ADF4351 Fractional N PLL Synthesizer IC Pin Diagram, Circuit and Applications

The ADF4351 is a widely used fractional-N frequency synthesizer integrating a PLL and VCO to generate precise RF output frequencies. It is suitable for signal generators, SDR platforms, wireless communication equipment, instrumentation, and microwave applications. This article explains the ADF4351 pin diagram, working, circuit implementation, specifications, and applications.

Key features of ADF4351

• Wide Output Frequency Range: 35 MHz to 4400 MHz, achieved via an integrated voltage controlled oscillator (VCO) with a fundamental range of 2200 MHz to 4400 MHz and programmable output dividers (divide-by-1/-2/-4/-8/-16/-32/-64).
• Dual Synthesizer Modes: Operates as both a fractional-N and integer-N phase-locked loop (PLL) synthesizer.
• Low Phase Noise & High Precision: Features a low phase noise VCO, typical rms jitter of 0.3 ps, and a typical Error Vector Magnitude (EVM) of 0.4% at 2.1 GHz.
• Programmable Output: Programmable output power levels (–4 dBm to +5 dBm in 3 dB steps) and an RF output mute function (both pin- and software-controllable) for applications requiring isolation.
• Auxiliary Output: Includes a secondary (auxiliary) RF output stage that can be used to drive another circuit or powered down when not in use.
• Fast Lock & Cycle Slip Reduction: Includes a switched bandwidth fast lock mode and cycle slip reduction to dramatically improve lock times without modifying loop filter circuitry.
• Control Interface: Simple 3-wire serial interface (SPI compatible).
• Power Supply & Logic: Operates on a 3.0 V to 3.6 V power supply with 1.8 V logic compatibility and features software/hardware power-down modes.

ADF4351 Pin Diagram

Here’s a look at the ADF4351 pin diagram:

Pin	Name	Function
1	CLK	Serial clock input for the 3-wire SPI-compatible interface.
2	DATA	Serial data input (MSB first) used to program internal registers.
3	LE	Load Enable. Transfers serial data from the shift register into the selected register.
4	CE	Chip Enable. High enables normal operation; Low powers down the device.
5	SW	Fast-lock switch output used with the external loop filter.
6	VP	Charge pump supply voltage.
7	CP OUT	Charge pump output connected to the external PLL loop filter.
8	CPGND	Ground for the charge pump circuitry.
9	AGND	Analog ground connection.
10	AVDD	Analog power supply (3.0 V to 3.6 V).
11	AGNDVCO	Ground for the VCO
12	RFOUTA+	Primary VCO output.
13	RFOUTA-	Complementary VCO output.
14	RFOUTB+	Auxiliary VCO output.
15	RFOUTB-	Complementary Auxiliary VCO output.
16	VVCO	VCO supply voltage.
17	VVCO	Additional VCO supply pin. Connect to the same supply as Pin 16.
18	AGNDVCO	Ground for the VCO
19	TEMP	Internal temperature sensor/output used during factory calibration. Normally bypassed with a capacitor.
20	VTUNE	Control Input to the VCO.
21	AGNDVCO	Ground for the VCO
22	RSET	Connect a precision resistor (typically 5.1 kΩ) to ground to set the charge pump current.
23	VCOM	Internal compensation node. Bypass with an external capacitor placed close to the pin.
24	VREF	Reference voltage decoupling pin. Connect a bypass capacitor to ground.
25	LD	Lock Detect output. High indicates PLL lock; Low indicates unlocked condition.
26	PDBRF	RF output power-down control. Low mutes the RF outputs. Can also be software controlled.
27	DGND	Digital Ground
28	DVDD	Digital logic supply voltage. Must be equal to AVDD.
29	REFIN	Reference clock input. Accepts CMOS, TTL, or AC-coupled sinusoidal reference signals.
30	MUXOUT	Multiplexed output configurable for lock detect, R-counter, N-divider, analog lock detect, and diagnostic signals.
31	SDGND	Digital Σ-Δ Modulator Ground. Ground return pin for the Σ-Δ modulator.
32	SDVDD	Supply voltage for the digital Σ-Δ modulator. Must be equal to AVDD.

Example Use Case : ADF4351 as Direct Conversion Modulator for Base Station Transmitters

A prominent system-level use case for the ADF4351 is acting as the Local Oscillator (LO) in a direct conversion transmitter architecture, heavily used in modern base stations. The ADF4351 is paired with a dual integrated digital to analog converter (like the AD9788 TxDAC) and an I/Q modulator (like the ADL5375).

The ADF4351 generates the LO frequency. Its complementary differential outputs directly drive the differential LO input ports of the ADL5375 modulator.

Applications of ADF4351

• Wireless Infrastructure: Base station applications including W-CDMA, TD-SCDMA, WiMAX, GSM, PCS, DCS, and DECT.
• Test Equipment
• Wireless LANs (WLANs)
• CATV Equipment
• Clock Generation

Summary

The ADF4351 provides flexible RF frequency synthesis with excellent phase noise performance. It is one of the most popular PLL synthesizer ICs for RF development.