Analog Devices Inc. LTC6948IUFD-1

Overview

LTC6948IUFD-1 from Analog Devices is an ultralow-noise 0.373–3.74 GHz fractional-N synthesizer with integrated VCO, featuring a 4th-order ΔΣ modulator, 18-bit fractional denominator, –226 dBc/Hz normalized in-band phase noise floor, and programmable output divider (1–6). It serves as the core frequency synthesis engine in wideband RF receivers requiring precise, low-jitter local oscillator signals.

For engineers reviewing the LTC6948IUFD-1 datasheet, LTC6948IUFD-1 pinout, LTC6948IUFD-1 application, or LTC6948IUFD-1 equivalent, key selection criteria include its guaranteed VCO range (2.24–3.74 GHz), output divider flexibility, reference input support up to 425 MHz, fast frequency switching capability, and compatibility with FracNWizard™ design software for loop filter optimization.

Technical Context

The LTC6948IUFD-1 implements a fully integrated PLL architecture comprising a phase-frequency detector (PFD), ultralow-noise charge pump (1–11.2 mA programmable), fractional feedback divider with advanced 4th-order ΔΣ modulation, and a high-linearity VCO tuned via analog voltage (TUNE pin). Its output buffer supports differential RF± outputs with 50% duty cycle and configurable mute control.

It operates with three independent supply domains: 3.3 V for digital/REF/RF circuitry (VREF+, VD+, VRF+), 5 V for VCO and charge pump (VVCO+, VCP+), and includes an internal LDO for the ΔΣ modulator. The device supports SPI-based configuration via CS/SCLK/SDI/SDO pins and provides status monitoring through the STAT pin.

Key Specifications

Pinout & Package

28-lead (4 mm × 5 mm) plastic QFN (UFD package) with exposed thermal pad (Pin 29 = GND). Requires soldering of exposed pad to PCB ground plane for thermal and electrical integrity.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar fractional-N synthesizer applications.

Compared with HMC703LP4E and ADF4371, the LTC6948IUFD-1 delivers superior in-band phase noise (–226 dBc/Hz) at lower power and smaller 4×5 mm footprint, making it optimal for compact, high-performance 2.2–3.7 GHz receiver designs where spectral purity is prioritized over ultimate frequency range.

Availability

LTC6948IUFD-1 is available at Aetrix Electronics and suitable for wireless basestations, microwave data links, military radios, and test equipment requiring stable component supply, full production traceability, and long-term lifecycle support.

Supply support for LTC6948IUFD-1 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Analog Devices (formerly Linear Technology) is a global leader in high-performance analog, mixed-signal, and RF ICs, serving precision instrumentation, communications, and defense markets.

The LTC6948 product line delivers ultralow-noise fractional-N synthesizers with integrated VCOs for demanding RF signal generation in 5G infrastructure, radar, and secure comms-designed to replace discrete PLL+VCO solutions with single-chip simplicity and verified phase noise performance.

FAQ

What is the guaranteed VCO frequency range for the LTC6948IUFD-1?

The LTC6948IUFD-1 is factory-configured for a VCO frequency range of 2.24 GHz to 3.74 GHz. This range is fixed per variant and cannot be extended beyond these limits by programming. Output frequencies outside this band are achieved only via the internal O_DIV (1–6), yielding final RF output from 0.373 GHz to 3.74 GHz. Operation outside the specified VCO range may result in loss of lock or degraded phase noise.

Does the LTC6948IUFD-1 support differential reference input?

Yes, the LTC6948IUFD-1 supports true differential reference input on REF+ (Pin 1) and REF– (Pin 28). These pins are internally self-biased and must be AC-coupled using 1 µF capacitors. When used differentially, the input accepts 10–425 MHz signals with 0.5–2.7 VP-P amplitude and 50% duty cycle. Single-ended use is also supported with appropriate REF– bypassing per datasheet guidelines.

How does the MUTE function operate on the LTC6948IUFD-1?

The MUTE pin (Pin 9) is an active-low CMOS input that disables the RF± differential outputs while preserving internal VCO and bias states. When asserted, RF output power drops to ≤–80 dBm within 110 ns; deassertion restores full output in ≤170 ns. This enables rapid channel blanking without disrupting PLL lock or requiring recalibration, making it ideal for TDD systems and frequency-hopping applications.

What is the purpose of the TB, CMA, CMB, and CMC pins on the LTC6948IUFD-1?

TB (Pin 16), CMA (Pin 18), CMB (Pin 19), and CMC (Pin 20) form the VCO bias network. TB is a buffered VCO bias output; CMA/CMB/CMC are VCO core bias inputs. They must be short-traced together and bypassed to GND with a shared 2.2 µF ceramic capacitor. Per datasheet layout guidance, no traces should run under the package between these pins-this prevents coupling-induced phase noise degradation and ensures specified VCO performance.

Can the LTC6948IUFD-1 generate frequencies below 373 MHz?

No, the LTC6948IUFD-1 cannot generate RF output frequencies below 0.373 GHz. Its minimum output frequency is defined by the lowest VCO frequency (2.24 GHz) divided by maximum O_DIV = 6, resulting in 0.373 GHz. Frequencies lower than this require external division or a different synthesizer variant (e.g., LTC6948IUFD-2 supports down to 0.513 GHz min). Attempting to program O_DIV values outside 1–6 or N-divider values outside spec will cause undefined behavior or unlock.