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Using Cheap Asian Electronic Modules Part 16: by Jim Rowe 35MHz-4.4GHz digitally controlled oscillator This programmable frequency module is based on the ADF4351 PLL (Phase-Locked Loop) IC and it can produce a sinewave from 35MHz to 4.4GHz, with crystal accuracy. It can even be used as a sweep generator and costs less than $30. T hat’s an impressive range of frequencies that can be produced by this surprisingly compact (48 x 36.5 x 10mm) module. It is available from various Chinese websites including Banggood (siliconchip.com.au/link/ aajb) and AliExpress, as well as eBay, for around $30. It’s essentially a smaller, lower-cost version of the ADF4351 development board sold by Analog Devices. It runs from 5V and has two RF outputs, one 180° out of phase with the other, allowing it to produce either single-ended or differential signals. It’s controlled using a serial bus that’s connected via a 10-pin header, which also makes connection to the 3.3V supply rail. The ADF4351 chip at the heart of the module is an advanced phaselocked loop (PLL) device. Before we delve into how the ADF4351 works, it’s a good idea to briefly cover the operation of PLLs. Fig.1 shows the block diagram of a basic PLL. It incorporates a negative feedback loop, similar to the one used to improve the performance of audio amplifiers. But in this case, rather than having a voltage divider providing the feedback signal, we have a frequency divider in the loop. The PLL’s output signal (Fout) is produced by the voltage-controlled 82 Silicon Chip oscillator (VCO) at upper right. The frequency divider divides this output frequency by a factor of N. The resulting signal (Ffb) is then fed to the negative input of phase detector PD, which compares its frequency and phase with Fref, the signal from a low-frequency reference oscillator, fed to its positive input. The PD output “error” pulses are fed to charge pump CP, which uses them to develop a fluctuating DC voltage with a polarity and amplitude proportional to the frequency/phase differences between Fref and Ffb. This voltage is then low-pass filtered and used to control the VCO’s frequency. This feedback action causes the VCO frequency (Fout) to stabilise at very close to N times the reference frequency, Fref. The PLL is then described as being “in lock”, since the feedback action keeps Ffb locked to Fref in both frequency and phase. So even if Fref is fixed, by changing the division ratio N, we can control the frequency of Fout. Basic PLLs like this have been in use for many decades but more elaborate versions have also been developed, to overcome some of the limitations of a basic PLL. One of these limitations is that the minimum change in Fout is equal to Fref, so you need quite a low reference frequency to have fine control over the output frequency. But it’s easier to produce accurate and stable reference oscillators at higher frequencies, so one of the first enhancements to PLLs was to add a reference frequency divider between the Fref input and the phase detector PD. Also, if the output frequency needs to be up in the GHz (Gigahertz) range, it’s not easy to provide a programmable divider working at these frequencies. So another early PLL improvement was to add a fixed “prescaler” to the feedback loop, between the VCO output and the input of the main (programmable) feedback divider. Fig.1: block diagram of a basic phase-locked loop. They’re typically used to generate a stable high-frequency signal from a fixed lowfrequency signal. Celebrating 30 Years siliconchip.com.au Fig.2: block diagram of the ADF4351 wideband synthesiser IC. The integrated voltage-controlled oscillator has an output frequency range of 2.2 to 4.4GHz, which, when combined with the RF divider, provides the ~35MHz to 4.4GHz range. The fractional-N PLL controls the frequency from its three registers via the equation: Fout = Ffb × (INT + FRAC ÷ MOD). Unfortunately, this reduces the output frequency adjustment resolution. However, this can be overcome by adopting what’s referred to as a “dual modulus prescaler”. This is essentially a prescaler with a division ratio that can be switched from one value (say P) to another (like P+1) by an external control signal. We don’t have space here to fully explain the operation of modern (and quite elaborate) PLLs but the prior description should be enough to understand how the ADF4351 works. Inside the ADF4351 The block diagram of the ADF4351 IC (Fig.2) is somewhat more complex than the basic PLL shown in Fig.1. The VCO part of the device is labelled “VCO CORE” and shaded pink. There are actually four VCOs inside the core, each used to generate a different frequency range. They are all tuned by the dual varicap diode shown to its right, using a tuning voltage fed in via the Vtune pin. Above the VCO core, you can see the phase comparator and charge pump, both blue. The charge pump output goes to the CPout pin, so that an external low-pass filter can be used to smooth the pulsating output of the charge pump before it is fed back into siliconchip.com.au the ADF4351 via the Vtune pin. The differential outputs from the bottom of the VCO core go to three different destinations. One of these is to the yellow “RF DIVIDER” block to its right. This programmable frequency divider can divide the VCO output frequency by 1 (ie, no division), 2, 4, 8, 16, 32 or 64. This lets the chip generate low output frequencies while the VCO core is operating at much higher frequencies (2.2-4.4GHz). The outputs from the RF divider are fed directly to the chip’s main RF output stage, which drives the A+ out and A- out pins. The RF divider outputs also go to the inputs of two different multiplexers (digital selector switches), shown in mauve. The auxiliary multiplexer on the right switches between the direct output lines from the VCO core and the outputs from the RF divider and so determines which is fed to the auxiliary RF output stage and then to the B+ and B- output pins. The PDBRF pin allows both RF output stages to be disabled when they are not needed, to save power. The feedback (F/B) multiplexer at left determines which of the same two signal pairs go to the feedback divider, in the yellow box. It’s also rather more Celebrating 30 Years complex than the simple feedback divider shown in Fig.1. That’s because the ADF4351 offers the ability to implement either an integer-N or a fractional-N PLL, as required. So the feedback divider needs three registers which hold the integer division value, the fractional division value and the modulus value, plus control circuitry labelled “third-order fractional interpolator”. This circuitry effectively allows the feedback frequency to be divided by a rational number (fraction). The output of this divider is then fed, via a buffer, to the phase comparator. The circuitry shown in the upperleft corner of Fig.2 takes the input from the external reference oscillator (fed into the REFin pin) and processes it before feeding it to the other phase comparator input. As mentioned earlier, one of the refinements to earlier PLLs was to add a reference signal frequency divider, so that high-frequency reference oscillators could be used; hence the 10-bit R counter. But the ADF4351 also provides a frequency doubler and an additional divide-by-two stage for the reference input, both of which can be switched in or out under software control. This gives the chip a great deal of flexibility. May 2018  83 Fig.3: complete circuit diagram for the ADF4351 frequency synthesiser module. The whole chip is controlled by means of a simple 3-wire serial peripheral interface (SPI), shown at centre left of Fig.2. Serial data from the PC or microcontroller is fed in via the DATA pin, clocked into the serial data register and function latch via clock pulses fed to the CLK pin, and then latched into the various control registers by feeding in a pulse via the LE (latch enable) pin. All functions of the ADF4351 chip are configured using six 32-bit control words, sent over this serial bus. Multiplexer C and the other blocks at the upper right of Fig.2 allow external monitoring of the ADF4351’s status. The “LOCK DETECT” block monitors the phase comparator and provides a high logic output on the LD pin when the PLL is locked. Multiplexer C allows either of the two phase comparator inputs or this lock status to be fed to MUXout pin. 84 Silicon Chip The fast lock switch provides a signal which can be fed into the external low pass filter (between the CPout and Vtune pins) when in “fast lock” mode. So that covers the operation of the IC itself. The synthesiser module The full circuit of the module is shown in Fig.3 and most of the real work is done by IC1. All of the programming and status monitoring signals to and from the module are available at CON1. This includes the DATA, CLK and LE lines (ie, the serial bus) and also the CE (chip enable), LD (lock detect), MUXout and PDBRF (power down RF buffer) lines. The reference signal is provided by a 25MHz crystal oscillator (XO), shown at upper left, with its output fed to the REFin pin of IC1 via a loading/coupling circuit comprising two 1nF capacitors and a 51W resistor. Celebrating 30 Years Note that there is also provision for feeding in a different reference signal, via the SMA socket labelled MCLK. In order to do this, you’d need to remove the 0W resistor connected to pin 3 of the onboard XO. If you are using the onboard XO, the MCLK socket can be used to monitor its output via a scope or frequency counter. The resistors and capacitors connecting the CPout and SW pins of IC1 (pins 7 and 5) to the Vtune pin (pin 20) form the low-pass loop feedback filter. The RF output signals from the RFOA+ and RFOA- pins (12 and 13) are taken to the RFout+ and RFoutSMA sockets via matching/filtering circuits using L2, L3, L5 and L6, plus two 1nF capacitors. Notice that the output pins are fed with the +3.3V supply voltage via L2 and L3. In this module, the auxiliary RF outputs RFob+ and RFob- (pins 14 and 15) are not wired up. siliconchip.com.au Fig.4: when connecting the ADF4351 synthesiser module to an Arduino-based device, a few extra resistors are needed. These resistors form a voltage divider, as the module can only handle 3.3V signals, while the Arduino’s outputs have a swing of 5V. Note the changes needed if using a V2 module at the end of this article. The whole module operates from a 3.3V supply, derived from the 5V input at CON2 via REG1, an ASM1117 low-dropout regulator. This AVdd rail powers all of the analog/RF circuitry directly. The digital supply rail, DVdd, is derived from AVdd using LC filters comprising inductors L4 and L1 and a number of bypass capacitors. There are two indicator LEDs. LED1 is connected between the DVdd line and ground and indicates when the module has power while LED2 is connected to the LD (lock detect) pin of IC1 (pin 25) and indicates when the PLL is in lock. All the remaining components are for bypassing and stability, apart from fuse F1 and diode D1, which prevent damage in the event that the 5V power source is connected with reversed polarity. Controlling it with an Arduino I initially hooked the module up to an Arduino Uno using the simple circuit shown in Fig.4. The three main control lines MOSI, SCK and LE are not taken directly to the DAT, CLK and LE pins of the module but instead via 1.5kW/3kW voltage dividers. This is because the inputs of the ADF4351 can only cope with 3.3V signals, whereas the Arduino outputs have a 5V output swing. The LD signal fed back from the module to the Arduino’s D2 pin does not need a divider because it’s going the other way and the Arduino inputs function well with a signal having a swing of 3.3V. siliconchip.com.au Note also that Fig.4 indicates that the 5V supply for the module can come from either a plugpack or from the 5V output of the Arduino. I adapted an Arduino sketch I found on the internet, written by French radio amateur Alain Fort F1CJN (siliconchip.com. au/link/aaje). Mr Fort’s sketch was written for an Arduino with an LCD button shield but I decided to adapt it so that it would work with the simple configuration shown in Fig.4, relying on the Arduino IDE’s Serial Monitor to send commands to the ADF4351 and to indicate the PLL’s output frequency and whether it was locked or not. I also connected one of the PLL module’s RF outputs to my frequency counter, via a prescaler, so I could monitor it. The results were quite impressive. I could type in any frequency between 35MHz and 4.4GHz, with a resolution of 0.01MHz (10kHz) and the module’s output would lock to that frequency in the blink of an eye. I also monitored the current drawn by the module and found that it varied between 110mA and 145mA, depending on the output frequency. I also checked the accuracy of the module’s 25MHz on-board reference XO and found it to be 25.0000734MHz or only 73.4Hz high. Since this equates to an error of just +2.936ppm, it seems quite accurate. So that’s one easy way to get the ADF4351 module going with an Arduino. The sketch (“ADF4351_and_ Arduino_SC_version.ino”) is available for download from the Silicon Chip website. Driving it from a Micromite I also hooked the module up to a Micromite LCD BackPack combination and wrote some code so that it could be controlled via the LCD touchscreen. The circuit is shown in Fig.5 and it’s about as simple as you can get. In this The bottom view of this module is shown at approximately twice actual size. The bottom of the board is populated by five 10kW 10kW pulldown resistors for the breakout pin connections. Celebrating 30 Years May 2018  85 Fig.5: when connecting the ADF4351 module to a Micromite no extra components are needed, unlike with an Arduino. However, the newer version of the module requires a 10kW resistor between CE and +3.3V. case, no resistive dividers are needed on the SCK, MOSI and LE lines because the Micromite’s logic pins have a swing of 3.3V. I used a “software” SPI port rather than the hardware one used by the Micromite to communicate with the LCD and touchscreen, to prevent possible interaction. The embedded C code (CFUNCTION) needed to provide this added port is included in the MMBasic program I wrote for this approach. Software SPI port performance is limited but that isn’t a problem as the amount of data to transfer is small. A USB charger was used to supply 5V to the ADF4351 module because its current drain is a little too high for the BackPack to provide. The software uses just two screens, as shown below. The initial screen (at left) displays the current frequency and gives you the option of touching the button at the bottom if you want to change it. You will then get the second screen, which allows you to key in a new frequency, displayed below the current frequency. When you’re happy with the new figure, simply touch the OK button and the module jumps to the new frequency. The program returns to the main screen, displaying the new frequency. So for those who would like to team up the module with a Micromite, this program (“Simple ADF4351 driver program.bas”) should get you off to a good start. Like the Arduino sketch, it’s available from the Silicon Chip website. Performance I checked the module’s RF output performance at quite a few different frequencies, using my Signal Hound USB-SA44B spectrum analyser controlled by Signal Hound’s “Spike” software. The results were quite impressive, as you can see from the two spectrum plots. One plot shows the output at 275MHz, with the only significant spurs visible being at ±50MHz with an amplitude of -57dBm. The other plot shows the output at 4.200GHz, with two spurs again visible but this time both on the low side: one at 4.150GHz (far left) with an amplitude of about -53dBm and the oth- The sample program running on a Micromite LCD BackPack. These are the only two screens the software uses, one to enter a specific frequency for the module to output and another to display the current frequency. 86 Silicon Chip Celebrating 30 Years siliconchip.com.au Spectrum analysis of the ADF4351 module’s output performance at 275MHz (left) and 4.2GHz (right). The RF output performance over the full range was good, with only a few visible spurs outside the programmed frequency. These normally correspond to beat frequencies or integer-multiples of the reference and oscillator frequency. er at 4.175GHz with an amplitude of -61dBm. In both cases, the amplitude of the main output carrier is very close to 0dBm. This turned out to be the case over most of the range, in fact. The only region where the carrier level did drop (to around -20dBm) was in the vicinity of 2.45GHz – perhaps by design, to minimise interference with WiFi and Bluetooth systems. Overall, the ADF4351 frequency synthesiser module is very impressive, especially when you consider its frequency range and price. It could even be used to make your own VHF/UHF signal and sweep generator, teamed up with a Micromite and a 4GHz digital attenuator module that we will describe in next month’s issue. These changes will be critical to successfully connect the module to a micro, so here are the main details listed below: 1. Many of the connections to the 10-way pin header (CON1) have changed, as shown in Fig.6. 2. There is now no on-board pullup resistor connecting IC1’s CE pin to the +3.3V (DVdd) line, nor are there pulldown resistors connected between the CLK, DATA and LE pins and ground. To ensure normal operation of the module with either an Arduino or a Micromite, an external 10kW resistor must be connected between the CE and +3.3V pins of CON1. To ensure maximum stability, it’s a good idea to also connect an external 10kW resistors between the LE pin and ground. Once the above changes are made, version 2 of the module performs just as well as the earlier version. Useful links The module from AliExpress: www. aliexpress.com/item//32848807357. html The module from eBay: www.ebay. com.au/itm/142521016834 ADF4351 data sheet: siliconchip. com.au/link/aajc Fundamentals of PLLs: siliconchip. SC com.au/link/aajd Fig.6: CON1 has been changed completely on the newer version of the ADF4351 module. Every signal, except for CLK, is connected to a different pin location. A new version of the ADF4351 synthesiser module Just recently we received a second ADF4351 Synthesiser module and discovered that it was a “V2” module which had been changed in a number of ways compared with the first version. Are Your S ILICON C HIP Issues Getting Dog-Eared? REAL VALUE AT $16.95 * PLUS P & P Keep them safe, secure & always available with these handy binders Order now from www.siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *See website for overseas prices. siliconchip.com.au Celebrating 30 Years May 2018  87