HFF Crystals | High Frequency Fundamental

HFF Crystals: High Frequency Fundamental Mode Crystal Resonators

When a design calls for a stable frequency reference above 50 MHz, engineers face a choice: use a standard AT-cut crystal running in overtone mode, or use a High Frequency Fundamental (HFF) crystal that hits the target frequency directly in fundamental mode. That choice has real consequences for phase noise, board space, and manufacturing yield — and it's why HFF crystals have become the standard component in telecom, GPS, and wireless modules that can't tolerate the tradeoffs of overtone operation.

FCD-Tech supplies a range of SMD HFF crystals — including the H3225, H5035, and H6035 series — engineered for 70 MHz to 250 MHz operation in true fundamental mode.

What Is an HFF Crystal?

An HFF crystal is a quartz crystal resonator built with an inverted mesa structure: the quartz wafer is chemically etched down to a thin raised platform at its center, surrounded by a thicker support rim. This mesa design reduces the vibrating mass of the crystal, which allows it to oscillate at a much higher fundamental frequency than a conventional flat AT-cut blank could reach.

The result is a crystal that reaches 70–250 MHz without relying on overtone modes (3rd, 5th, or 7th overtone), which is what separates an HFF crystal from a standard high-frequency AT-cut part.

Why Fundamental Mode Matters

Overtone crystals work, but they carry known drawbacks that fundamental-mode HFF crystals avoid:

• Better phase noise performance. Fundamental-mode operation avoids the additional noise contribution that comes from overtone excitation, which matters directly in RF and clock-reference applications.
• Simpler oscillator circuit design. Overtone crystals require additional circuitry to suppress unwanted fundamental-mode oscillation. HFF crystals don't need it.
• More predictable manufacturing yield. Overtone-mode operation is more sensitive to circuit parasitics; fundamental-mode HFF crystals are less finicky to design around.

HFF Crystal Applications

HFF crystals are the go-to component wherever a compact, high-frequency, low-phase-noise reference is needed in a fundamental-mode design:

• Wireless modules — crystal for NFC module, Bluetooth, WLAN, and Zigbee designs, where board space and RF cleanliness both matter
• Telemetry systems — remote sensing and data-acquisition hardware needing a stable high-frequency clock
• Optical network equipment — timing references for high-speed optical transceivers
• Mobile digital TV tuners — frequency references for broadcast reception hardware
• GPS and radio communication systems — precise timekeeping and signal synchronization

FCD-Tech HFF Crystal Range

All parts are hermetically sealed with gold-tin construction for high stability and low aging, 100% Pb-free, and RoHS-compliant.

How to Choose the Right HFF Crystal

Three factors typically drive the selection:

1. Target frequency — confirm the required frequency falls within the 70–250 MHz fundamental-mode range rather than needing a standard overtone crystal instead.
2. Board space — the H3225's 3.2 x 2.5 mm footprint suits the tightest designs; the H6035 offers more package area if thermal or mechanical robustness is a priority.
3. Application environment — telemetry and aerospace-adjacent designs benefit from the shock and vibration resistance built into FCD-Tech's HFF line; consumer wireless modules (NFC, Bluetooth, WLAN) can prioritize the smaller footprint instead.

Frequently Asked Questions

What's the difference between an HFF crystal and a standard overtone crystal? An HFF crystal reaches high frequencies (50 MHz+) in fundamental mode using an inverted mesa structure, while a standard crystal relies on overtone modes (3rd, 5th overtone) to reach the same frequency range. Fundamental mode delivers better phase noise and simpler oscillator circuit design.

What frequency range do HFF crystals cover? FCD-Tech's HFF crystal range covers 70–250 MHz in fundamental mode, depending on the specific package (H3225, H5035, H6035).

Are HFF crystals suitable for Bluetooth and NFC designs? Yes — the compact SMD package and fundamental-mode stability make HFF crystals a common choice for NFC, Bluetooth, WLAN, and Zigbee modules where board space and RF cleanliness are both constraints.

Request a Quote

FCD-Tech supplies HFF crystals in standard and custom frequencies, sizes, and specifications. Contact us via the Request a Quote form for datasheets, pricing, or a custom HFF crystal built to your design requirements.

Model	Frequency Range Mode of operation	Size	Features	Data Sheet
H3225 HFF crystal 2-pad SMD	70 MHz ~ 250 MHz Inverted Mesa AT-cut Fundamental	3.2 x 2.5 x 0.8 mm	Hermetic sealing with gold-tin ensures high stability and low aging. Inverted Mesa AT-cut crystal range 70 MHz - 250 MHz Operates in its fundamental mode. High shock and vibration resistant.  This product can operate at a wider range of temperatures. 100% Pb-free, RoHS-compliant. Applications: Telemetry, Optical Network, Radio Communication...
H5035 HFF crystal 4-pad SMD	70 MHz ~ 250 MHz Inverted Mesa AT-cut Fundamental	5.0 x 3.5 x 0.7 mm	Small size area with the low height Inverted Mesa AT-cut crystal range 70 MHz - 250 MHz Hermetic sealing Suitable for NFC, Bluetooth, WLAN and Mobile Digital TV Tuner Zigbee Product is lead free in construction HFF Frequencies available
H6035 HFF crystal 4-pad SMD	80 MHz ~ 200 MHz Inverted Mesa AT-cut Fundamental	6.0 x 3.5 x 1.1 mm	Hermetic sealing with gold-tin ensures high stability and low aging. Inverted Mesa AT-cut crystal range 80 MHz - 200 MHz Operates in its fundamental mode. High shock and vibration resistant.  This product can operate at a wider range of temperatures. 100% Pb-free, RoHS-compliant. Applications: Telemetry, Optical Network, Radio Communication...