ADF41513 driver

This driver supports Analog Devices’ ADF41513 and similar SPI PLL frequency synthesizers.

1. Supported devices

The ADF41513 is an ultralow noise frequency synthesizer that can be used to implement local oscillators (LOs) as high as 26.5 GHz in the upconversion and downconversion sections of wireless receivers and transmitters. The ADF41510 is a similar device that supports frequencies up to 10 GHz.

Both devices support integer-N and fractional-N operation modes, providing excellent phase noise performance and flexible frequency generation capabilities.

Key Features:

  • ADF41510: 1 GHz to 10 GHz frequency range

  • ADF41513: 1 GHz to 26.5 GHz frequency range

  • Integer-N and fractional-N operation modes

  • Ultra-low phase noise (-235 dBc/Hz integer-N, -231 dBc/Hz fractional-N)

  • High maximum PFD frequency (250 MHz integer-N, 125 MHz fractional-N)

  • 25-bit fixed modulus or 49-bit variable modulus fractional modes

  • Programmable charge pump currents with 16x range

  • Digital lock detect functionality

  • Phase resync capability for consistent output phase

2. Device attributes

The ADF41513 driver provides the following IIO extended attributes for frequency control and monitoring:

Each IIO device has a device folder under /sys/bus/iio/devices/iio:deviceX, where X is the IIO index of the device. Under these folders reside a set of device files that provide access to the synthesizer’s functionality.

The following table shows the ADF41513 related device files:

Device file

Description

out_altvoltage0_frequency

RF output frequency control and readback (Hz)

out_altvoltage0_frequency_resolution

Target frequency resolution control (Hz)

out_altvoltage0_powerdown

Power management control (0=active, 1=power down)

out_altvoltage0_phase

RF output phase adjustment and readback (radians)

2.1 Frequency Control

The out_altvoltage0_frequency attribute controls the RF output frequency with sub-Hz precision. The driver automatically selects between integer-N and fractional-N modes to achieve the requested frequency with the best possible phase noise performance.

Supported ranges:

  • ADF41510: 1,000,000,000 Hz to 10,000,000,000 Hz (1 GHz to 10 GHz)

  • ADF41513: 1,000,000,000 Hz to 26,500,000,000 Hz (1 GHz to 26.5 GHz)

The frequency is specified in Hz, for sub-Hz precision use decimal notation. For example, 12.102 GHz would be written as “12102000000.000000”.

2.2 Frequency Resolution Control

The out_altvoltage0_frequency_resolution attribute controls the target frequency resolution that the driver attempts to achieve. This affects the choice between integer-N and fractional-N modes, including fixed modulus (25-bit) and variable modulus (49-bit) fractional-N modes:

  • Integer-N: Resolution = f_{PFD} (same as PFD frequency)

  • Fixed modulus: Resolution = f_{PFD} / 2^{25} (~3 Hz with 100 MHz PFD)

  • Variable modulus: Resolution = f_{PFD} / 2^{49} (µHz resolution possible)

Default resolution is 1 Hz (1,000,000 µHz).

2.3 Phase adjustment

The out_altvoltage0_phase attribute allows adjustment of the output phase in radians. Setting this attribute enables phase adjustment. It can be set from 0 to 2\pi radians. Reading this attribute returns the current phase offset of the output signal. To create a consistent phase relationship with the reference signal, the phase resync feature needs to be enabled by setting a non-zero value to the adi,phase-resync-period-ns device property, which triggers a phase resynchronization after locking is achieved.

3. Operating modes

3.1 Integer-N Mode

When the requested frequency can be achieved as an integer multiple of the PFD frequency (within the specified resolution tolerance), the driver automatically selects integer-N mode for optimal phase noise performance.

In integer-N mode:

  • Phase noise: -235 dBc/Hz normalized floor

  • Frequency resolution: f_{PFD} (same as PFD frequency)

  • Maximum PFD frequency: 250 MHz

  • Bleed current: Disabled

3.2 Fractional-N Mode

When sub-integer frequency steps are required, the driver automatically selects fractional-N mode using either fixed or variable modulus.

Fixed Modulus (25-bit):

  • Used when variable modulus is not required

  • Resolution: f_{PFD} / 2^{25}

  • Simpler implementation, faster settling

Variable Modulus (49-bit):

  • Used for maximum resolution requirements

  • Resolution: f_{PFD} / 2^{49} (theoretical)

  • Exact frequency synthesis capability

In fractional-N mode:

  • Phase noise: -231 dBc/Hz normalized floor

  • Maximum PFD frequency: 125 MHz

  • Bleed current: Automatically enabled and optimized

  • Dithering: Enabled to reduce fractional spurs

3.3 Automatic Mode Selection

The driver automatically selects the optimal operating mode based on:

  1. Frequency accuracy requirements: Determined by frequency_resolution setting

  2. Phase noise optimization: Integer-N preferred when possible

  3. PFD frequency constraints: Different limits for integer vs fractional modes

  4. Prescaler selection: Automatic 4/5 vs 8/9 prescaler selection based on frequency

4. Usage examples

4.1 Basic Frequency Setting

Set output frequency to 12.102 GHz:

root:/sys/bus/iio/devices/iio:device0> echo 12102000000 > out_altvoltage0_frequency

Read current frequency:

root:/sys/bus/iio/devices/iio:device0> cat out_altvoltage0_frequency
12101999999.582767

4.2 High Resolution Frequency Control

Configure for sub-Hz resolution and set a precise frequency:

# Set resolution to 0.1 Hz (100,000 µHz)
root:/sys/bus/iio/devices/iio:device0> echo 0.1 > out_altvoltage0_frequency_resolution

# Set frequency to 12.102 GHz (1 µHz precision)
root:/sys/bus/iio/devices/iio:device0> echo 12102000000 > out_altvoltage0_frequency
root:/sys/bus/iio/devices/iio:device0> cat out_altvoltage0_frequency
12101999999.980131

4.3 Monitor Lock Status

When lock detect GPIO is configured, check if PLL is locked:

# Read frequency - will return error if not locked
root:/sys/bus/iio/devices/iio:device0> cat out_altvoltage0_frequency

If the PLL is not locked, the frequency read will return -EBUSY (Device or resource busy).