correspond to data storage. that you don’t really need to display This chip serves three purposes. the entire frequency at any one time. X,XXX,XXX Hz. www.siliconchip.com.au May 2003  33 Fig.1: inside the AD9835 DDS IC. Its operation is explained in the text. On the facing page is Fig.2, the circuit diagram. One is to control the display and switches, the second is to send the necessary control commands to the AD9835 DDS chip and the third is to store the output frequency in the internal EEPROM. The frequency set by the user is stored in the internal EEPROM and this same frequency is automatically set when the project is next powered up. The 4MHz internal RC oscillator is used to lower system cost and to free the extra pins for I/O functions. There are no critical absolute timing requirements in the project, so a crystal oscillator is not required. How DDS frequency generation works The internal block diagram of the AD9835 (Fig.1) shows its operation as well as that of a basic Direct Digital Synthesis (DDS) generator. A DDS generator consists of three major components – a phase accumulator, a sine-wave lookup table (usually con34  Silicon Chip tained in ROM) and a digital-to-analog convertor (DAC). The phase accumulator is also commonly referred to as a Numerical Control Oscillator (NCO), although no part of a DDS actually “oscillates”. As sinewaves are non-linear in nature, they are not that easy to generate accurately, even using standard sampling techniques. On the other hand, the “phase” of a sinewave is completely linear in nature, from 0-360°, and thus lends itself to be more easily generated. Using a reference clock and the linear aspect of the phase, a DDS generator can generate very accurate sine- waves of almost any frequency, completely in the digital domain. The phase accumulator basically performs an integration function and generates a linear phase “ramp” in proportion to the desired frequency, which is contained in the 32-bit register FREQ0. The sine (or cosine in the case of the AD9835) lookup table converts the linear phase ramp into a sinewave. As sinewaves are completely symmetrical every 90°, the lookup table only needs to store one quarter of the waveform, with some appropriate control logic to map this over the full cycle. (Different DDS devices have differing ways to do this). Even though the AD9835 contains a 32-bit frequency control register, a 32-bit (232 memory bits) lookup table is not required, as the AD9835 only has a 10-bit DAC. So the (co)sine lookup table does not need to be much more accurate than this; in this case it is only 12 bits. This ensures that the accuracy of the signal is determined entirely by the DAC resolution and linearity. How it works: As you can see from the schematic (Fig.2) there isn’t much to the design. IC3 does all the frequency generation, IC2 handles the control and display, while IC1 and IC4 provide output drive. IC3 is programmed by a custom www.siliconchip.com.au www.siliconchip.com.au May 2003  35 All components mount on the one single-sided PC board, although some are on the back (track side). Note how the LEDs stand up to poke through the case lid. The PIC chip should be socketed to allow for any future firmware upgrades. This pic is of an early protoype – the one presented is V2.0. 3-wire serial control bus from the micro- controller (IC2). Data and commands are transferred in 16-bit words. The serial interface is run asynchronously to the main clock and can be run at any speed determined by the host micro. IC3 has various modes and internal registers that must be defined before it will output a frequency, and these are explained in the data sheet for those who are curious. IC3 is also capable of frequency and phase modulation of the output signal, both of which can be controlled by either the serial bus or an external pin. For the sake of simplicity and cost reduction, these features have not been implemented into this design. The actual output frequency is determined by the value in the 32-bit FREQ0 register. The value in FREQ0 will be equal to F/(50MHz/232), where F is the desired output frequency. From this equation you will see that with our 50MHz oscillator we can get approximately 12mHz resolution and this value will also equal our output frequency uncertainty (ignoring crystal accuracy). In the case of a 1Hz output frequency, the FREQ0 register will contain the value 86 which equates to 1.001Hz, not quite 1Hz but close enough! At the low end of the frequency range the frequency accuracy will be 0.1% worst case, while at the high end the frequency accuracy can be controlled to a staggering one part in four billion. Low distortion was not a major de36  Silicon Chip sign requirement, so for simplicity no measures were taken to improve this. The total harmonic distortion (THD) is around the 1% mark, which is adequate for most general applications. External output filtering can be added to improve this if desired. IC3 is clocked by XTAL OSC1, an industry standard 50MHz TTL/CMOS 8-pin DIP crystal oscillator. The stability of the generated output signal will be dependant upon this clock but for this low-cost application any grade oscillator will suffice. There are many brands of oscillator that match this standard footprint – some come in plastic DIP, while others come in a metal can package. IC3 requires digital and analog power and ground pins. These are individually decoupled at the power pins and run as separate lines from the regulators. IC3 provides a current output on pin 14, which is converted into a voltage by R4. C11 is an optional filter capacitor and is not fitted in the standard design. R2 sets the full scale DAC output signal current level, which in this case is approximately 3.9mA. This is already at a maximum value that will not compromise the performance of the chip. Thus the output voltage across R4 will be approximately 1V p-p referenced above ground, as IC3 is powered from a single power rail. This signal is AC-coupled by C12 and referenced to ground and user adjusted by VR1. The signal is then amplified by IC1 which operates with a gain of 4.9, as set by R5 and R6. This gives an The early prototype board has a capacitor and two resistors strapped on to the back. In the final version these are “on top”. The main purpose of this photograph is to show the components which are soldered to the back of the board, especially the tiny AD9835 TSSOP IC (top, centre) which requires great care. www.siliconchip.com.au Fig.3: front (top) and back (bottom) sides of the PC board showing component placement. It is a single-sided board as far as tracks are concerned; some components are on the track (copper) side. The majority of components are polarised or need to be mounted a certain way (for example, the three pushbutton switches). Note carefully the comments about soldering IC3 (and IC4 for that matter). approximate maximum output signal level (at all frequencies) of ±2.5V peak. Thus the final sinewave output level is adjustable from 0-5V p-p. IC1 provides a low impedance buffered output and R7 provides a nominal 50Ω output impedance. IC1 can be either a National Semiconductor LM6361 or an Elantec EL2044. The EL2044 is the recommended device as it has a slightly higher bandwidth, so will provide a higher signal level output at the high end of the frequency range. IC4 is a hex Schmitt inverter, with C13, R1 and R3 providing a level shift function to bias the ground-referenced sinewave input signal to half the supply rail (suitable for a 5V TTL input). The Schmitt input squares up the sinewave and gives a CMOS/TTL output. The 7-segment display and digital LEDs are multiplexed onto the same output pins on IC2. The common cathode line for both displays goes back to a separate pin on IC2. IC2 is thus able to “switch” alternately between displaying the 7-segment display and the digit LEDs. This is done in firmware and each display is turned on for a 5ms burst at a rate determined by the main loop. As long as it is greater than 50Hz or so you won’t see any flicker – all the LEDs appear to be continuously on. The pushbutton switches are debounced in the firmware by a small delay of a few hundred milliseconds after each key press. The power supply is a typical halfwave rectified AC input with positive and negative 5V regulators (REG1 & REG2). Heat dissipation in the regulators Parts List – 10MHz DDS Function Generator 1 PC board, 123 x 56mm; coded 04105031 1 UB3 Jiffy box (130 x 67 x 44mm) 1 18-way IC socket (for IC2) 3 momentary action PC-mounting switches 2 RCA sockets, PC-mounting (with BNC adaptor if required) 1 knob to suit potentiometer shaft 4 12mm x 3mm tapped spacers 4 20mm x 3mm bolts, nuts and washers www.siliconchip.com.au Semiconductors 1 EL2044 or LM6361 opamp (IC1) 1 16F628 PIC microcontroller, programmed with DDSFRQ20.HEX (IC2) 1 AD9835 DDS generator (IC3) 1 74HC14 hex Schmitt trigger (IC4) 1 LM7805 +5V regulator (REG1) 1 LM79L05 -5V regulator (REG 2) 1 50MHz TTL oscillator (XTAL OSC1) 1 FND500 7-segment LED display (DISP1) 7 3mm red LEDs (LED1-7) 2 1N4001 silicon diodes (D1, D2) Capacitors 2 470µF 10V PC electrolytic (C12,13) 1 470µF 25V PC electrolytic (C14) 1 100µF 25V PC electrolytic (C15) 2 10µF 10V PC electrolytic (C1,2) 2 100nF MKT SMD (0.2p) (C6,7) 5 100nF ceramic (C3,9,10,16,17) 2 10nF ceramic SMD (0.2p) (C4,5) Resistors (1/4W 1% unless noted) 1 47Ω 1 120Ω 1 270Ω (SMD) 3 470Ω 2 1.5kΩ (SMD) 1 3.9kΩ (SMD) 2 470kΩ 1 1kΩ potentiometer May 2003  37 The PC board is connected to the case lid via four tapped spacers. Suitable cutouts and holes in the lid allow the switches, LEDs and pot shaft to poke through, while the 7-segment display can also be seen through a cut-out. will depend on the input voltage level from the plugpack. Ensure that this is not too high – a 9V AC plugpack is recommended as a maximum. The plugpack should be rated at 100mA or greater. Parts availability All of the components are available off-the-shelf for those who wish to con- struct the project from scratch. Farnell carry the “hard to get” bits like the AD9835, AD6361, 50MHz oscillator and surface mount 74HC14. As far as the PC board is concerned, it is not out of reach of home manufacture using good lithography techniques, despite the TSSOP track/pad pitch being extremely small. It is however recommended that a proper solder-masked PC board be obtained as this will make construction a lot easier. The HEX file is available for download for non-commercial use from the SILICON CHIP website (www. siliconchip.com.au) and the author’s website at www.alternatezone.com Short form kits and PC boards may also be available from the author. Given the popularity of the original design, it is probable that kits will be made available by major suppliers in due course. Construction Start construction with the AD9835 TSSOP IC (see separate panel). This way, there will be no other components to obstruct you and the board will sit flat and steady on your bench. This IC is by far the most difficult component to solder in this kit; indeed, it may be the most difficult com- Soldering the AD9835 TSSOP IC Soldering the AD9835 is the most difficult aspect of project construction and unless you have the right tools and experience it is likely that you will have problems. Don’t underestimate how hard this will be: the pin pitch is 1/4 that of a regular IC and half that of a standard SOIC. If you have a solder-masked PC board then your job will be a lot easier, as the solder mask will help stop the solder bridging between pins. As a minimum you will need a good temperature-controlled soldering iron with a fine tip suitable for surface mount soldering, 0.45mm solder and tweezers. A chiselled tip is much better than a conical tip, which will have difficulty making good thermal contact. The ideal tip to use is the “wicking” chisel type which has a small cavity in the middle of the tip to help “wick” the solder back off the joint, this helps to keep the amount of solder on the pins to an absolute minimum. Proper 0.45mm surface-mount solder should be used – anything bigger will be a nightmare. Alternatively (if you have them available), proper surface-mount solder paste and a hot-air surface-mount soldering gun will give you a first class job. A magnifying lamp will come in very handy for this job – in fact, it is probably essential as many people will have to do the soldering under a magnifying lamp. You would need really good eyesight to solder and inspect the job without a magnifying lamp. The best approach is to apply a small amount of solder to one of the corner pads of the chip. Then use tweezers to place the chip over the pad (ensure correct orientation!). You can then reheat this pin and move the chip with the tweezers to get it properly centred. Before you solder any more pins, double check the orientation of the chip: desoldering the chip later is an option you don’t even want to think about. 38  Silicon Chip Once the chip is aligned on the pads correctly then solder the pin on the opposite corner so the chip will hold in place. A “wicking” motion of the soldering iron away perpendicular from the pin is your best shot at avoiding bridges between pins. The regular soldering technique of applying the iron to the pin and pad and then applying the solder on the other side of the joint will not work in this situation. Neither the soldering iron tip nor solder are small enough to allow this. You will find that excess solder will form around the joint no matter how hard you try to control how much you use. So you will have to just try and “wick” it away from the joint. The biggest killer when hand soldering surface mount components is too much heat. Not only can excessive heat lift pads and crack tracks but worse, it can crack or destroy the component internally. Only solder one joint at a time and let the device cool before moving on. Do not apply heat to any joint for more than one or two seconds at most. Patience here could help prevent a big headache later. www.siliconchip.com.au These two ’scope shots show both analog and digital outputs at two different frequencies. The shot on the left is at 1kH and shows a pretty good result on both sine and square waves. The other shot is at 5.736MHz (no, this frequency is nothing special!) and shows a still-quite-usable waveform in both cases. The ringing on the square wave, in particular, is due to the signal being unterminated – it would be significantly better into a load. ponent you will ever have to solder! Having successfully soldered and checked the TSSOP, the other 0805 passive components and the SOIC package will be much easier to solder – but similar rules apply. Watch the orientation of the SOIC. Finish construction with the usual through-hole components. The PIC chip should be socketed to allow for firmware updates. The 7-segment display will require a regular wide IC socket cut to size to stand it off from the PC board. The LEDs should be mounted about 10mm proud of the PC board. The easiest way to do this and to ensure alignment of all the LEDs is to cut a 10mm strip of cardboard and place this between the legs of all the LEDs while soldering. Watch the LED polarity. The PC board is mounted behind the front panel on four 12mm spacers You may notice on the prototype that R1, R3, and C13 are retrofitted on the underside of the board. The published design has these components added to the top of the PC board, as shown on the overlay. escaping from any of the components! Next, measure the outputs of the regulators, they should be plus and minus 5V (respectively). In use When first powered up, the project will most likely be set to 0Hz, so there will be no output waveform. This could vary though, depending on the initial contents of the EEPROM bytes in the PIC chip. In either case, press the SFT (SHIFT) button to select which digit will be displayed on the 7-segment display. Use of the SFT button allows you to quickly determine the output frequency. The INC button will increment the currently displayed digit and will wrap back to zero. There is no ability to decrement the number other than the wrap around. Using SFT and INC does not update the frequency at the output. To do this you must push the SET button. This sets the output frequency and also stores the frequency in the internal EEPROM memory, so this frequency will be automatically reloaded when the project is next powered up. Remember that the output frequency will only change when you press the SET button, so the display will only reflect the actual output frequency when you have not touched the INC button since the last time the output frequency was set. When using the TTL/CMOS output, ensure that the output level control is set to maximum, as this output is generated directly from the amplified sine -wave output. SC Happy generating. * david<at>alternatezone.com Testing Before you power up the project, check for any shorts, especially on the surface-mount devices. Do a visual inspection and a multi-meter check. Don’t try to probe the pins of the TSSOP chip directly; instead probe the other end of the appropriate track on a larger component. Apply power to the project and ensure that the vital smoke is not www.siliconchip.com.au Full-size artwork for the Function Generator front panel can also be photocopied and used as a drilling template for the case lid. May 2003  39