kc4ri.comkc4ri.com/...and_Easy_Way_to_Get_Started_with_The_ADF4351_Synt…  · Web viewNote: Since the VCO (Voltage Controlled Oscillator) of the ADF4351 Synthesizer operates from

A Quick and Easy Way to Get Started with The

Analog Devices ADF4351 Synthesizer

Mike Stipick - KC4RI © 2017

[email protected]

Driven by the demands of the Wireless, Aerospace, and Satcom industries, advances in semiconductor technology during the past few years has brought to market many new semiconductor devices that are available for use in Amateur Radio. Many of these devices operate well into the VHF/UHF and Microwave Spectrum. Advances in frequency synthesizer chips have enabled designers to easily and inexpensively produce clean and stable high frequency signals. Most manufacturers of these chips provide Evaluation Boards to allow circuit designers to “Test Drive” the chip without spending the time and expense to design a circuit board for the evaluation of the chip in their system. One such chip is the Analog Devices ADF4351. This chip can be used as both an Integer and Fractional-N Synthesizer to produce signals from 35 MHz to 4.4 GHz. It is a very versatile chip and is fully programmable by a three wire Serial Peripheral Interface (SPI).

There are many Amateur Radio applications where a synthesizer like this may be used. One primary application is to use it for an LO (Local Oscillator) with a Mixer to Up or Down Convert. The software program described in this article demonstrates a simple technique that allows users to select a low side LO with an intended Intermediate Frequency (IF) of 28 MHz for the Amateur Bands of 2 meters through 23 cm. The user can easily modify this program to produce frequencies other than those chosen in the original program and hopefully add additional features to their project.

When one initially reads the ADF4351 Datasheet, one will notice that the Synthesizer Chip has 6 - 32 Bit Registers that must be accurately programmed. These registers have a variety of multibit values that are not word or byte aligned (See Figure #1). This means that one must execute a lot of complex bit manipulation to ensure that the values in these registers are correct. One must also make a series of calculations to determine values of INT, FRAC, MOD, and the R Counter based on their relationship (See Figure #2).

Figure 1 REGISTER MAP FOR REGISTER #2 ADF4351

RFOUT = [INT + (FRAC/MOD)] × (fPFD/RF Divider)

Figure 2 - RF N DIVIDER

At first this may seem like a daunting task for those not experienced with embedded programming. Fortunately, there is a free tool provided by Analog Devices that was designed for their Evaluation Board. This tool has a very useful GUI (Graphical User Interface) that was designed to program the ADF4351 Registers (See Figure #5). It can be used offline to generate the Hex Values for these registers, which can then be inserted into the program code of a microprocessor to program the synthesizer chip. The Evaluation Tool also checks to ensure that there is not a parameter, or register violation. This makes the microprocessor code very simple, compact, and efficient compared to computing the register values and programming all of the registers. This does require the user to select discreet frequencies and amplitudes to be programmed. In most cases such as UP/DOWN Converters this is not a problem since signal tuning is usually done at the IF frequency.

The ADF4351 is sold by many of the major parts distributors and cost less than $14.00 in single quantities. The Evaluation Board from Analog Devices is available for about $165.00 (See Figure #4). Because the chip comes in a 5 mm X 5 mm 32-Lead Lead Frame Chip Scale Package [LFCSP_VQ] it requires a “hot air” soldering station or reflow oven to solder it on a printed circuit board. The parts placement on a high frequency synthesizer board is fairly critical and requires small Surface Mount Devices (SMDs).

Figure 3 – 5 mm x 5 mm LFCSP

Figure 4 - ANALOG DEVICESEVALUATION BOARD

Figure 5 – ANALOG DEVICES EVALUATION TOOL SOFTWARE

Fortunately, one can find Chinese Evaluation boards on eBay at a cost of $26.00 to $35.00. These boards do not have a USB interface and require some type of microprocessor to program them.

Figure 6 - CHINESE EVALUATION BOARD

For this project, I chose to use an Arduino Nano (See Figure #7). This is just one member of the Arduino family. You can use other members of the Arduino family if you wish. The Arduino processors are great for projects like this because they are inexpensive and have a great IDE (Integrated Development Environment). The IDE also supports Open Source Libraries for many parts such as LCDs, various serial interfaces, and many sensors and devices. A web search will also reveal many example programs and forums where one can post questions and receive help.

The cost of the Arduino Nano starts at just under $4.00 from Chinese Suppliers.The Nano is a 5 Volt part and the ADF4351 is a 3.3 Volt part. Therefore, one must use a resistive divider or a voltage translator between the two devices. I chose to use a SparkFun BOB - 12009 FET Voltage Translator (Cost $2.95) (See Figure #8).

Connect the Arduino Nano, the SparkFun Voltage Translator, and the ADF4351 Synthesizer Board as indicated in the schematic diagram (See Figure 9). I used a leftover PCB from another project to mount the Arduino Nano and the SparkFun

Voltage Translator and then I used Arduino style jumpers to connect to the ADF4351 Synthesizer Board (See Figure 10). Eventually, I hope to have boards available for a complete Synthesizer.

Figure 7 - ARDUINO NANO V3.1

Figure 8 - FET VOLTAGE TRANSLATOR 5V TO 3.3V

Figure 9 - SCHEMATIC FOR QUICK AD EASY ADF4351

Figure 10 - PROTOTYPE CONNECTIONS

Note: Make sure to use a 50 Ohm Load on the unused RF Output (OUT - ) from the ADF4351.

Note: Power required for the Arduino Nano is supplied by a standard USB Mini Connector. Power for the ADF4351 Board is supplied by a USB to 3.5mm Barrel Jack 5V DC Cable Plug.

Once you have your hardware assembled you will need to download three pieces of software and install them:

1. Arduino IDE - https://www.arduino.cc/en/Main/Donate

2. ADF4351 Software Evaluation Tool - http://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/eval-adf4351.html#eb-over view

3. Arduino Quick Quick_And_Easy_ADF4351 Program – Website http://kc4ri.com . Look under the Projects tab for Synthesizers or email [email protected]

You will also want to download the ADF4351 Datasheet and other information from Analog Devices.

http://www.analog.com/en/products/rf-microwave/pll-synth/fractional-n-plls/adf4351.html#product-overview

Once you have installed the Arduino IDE, Under the “Tools” Tab Select your Board Type from the Board Manager (IE Arduino Nano).

From the “Tools” Tab select the COM Port that you are using to connect to your Arduino.

From the “File” Tab select “Examples” then “01 Basics” then “Blink”.

This loads a simple Sketch that Blinks an onboard LED.

Note: Programs are called “Sketches” in the Arduino World.

Once you have loaded “Blink”, select “Verify/Compile” from the “Sketch” Tab.

This should compile correctly and verify that everything is working to this point.

Next select “Upload” from the Sketch Tab and Upload the compiled code for “Blink” to your Arduino.

Once uploaded one should see a blinking LED on your Arduino Board.

Note: The most common problem encountered in “Uploading” is that the correct Com Port is not selected.

The next step is to copy the “Quick_And_Easy_ADF4351.ino” file into your Arduino Sketches folder.

C:\Users\your_user_id\Documents\Arduino\Sketches\Quick_And_Easy_ADF4351

You can create a folder, or the first time you run “Quick_And_Easy_ADF4351.ino” the IDE will ask if you want it to create a folder for you.

Your IDE should look like Figure #11.

Select “Verify/Compile” from the “Sketch Tab” and verify that the program compiles correctly.

Then Select “Upload from the “Sketch” Tab and verify that the file uploads correctly.

Once the file uploads then select “Serial Monitor” from the “Tools” Tab.

The “Serial Monitor” should open a New Window and “Reset” the Arduino.

Check that the Baud Rate for the Serial Monitor Window is set to “115200”.

Failure to have this set properly will result in an unrecognizable display.

You should now see a display similar to that in Figure #12.

Simply type in a valid frequency from the list (116, 194, 404, 897, 1267) and the Arduino will program the synthesizer registers to the selected frequency.

To verify proper operation, one can use a spectrum analyzer, frequency counter, or a receiver tuned to the desired frequency.

Once the basic hardware and software are functioning properly; one can compare the results from the ADF4351 Evaluation Software Tool with the values displayed in the Serial Monitor Window.

If there is a difference check that the following in the Evaluation Tool:

1. Reference Frequency and the PFD frequency are set to 25 MHz. This should be the onboard crystal frequency.

2. Check the Amplitude setting in Register 4. This should be -1dBm.3. Check the Prescaler value in the RF Settings Section. It should be set to

“4/5”.

If you want to change or add a frequency. Simply type in the desired frequency into the Evaluation Tool and it will provide you with the correct Register Values in Hex.

Use the Arduino IDE Editor to make changes to the following parts of the Quick_And_Easy_ADF4351 program.

Lines 44 – 48 Defines the Register Arrays and the associated values. Add or replace definitions here.

Line 121 Defines the valid input values for frequencies. Add or replace here.

Lines 142 – 184 Is the Decode Frequency Select Function. This Function contains a series of “else if” statements. This is where you would modify or add a new frequency.

Feel free to experiment and try different things. In the future, I hope to post additional software on my website: http://kc4ri.com . I can also be contacted at [email protected] .

Figure 11 - ARDUINO IDE

Figure 12 - ARDUINO SERIAL MONITOR DISPLAY

Note: Since the VCO (Voltage Controlled Oscillator) of the ADF4351 Synthesizer operates from 2200 to 4400 MHz, all other frequencies are divided down from a frequency in that range. These signals have high harmonic content and should be Low Pass Filtered. Viewing the unfiltered output on a spectrum analyzer will show the very strong odd harmonics.