Analog Devices ADF4107 Based RF Frequency Synthesizer

I have had a few projects recently needing a stable local oscillator (LO) source for downconversion, notably my hydrogen line radio telescope and a few receivers for low earth orbit high frequency satellite transmitting in the 5.8Ghz range. I have attempted the use of various VCOs along with a very stable power supply to produce a stable LO frequency, but this over the long term has proven unsuccessful. My previous designs used a  precision adjustable voltage regulator trying both analog and digital precision trim-pots, but temperature drift among other things resulted with it being too difficult to keep a stable frequency. These solutions would need constant calibration which proved to be unreliable for a true stable LO source. An alternate good solution could be the use of a precision frequency synthesizer / phase-locked loop from a reference source to produce a stable frequency. Researching frequency synthesizers, I came across the Analog Devices ADF series of PLL frequency synthesizers which I ultimately ended up choosing for my design.

I wanted the design to be flexible, that is with a single board I could populate it with a variety of VCOs only having to update the ADF4107 register configuration and the loop filter component values to produce a fixed or tunable frequency. My final design achieved this goal.

ADF4107 based frequency synthesizer

This design was based on the Analog Devices ADF4107 frequency synthesizer. The ADF4107 consists of a low-noise phase frequency detector that uses a programmable reference divider and prescaler to run a VCO via a precision charge pump to produce a stable locked frequency from a fixed reference oscillator. There are many variations within the ADF frequency synth family, I chose the 4107 as it met my needs with a maximum frequency of 7Ghz.

The ADF4107 can utilize a variety of VCOs, I chose to go with a Z-Comm device which I have used in many previous designs. The specific VCO used in this version is a V940ME02. It has an adjustable output frequency range of 5220 to 5420Mhz which fit well into the 5.40Ghz LO needed for my downconverter. The reference input to the PLL is provided by a 32Mhz TCXO crystal, which was a last minute change as I ordered the wrong package type for the original 20Mhz TCXO I had specified for the design. Only some reference counter changes were needed to use the alternate crystal that I had in my parts bin.

The ADF4107 itself is a very complex device looking at both its usage and programming. To help integrate this family into your design, Analog Devices has a great tool called ADIsimPLL to assist with the initial design regarding calculations of output frequency range based on the reference frequency input. It will also assist with selecting the loop filter component values based on a given design among many other useful things.

ADIsimPLL screenshot

My ADF4107 synthesizer is controlled via a SPI bus from a Microchip PIC18F14K50 microcontroller. After the initial power up of the ADF4107, multiple registers must be sequentially loaded to define the prescaler counters among other configuration flags. The PIC stores and handles this configuration. I have also included an external serial interface into the design to allow remote tuning and control of the frequency synthesizer via the PIC, although in this specific design it is not being used as it only needs to provide a single hard-coded fixed frequency.

The design itself follows good mixed-signal design practices. The board is a four-layer design that I had received from my favorite fab house, OSH Park. Upon receiving, I populated by first board by hand and had a working board on power up. ADF4107 register programming took a bit as the specific loading order of the registers was tricky, but once mastered, the PLL immediately locked onto 5.40Ghz according to my frequency counter. I couldn't view it directly on my spectrum analyzer as it only goes to 3Ghz, but the frequency counter was nicely stable at 5.4 Ghz plus or minus a few hundred hz. Each subsequent PLL lock is almost instantaneous. I added two LEDs to some spare PIC outputs, one of them is used to indicate when there is PLL lock and a spare that can be used for any additional status I wish to see.

An output filter is included in the design, in this case it has a center frequency of 5.4Ghz. Filters of this frequency are easy to find as they are used a lot in 802.11a WLAN hardware. I did not include an output amplifier on this board as I had planned to keep the design as simple as possible, instead I would be using an external amplifier. This will need to be the case as the output of this VCO at 5.4Ghz is -15dBm, which is not high enough to provide a LO for most mixers. So on any future redesigns, I may add an amplifier back in.