KF5OBS /IP

Not too long ago I reviewed ADI’s wideband PLL synthesizer ADF4351 with integrated low phase noise VCO. The ADF4360 series family of synthesizer chips is very similar. The primary difference is their restricted frequency coverage and thus much lower price.

The ADF4360 is an integrated integer-N synthesizer with an integrated voltage controlled oscillator (VCO). The center frequency is set by external inductors. There are 9 chips in the family with 8 different frequency ranges. The frequency range are as follows:

ADF4360-0: 2400 to 2725 MHz
ADF4360-1: 2050 to 2450 MHz
ADF4360-2: 1850 to 2170 MHz
ADF4360-3: 1600 to 1950 MHz
ADF4360-4: 1450 to 1750 MHz
ADF4360-5: 1200 to 1400 MHz
ADF4360-6: 1050 to 1250 MHz
ADF4360-7: 350 to 1800 MHz
ADF4360-8: 65 to 400 MHz
ADF4360-9: 65 to 400 MHz

Even though the ADF4360-8 and ADF4360-9 have the same frequency range, they are a bit different. The ADF4360-9 has an auxiliary divider with division ranges from 2 to 31 on board. The ADF4360-8 has – just like all other ADF4360 – a hardware power down input (CE).

Analog Devices kindly sent me one of their Evaluation Kits for the ADF4360-9 (EV-ADF4360-9EB1Z) for review and evaluation [1].

EV-ADF4360-9EB1Z evaluation board from Analog Devices

What I intend to use this chip in is called a Fox-Hunt transmitter. It has very little to do with hunting actual foxes and actually relates to a common radio direction finding exercise. The way this works is that a small transmitter (the “fox”) is hidden somewhere and a group of people will attempt to locate the transmitter. Whoever finds the transmitter first, wins the game.

I want to build a very small transmitter for the 2m amateur radio band (~145 MHz) with just a few miliwatts of output power. With such a low power VHF transmitter, a radio direction finding exercise could be conducted in a small area like a park, or even more challenging, with several transmitters at the same time. A microcontroller is supposed to be in charge of setting the frequency and keying the required station identification (call-sign of the control operator) as required by the FCC.

But now back to the ADF4360. The chip has an SPI compatible 3-wire interface, operates between 3 – 3.6 Volts and its inputs are 1.8 V logic compatible. In other words: this chip will interface with pretty much any microcontroller out there. My project will probably be Atmel AVR or MSP430 based and I program in C. However, I will write example code for Arduino (AVR) / Energia (MSP430) for folks who would like to experiment with it more easily.

I looked at the output spectrum of the ADF4360-9 set to 400 MHz on a Teledyne LeCroy HDO6054. The phase frequency detector (PFD) frequency is 200 kHz and you can clearly see spurs 200 kHz spaced to both sides of the carrier. The spurs are smaller than -70 dBc and, to be fair, the ADF4360-9 is not correctly terminated. The IC has a differential output and the datasheet warns that the performance of the output signal may be degraded if not both ports are properly terminated with 50 Ohms. In my case, only one port is fed into the 50 Ohm port of the scope. The other port is open.

Output spectrum of a ADF4360-9 at 400 MHz with a 200 kHz PFD frequency. Clearly visible, the spurs 200 kHz left and right from the carrier

In any case, -70 dBc is a lot of attenuation. As a matter of fact, the output signal could be transmitted the way it is over the air. The FCC demands in 74 CFR 97.307 (e) that “the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency between 30 – 225 MHz must be at least 60 dB below the mean power of the fundamental.” This is clearly the case.

The eval board comes with a very comfortable software, just like the ADF4351 did. It is very nice to be able to manipulate all registers and parameters and watch what happens right away.

So what’s next? I will design the circuit, design a PCB, and write the necessary software code for the little VHF tracker (“fox”). The project will be an open hardware project. That means you will be able to use my project free of charge for personal use. As soon as that is done, I will post a new article with the entire project in it. Stay tuned!

Links and Sources:

[1] ADF4360-9, ADI: http://www.analog.com/

The ADF4351 from Analog Devices (ADI) is a modern wideband PLL synthesizer with integrated low phase noise VCO. It is capable of generating signals between 35 MHz to 4400 MHz with a very low jitter of typical 0.3 ps. ADI agreed to send me the EVAL-ADF4351EB1Z, an evaluation board for the ADF4351, for review purposes. Let’s check it out!

The ADF4351 has an integrated voltage-controlled oscillator (VCO) with a fundamental output frequency ranging from 2200 MHz to 4400 MHz. In addition, divide-by-1/-2/-4/-8/-16/-32/-64 circuits allow the user to generate RF output frequencies as low as 35 MHz. For applications that require isolation, the RF output stage can be muted. The mute function is both pin- and software-controllable. An auxiliary RF output is also available, which can be powered down when not in use. Control of all on-chip registers is through a simple wire interface. The device operates with a power supply ranging from 3.0 to 3.6 V and can be powered down when not in use.

ADF4351 evaluation board

ADI ships the EVAL-ADF4351EB1Z with a USB cable and a CD. The CD contains all software necessary to get started right away. The board itself makes a very clean impression. Despite the surface mount technology, all case styles of the components used can comfortably be handled with appropriate hand soldering tools. This allows for easy application specific modifications.

There are two 4 mm jacks for power (3.75 V to 5.5 V), a small USB connector, and 3 SMA connectors on the board. The first SMA connector serves as reference input for an external reference signal. Alternatively, it can be used as an output for the on-board reference (25 MHz). The other two SMAs are the primary RF output of the ADF4351. It is a differential pair. For best performance, make sure to terminate both outputs correctly (50 Ohms) even if just one output is being used.

Power can either be supplied through the 4 mm banana jacks or the USB port. Switches S1 and S2 are used to select the power source. Switch S1 is used to power the ADF4351 from the external DC connector and switch S2 to power from the USB port. The USB clock can cause spurs in the RF signal if power is derived from the USB port.

Plenty of test points on the board allow easy troubleshooting. An additional 100 mil / 2.54 mm header can be populated to gain easier access to the most important logic signals.

The external loop filter on the eval board has a bandwidth of 35 kHz. This value can easily be changed by changing the value of the corresponding components. The software package ‘ADIsimPLL’ from Analog Devices is a great tool for designing an application specific loop filter.

Make sure to install the software from the CD before connecting the evaluation board to the PC for the first time. Once the software is installed properly, connect the board to the PC and start the ADF4351 software package. If the two power LEDs on the board (D5 and D6) do not light up upon connection, verify S1 and S2 for proper selection of the desired power source.

ADF4351 Application Software

The software is self-explanatory. It allows accessing and manipulating of all functions and registers of the ADF4351. Additionally, the software allows you to use the evaluation board as a sweep generator and it can do frequency hopping between two frequencies.

After trying this board out for a while, I highly recommend this chip. It is small, inexpensive ($8.25, 100 QTY) and extremely simple to integrate.

I can think of many applications for the ADF4351 in the amateur radio community. The ADF4351 a perfect 21st century alternative for older SP5055 based designs. The SP5055 was a very popular synthesizer chip used in many older amateur radio projects.

The chip is predestined to be used as a flexible Local Oscillator (LO) in amateur radio transverters. A flexible LO frequency allows to cover more bandwidth in the target frequency range than the IF transceiver itself offers. Paired with a baseband processor and a power amplifier, this chip easily transform into an inexpensive amateur television (ATV) transmitter. I will show some practical designs and applications in future articles. Stay tuned!

Links and Sources:

[1] ADF4351, ADI: http://www.analog.com/

[2] ADF4351 Datasheet, ADI: http://www.analog.com/

[3] Evaluation Board User Guide, ADI: ftp://ftp.analog.com/