Monolithic Crystal Filters (MCF) - Packaging, Frequencies, and Applications

Introduction to Monolithic Crystal Filters (MCF)

Monolithic Crystal Filters (MCF) are passive bandpass filters fabricated on a single piezoelectric crystal substrate (e.g., quartz, lithium niobate, or lanthanum gallium silicate). They leverage the mechanical resonance properties of the crystal to achieve excellent selectivity, low insertion loss, and high temperature stability. MCFs are widely used in communication systems, navigation, broadcasting, and instrumentation where precise frequency control is critical 43.

Key Technical Specifications

1. Packaging and Dimensions

MCFs are available in through-hole and surface-mount (SMD) packages. Common types include:

• 49T (Through-Hole,DIP):

  • Dimensions: ~11.0 × 4.5 mm (industry-standard).

  • Applications: Legacy systems, industrial equipment.

• UM-1/UM-5 (Through-Hole,DIP):

  • Dimensions: UM-1 (12.5 × 5.0 mm), UM-5 (11.0 × 4.0 mm).

  • Applications: RF modules, test instruments.

• SMD 7050 (Surface-Mount SMD):

  • Dimensions: 7.0 × 5.0 × 1.4 mm (most compact) 6.

  • Applications: Space-constrained designs (e.g., IoT devices, wearables).

• UM-1MJ/UM-5MJ (Surface-Mount SMD):

• Dimensions: UM-1 (11.8*7.8*3.5mm), UM-5 (9.8*7.8*3.5mm).
• Applications: RF modules, test instruments.

2. Frequency Ranges and Pole Configurations

MCFs cover frequencies from 10 MHz to 250 MHz, with pole counts (2–8 poles) determining selectivity:

• Low-Frequency MCFs (10–50 MHz):

  • Examples: 10.7 MHz (49T), 21.4 MHz (UM-1), 45.0 MHz (SMD 7050) 46.

  • Bandwidth: ±1 kHz to ±20 kHz (narrowband).

• High-Frequency MCFs (50–250 MHz):

  • Examples: 58.05 MHz, 73.35 MHz (SMD 7050).

  • Bandwidth: ±20 kHz to ±100 kHz (wideband) 8.

• Pole-Dependent Performance:

  • 2–4 poles: Shape factor 5:1 (20 dB to 3 dB bandwidth), insertion loss 3–4 dB.

  • 6–8 poles: Shape factor 2.5:1–2.0:1, insertion loss 4.5–5 dB, superior out-of-band rejection (>65 dB) 8.

3. Electrical Characteristics

• Insertion Loss: 3–5 dB (typical).

• Impedance: 800 Ω (standard), customizable (50 Ω–1 kΩ).

• Passband Ripple: ≤1 dB.

• Temperature Stability: ±10 ppm over -40°C to +85°C 68.

4. Applications

MCFs enable frequency discrimination in:

• Wireless Communication: Channel selection in radios (e.g., 10.7 MHz for FM receivers) 4.

• Satellite & Navigation Systems: Low-phase-noise filtering for GPS/GLONASS.

• Broadcast Equipment: TV transmitters (45 MHz, 70 MHz).

• Industrial Controllers: CAN bus, Ethernet interfaces 9.

MCF Selection Table by Package and Frequency

The table below summarizes industry-standard MCFs:

<

Pictures	Package Type	Dimensions (mm)	Frequency (MHz)	Poles	Bandwidth	Insertion Loss	Applications
	49T	11.5*4.5*11.2mm	10.7MHz	2 poles 4 poles 6 Poles 8 Poles	±3.75KHz~±15KHz	3.0 dB	FM Radios, Industrial Comms,Telecom
	49TMJ	14.7*11.5*5.2mm	10.7MHz	2–8	±3.75KHz~±15KHz	3.0 dB	FM Radios, Industrial Comms,Telecom
	UM-1	7.8*3.1*8.0mm	21.4MHz, 45.000MHz 38.855MHz 29.255MHz	2–8	±3.75KHz~±15KHz	3.5 dB	RF Transceivers,Telecom
	UM-1MJ	11.8*7.8*3.5mm	21.4MHz, 45.000MHz 38.855MHz 29.255MHz	2–8	±3.75KHz~±15KHz	3.0 dB	Telecommunication
	UM-5	7.8*3.1*6.0mm	21.4MHz, 45.000MHz 38.855MHz 29.255MHz	2–8	±3.75KHz~±15KHz	3.0 dB	Telecommunication
	UM-5MJ	9.8*7.8*3.5mm	21.4MHz, 45.000MHz 38.855MHz 29.255MHz	2–8	±3.75KHz~±15KHz	3.0 dB	Telecommunication
	SMD 7050	7.0*5.0*1.4mm	21.4MHz, 21.7MHz, 45.000MHz 109.65MHz	2–8	±3.75KHz~±15KHz ±7.5KHz~±15KHz	3.0 dB	5G IoT, Wearables, Aerospace 46,Telecom

Design Considerations

• Shape Factor: Critical for adjacent-channel rejection. Lower ratios (e.g., 2.0:1 for 8-pole) enhance selectivity 8.

• EMI Mitigation: Use ground shielding for SMD MCFs in high-noise environments.

• Impedance Matching: Ensure system impedance (e.g., 50 Ω) aligns with MCF specifications to minimize reflections.

Future Trends

• Miniaturization: Sub-5.0 × 3.2 mm packages for 6G devices.

• Hybrid Filters: Combining MCF with SAW/BAW for ultra-wideband (0.5–5% relative bandwidth) 3.

• AEC-Q200 Compliance: Automotive-grade MCFs for vehicle-to-everything (V2X) systems 9.

Conclusion

MCFs deliver unmatched frequency stability and Q-factor in compact packages. Designers should prioritize pole count for selectivity, package size for integration, and frequency tolerance for environmental resilience. For advanced needs (e.g., >100 MHz, 8-pole).