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Basic RF Signal Generator My AM/FM/DDS Signal Generator design (May 2022) is a very convenient piece of test equipment, but it’s overkill for many tasks. If you just need a basic test signal from 10Hz to 25MHz, this Generator is it; it’s compact, cheap to build and doesn’t involve many parts. T his design came about because my Q Meter (January 2023; siliconchip.au/Article/15613) needs a 100kHz to 25MHz signal at close to 0dBm to function. Many constructors may already have a suitable signal generator, such as my May 2022 design (siliconchip.au/Article/15306). Still, I decided to create a simpler version that does the job with minimal components and at a lower cost. A DDS design is the most sensible option, and the Analog Devices AD9834 is a good DDS chip, but it costs $27 plus delivery. It makes far more sense to purchase a ready-made module, which costs less and comes with most of the necessary parts already assembled onto a PCB. There are a variety of suitable DDS modules available on AliExpress and similar; I used (and can recommend) the one at siliconchip.au/link/abjo Using a module like this takes a lot of the hard work out of the design. By itself, the module will not do anything; it requires the power and control signals through the 10-pin header. It only took me a little while to design a control module for it. This has a microcontroller with a small display to show the frequency and a knob to set it. I kept the same display and appearance as the Q Meter, the earlier Signal Generator and associated projects. Circuit details The resulting circuit is shown in Fig.1. Microcontroller IC1 is a 28-pin 26 Silicon Chip DIP ATmega168 or ATmega328. Speed is not critical, so I am using the internal 8MHz RC clock source; no external crystal is needed. The display is the same SSD1306-based 128×64 pixel OLED screen as in my other designs, and the frequency is changed by a rotary encoder with a built-in pushbutton switch. IC1 updates the display over a twowire I2C bus with the usual 4.7kW pull-up resistors. The rotary encoder terminals are pulled up by 4.7kW resistors, with 100nF and 470nF debouncing capacitors. The differing time constants make it easier for the micro to detect the encoder rotation reliably. The Generator could run from any standard 5V plugpack, but as the current drain is not high, I decided to use two AA cells and a switch-mode boost converter to generate 4.4V DC. This boost converter is the same MCP1661 or MP1541 chip used in my LC Meter (November 2022 issue; siliconchip.au/ Article/15543). Why 4.4V instead of 5V? The resulting current consumption is lower, extending battery life. The AA cells should operate down to 1V each before the up-converter drops out. This voltage is set by the ratio of the 330kW and 120kW resistors to the feedback (FB) pin of REG1, which is maintained at 1.25V. Since 1.25V × ([330kW ÷ 120kW] + 1) = 4.4V, the voltage at the cathode of D1 will increase until it reaches 4.4V, then REG1 will adjust its duty cycle to maintain that. The switch interrupting power from the battery to REG1 (S1) is Australia's electronics magazine By Charles Kosina onboard, making construction easier. The AD9834 module is powered and controlled by IC1 via 10-pin header CON1. It has an onboard 75MHz oscillator, so the maximum output frequency (the Nyquist limit) is half that, ie, 37.5MHz. But it is best to operate it lower than that, so I chose a maximum of 25MHz. As for the low end, the Q Meter needs a minimum frequency of 100kHz, but the module can go as low as 1Hz. I decided that 10Hz was a reasonable lower limit, spanning the full range of useful audio frequencies. The resolution of the signal generator is 1Hz; pressing the pushbutton on the encoder toggles through step sizes of 1Hz, 10Hz, 100Hz, 1kHz, 10kHz, 100kHz and 1MHz. On power-up, the default step size is 1MHz. CON4 is a standard Atmel six-pin ICSP header that allows you to program IC1 in-circuit if fitted. There’s also an optional serial debug interface at CON3; if you aren’t using that, you can leave off Mosfet Q1 and its 1kW pull-up resistor. However, CON3 should be fitted as it is also used to trigger calibration when S2 is closed or its pins 1 & 3 are shorted. Output frequency response Once the firmware was working, I plotted the output level against frequency, shown as the red trace in Fig.2; two problems are apparent. The output was about -11dBm, which is too low, and it falls off rapidly above 18MHz. The output level is set by one siliconchip.com.au Fig.1: the circuit is simple because the DDS signal generator is a prebuilt module that plugs into CON1. It’s controlled by micro IC1, which monitors rotary encoder RE1 and displays the status on the OLED1 screen. Power comes from a pair of AA cells via boost converter REG1 that generates a steady 4.4V. Fig.2: the output frequency response of the Signal Generator with the original resistor R2 (red) and new value (green). siliconchip.com.au Australia's electronics magazine June 2023  27 resistor, R2, which is 6.8kW on the supplied module. By changing this to 1.2kW, the output increased to near 0dBm over the flat part of the range, shown in green in Fig.2. The resistor on the module is an M1608/0603 size SMD type, but a larger M2012/0805 size resistor will also fit. I measured the output power three ways, and they did not quite agree. The most reliable method is to measure the peak-to-peak voltage on an oscilloscope with an accurate 50W RF load (how I plotted Fig.2). The other methods used the tinySA spectrum analyser and the Analog Devices AD8318 power meter. Those two methods gave values between 1dBm & 4dBm lower. This still leaves the problem of frequencies above 18MHz having a reduced level. If this is sufficient for your needs, no further modifications are needed. However, I decided that it was worthwhile to improve the frequency response. If you look at my photos, you will see that the two outputs on the module each have a low-pass filter (LPF) consisting of three inductors and three capacitors. We can fix the drop-off by replacing L4-L6 & C7-C9 with different value components, giving a cutoff frequency of 35MHz. The inductors are M2012/0805-size, and the capacitors are M1608/0603size SMDs, but again, M2012/0805 size capacitors will fit. The new 5th-order Chebyshev LPF is shown in Fig.3. You will note that C8 is not needed in this topology. The new frequency response is shown in Fig.4. The other output can be left as-is, as it is unused. Despite the 35MHz cutoff frequency, there is still a reduction at 25MHz due to the relatively low Q of the Coilcraft chip inductors I used; their rated Q factors are not high. At 25MHz, the 820nH inductor has a Q of 23, the 1.5µH has a Q of 10 and the 1.8µH has a Q of 15. These are not very impressive figures! Fig.3: this new Chebyshev LPF arrangement provides a much flatter response than the one that comes with the module. I tested some other SMD inductors that supposedly had a higher Q but they actually made the output level slightly lower. So the Coilcraft inductors are good if you can get them; the parts list includes some close alternatives that might be easier to get. Harmonics As with all DDS systems, the output is not pure, with multiple spurs. These are shown in Plots 1-3. Of the five frequencies I tested (5MHz, 10MHz, 15MHz, 20MHz & 25MHz), 15MHz and 25MHz give the purest output as they are one-fifth and one-third of the clock frequency. The only spurs are harmonics of the fundamental. All others had multiple spurs, mostly more than 20dB down compared to the output frequency (the other two not shown are similar in appearance to Plot 2). Housing it I used a 105 × 75 × 40mm ABS enclosure with a clear lid, Altronics Cat H0321. An alternative is the Altronics Cat H0323 which is deeper at 55mm. Using the shallower H3021 case, there is only just enough spare room for the battery holder on the left side. The larger one has more room for the battery holder, allowing it to be attached to the bottom of the case. Another advantage of the larger (H0323) case is that there is enough room to fit a potentiometer to allow you to adjust the output level from around -23dBm to 0dBm, to be detailed at the end of the article. Actually, you can fit a small (9mm or 10mm body size) potentiometer in the smaller (H0321) case, but using the larger case gives you more room and choices for that pot. Apart from the AD9834 module, two circuit boards are used. One contains the control circuitry, and the other is the front panel with a cutout for the display and two holes for the switch and tuning shaft. This panel is a snug fit into the detent on the front panel, and it is held in place by the nut on the switch shaft. Construction The control board is built on a 59 × 65mm double-sided PCB coded CSE221001 that attaches to the clear lid by two screws in opposite corners. Countersunk holes must be drilled 28 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.4: the measured response of the new filter. It isn’t quite as good as estimated in Fig.3 due to the limited inductor Q values, but it’s a vast improvement on the original. We calculate that the response to be significantly flatter than this using the specified components. in the clear panel for these screws – the best way to position the holes is to place the blank PCB inside the top cover hard up against the right side and use it as a template for drilling the holes for the switch, shaft encoder and two diagonal mounting holes. After that, assemble the control board (see Fig.5), starting with the surface-mounting components. The resistors and capacitors are all 2.0 × 1.2mm, so they aren’t too difficult to solder. However, the 5-pin SOT-23 chip (regulator REG1) requires some care due to its close pin spacing. It really helps to have some flux paste to solder REG1. Put a little over the pads, then place the IC over them and tack one of the pins on the side with only two pins by loading a little solder on the clean tip of a soldering iron and touching it to both the pin and pad. Check that all the other pins are correctly located over their pads; if they aren’t, re-heat the joint and gently nudge the regulator into position. Once it’s aligned, put a bit more solder on the iron and, after adding a little more flux paste, touch it to the three pins close to each other so that three good joints are formed. Check for bridges between the pins. If any have formed, add more flux paste and then use some solder wick to remove the excess solder. Finally, solder the last pin on the other side. After that, all the through-hole components on the front side can be mounted. The OLED plugs into a 4-pin socket strip and is attached by 16mm screws through 8mm untapped spacers. Depending on the exact OLED siliconchip.com.au Plot 1 5MHz Plot 2 10MHz Plot 3 25MHz Australia's electronics magazine June 2023  29 Fig.5: fit the components to the control board as shown here; note that there are two 4.7kW resistors under the OLED screen and a 100nF capacitor on the underside of the PCB. screen used, the screws may need to be either 2mm or 2.5mm in diameter; most will accept M2 screws. So the OLED sits at the right height, carefully slide off the plastic strip on the 4-pin header soldered to the OLED and cut the pins to suit the depth of the socket. Use a 28-pin DIL socket for the ATmega168/328. Finally, attach the three connectors on the underside of the board. Transistor Q1 and its associated 1kW resistor can be omitted if you don’t need the serial debug feature. If you’re using the microcontroller purchased from the Silicon Chip Online Shop, it will come pre-­ programmed. In theory, you could solder it straight to the board, but using a socket will make replacing it easier in future, should that be necessary. If you have a blank chip, it is easiest to program it in-circuit using CON4. You will need an Atmel serial programmer; an Arduino can be used in this role. First, use the Arduino IDE to upload the ArduinoISP sample code to the Arduino to be used as a programmer. Next, wire up CON4 to the six-pin programming header on the Arduino, except for the RST signal on pin 5. Assuming you’re using the Uno, pin 5 on CON4 goes to its D10 digital pin instead. After that, you can use the free software AVRDUDE (Linux or Windows command-line) or AVRDUDESS (Windows GUI) to upload the HEX file (available from the Silicon Chip website) using “Arduino” as the programmer and 19,200 as the baud rate. Make sure you select the correct COM port (the one the Arduino programmer 30 Silicon Chip board is using) and the target chip (ATmega168, ATmega328 etc). Modifying the DDS module First, desolder and remove the small SMDs labelled L4-L6 & C7-C9 from the board. You can do this with a standard iron by grabbing one component at a time with some reasonably solid tweezers, then alternately heating one side and the other while pulling up gently until the part lifts off the board. It usually helps to melt a little extra tinlead solder into the pad on each side before doing this. Once the parts are off the board, squirt a small blob of flux paste onto each pad, place some solder wick on top, press down with the iron, and, when it’s hot enough, slide it off the pad. That should remove all but a very thin layer of solder. Clean up the flux residue with some flux cleaner or pure alcohol and a lint-free cloth or cotton bud. You can then install all the new components: L4 = 820nH, L5 = 1.8μH, L6 = 1.5μH, C7 = 33pF & C9 = 30pF. Do not install a new capacitor on the pads for C8. Note that one of the specified inductors has an open side which should face towards the PCB while the other inductors and the capacitors can be fitted in any orientation. Making the cable A short 10-pin flat ribbon cable with IDC connectors at each end joins the two modules. Crimp the IDC connectors as shown in Fig.6; if in doubt, check the photos. You can use a vice to close down the connector on the flat cable, making sure that it is exactly square, although it’s better to use a dedicated IDC crimping tool (eg, Altronics T1540). This photo shows nearly all parts required to build the Basic RF Signal Generator, except for the replacement components for the DDS module (see the text above). Australia's electronics magazine siliconchip.com.au Inductors L4-L6 and capacitors C7-C9 have been replaced on the AD9834 module to provide a 35MHz cutoff frequency. The output level is adjusted by changing R2, which I replaced with a potentiometer. There is no room for the strain relief clips on the connectors, so leave them off if supplied. Testing For initial testing, before assembling it into the case, connect the battery and switch it on. The OLED should come up with an initial message showing the version number. After two seconds, the display will show the frequency, step size and battery voltage. The default frequency on power-up is 10MHz, and the step size is 1MHz. Check the VCC voltage at pin 7 or 20 of IC1; it should be close to 4V. You can use the labelled pad near the bottom edge of the PCB as a GND reference. Rotating the knob should increase or decrease the frequency. Depending on the shaft encoder, it may operate backwards. If so, plug a jumper on the programming header between pins 4 and 6 of CON4. If you haven’t fitted the header, you can do it now or solder a short component lead off-cut between those pins. The firmware reads the level on digital input PORTB.3, which determines the encoder direction sensing. Adding a jumper between pins 4 & 6 of CON4 pulls that pin to GND. If all is well, connect the AD9834 module, being careful with the orientation of the flat cable, ensuring that pin 1 goes to GND at both ends. A green LED on that module should light up when power is applied. Check the output on the two SMA connectors with an RF power meter or oscilloscope. The output of the LPF requires 50W termination; without it, there may be some distortion of the output waveform. Final assembly Attach the control board to the transparent lid by two screws on opposite Fig.6: the ribbon cable is simple to make but ensure that the pins are fully pushed into the plastic housing, or you might end up with bad connections. Fig.7: where to drill the holes in the side of the box for the SMA connectors. siliconchip.com.au Australia's electronics magazine corners with 12mm-long M3 tapped spacers, into the countersunk holes you made earlier. The AD9834 module attaches to the bottom of the case with M2/M2.5 × 12mm CSK screws and nuts plus 5mm untapped spacers. First, two holes need to be drilled in the side for the SMA connectors, as shown in Fig.7. The square wave output connector is not accessible and is not used in this design. Next, slide in the module and use it as a template to mark the position of the two holes in the bottom. Drill these to 2.0mm or 2.5mm to suit your screws and countersink them on the bottom. Calibration The output frequency accuracy depends on the exact frequency of the 75MHz oscillator on the module. I found the error at 10MHz to be about 140Hz. This is of little importance for some applications, such as driving the Q meter. However, there is a calibration procedure built in. Set the frequency to precisely 10MHz and measure the output with a frequency counter. Turn on S2 or plug the jumper across CON3 and rotate the tuning knob until the readout on the counter is 10MHz ±1Hz, then press the knob. This sets a correction factor into an EEPROM which is read on power-up. As there is no temperature compensation in the 75MHz crystal oscillator, you can expect this frequency to drift slightly, but it is likely to remain within ±20Hz at 10MHz. Recalibration may be needed from time to time as the crystal oscillator ages. June 2023  31 Parts List – Basic RF Signal Generator 1 double-sided PCB coded CSE221001, 59 × 65mm 1 black PCB coded CSE220902B, 77.5 × 64mm, 1mm thick (front panel) 1 0.96in OLED screen, SSD1306-compatible controller (OLED1) [SC6176] 1 AD9834-based RF DDS signal generator module (MOD1) [AliExpress siliconchip.au/link/abjo] 1 vertical-mount rotary encoder with integral pushbutton and 20mm-long shaft (RE1) [SC5601] 1 105×75×40mm or 105×75×55mm ABS case [Altronics H0321 or H0323] 1 3.3uH axial RF inductor (L1) 1 820nH SMD inductor, M2012/0805, Q = 100 <at> 25MHz (L4 on MOD1) [Coilcraft 0805HP-821XJRC or Vishay Dale IMC0805ERR82J01] ● 1 1.8μH SMD inductor, M2012/0805, Q ≈ 40 <at> 25MHz (L5 on MOD1) [Coilcraft 0805CS-182XJRC or Murata LQW21HN1R8J00L] ● 1 1.5μH SMD inductor, M2012/0805, Q ≈ 40 <at> 25MHz (L6 on MOD1) [Coilcraft 0805CS-152XJRC or Murata LQW21HN1R5J00L] ● 1 2×AA cell holder with flying leads (BAT1) 2 AA alkaline cells 1 2×5 pin header (CON1) 1 2-pin polarised header with matching plug and pins (CON2) 1 3-pin polarised header with matching plug and pins (CON3) 1 2×3 pin header (CON4; optional; for in-circuit programming of IC1) 1 jumper shunt (optional; to set the direction of RE1) 1 4-pin female header (for OLED1) 2 SPDT chassis-mount toggle switches with solder tags (S1 & S2; S2 is optional, for calibration) 1 28-pin DIL IC socket (for IC1) 2 10-way IDC crimp sockets Cable & hardware 1 knob to suit RE1 2 M3 × 12mm tapped spacers 2 M3 × 6mm panhead machine screws 2 M3 × 6mm countersunk head machine screws 2 M2.5 or M2 × 12mm countersunk head machine screws 2 M2.5 or M2 × 16mm panhead machine screws 4 M2.5 or M2 hex nuts 2 3mm ID, 8mm long untapped spacers 2 3mm ID, 5mm long untapped spacers 1 70mm length of 10-way ribbon cable 1 double-sided foam tape pad or strips (to secure the cell holder) Semiconductors 1 ATmega168P or ATmega328P programmed with CSE22100A.HEX (IC1) 1 MCP1661T-E/OT or MP1541DJ-LF-P integrated high-voltage boost regulator, SOT-23-5 (REG1) 1 2N7002 N-channel signal Mosfet, SOT-23 (Q1; optional, debug interface) 1 MBR0540 50V 500mA schottky diode, SOD-123 (D1) Capacitors (all SMD ceramic, M2012/0805 size, unless noted) 2 10μF 6.3V X5R or X7R 1 33pF 50V C0G/NP0 (C7 on MOD1) ● 1 470nF 6.3V X7R 1 30pF 50V C0G/NP0 (C9 on MOD1) ● 4 100nF 50V X7R Resistors (all 1% SMD M2012/0805 size, unless noted) 1 330kW 1 120kW 5 4.7kW 1 1kW (optional, for debug interface) ● replacement parts for the AD9834 DDS module Additional parts for adjustable output level 1 100nF 50V X7R SMD M2012/0805 size ceramic capacitor 1 1.2kW 1% SMD M2012/0805 size resistor 1 50kW chassis-mounting single-gang linear potentiometer [Altronics R2245 or Jaycar RP8516] 1 short length of light-duty figure-8 wire (eg, stripped from ribbon cable) 32 Silicon Chip Australia's electronics magazine Battery life With fresh AA alkaline cells, the input voltage is about 3.2V. The current drain starts at 80mA and increases as the battery voltage drops (because the boost regulator maintains a constant output voltage). By the time the battery drops to 2.7V, the current is about 95mA. The best alkaline AA cells are 3000mAh, but that rating is for a light load. It has to be derated to 2000mAh or so at the expected current drain. This gives an expected operational life of about 20 hours. Adjusting the output level Depending on component values and settings, the Q meter can be fussy about its input signal level. Sometimes the 0dBm value is too high. We can use external attenuators, but this makes the setup rather complicated. The output level is set by resistor R2 on the DDS module, so I thought why not use a potentiometer in its place? The wires to the potentiometer could pick up noise that would amplitude-­ modulate the output. However, if the wires are short, that might not be a problem. The previous photos shows how I did this on the prototype. I started by replacing R2 with a 100nF M2012/0805 SMD capacitor, providing noise filtering and a firmer base to the connecting wires. Connect a 1.2kW M2012/0805 SMD resistor to one end of this capacitor, then use short wires to connect a 50kW pot between the wiper and the anti-clockwise end of the track. Mount this on the right-hand side of the enclosure so the wires are very short. Take great care in attaching the wires to the module to prevent any damage to the SMD connections. With the maximum resistance, the output becomes about 22mV peak-topeak, corresponding to about -29dBm. At minimum resistance, the output is close to 0dBm. I saw no evidence of noise pickup in the output signal. If adding this output control, using the larger case (H0323) gives you more options; you could use a 16mm, 10mm or 9mm potentiometer. With the smaller case, you’ll have to use a 9mm or 10mm potentiometer to have SC any chance of it fitting. KIT (SC6656) – $100 + P&P Includes everything except case, cells and optional 50kW pot siliconchip.com.au