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High-Q Ceramic Resonator Filters for GNSS Applications

Giri Krishnamurthy, Mini-Circuits Engineering

Boris Benger, Mini-Circuits Applications

Introduction: The History & Modern Uses of GNSS Applications

GNSS (Global Navigation Satellite Systems) has become an increasingly prevalent RF application for both military and consumer use. Early GNSS technology, or location-based services (LBS), was developed exclusively for military use and operated with a margin of error of about 10-yards (9.14m), which was sufficient at the time, but limited its suitability for end uses requiring a higher degree of precision. In the time since the first launch of GPS more than 40 years ago, the evolution of the technology has improved precision to the order of 2-3 yards (1.83-2.74m). This advancement of the technology combined with significant miniaturization and cost reduction of LBS-enabled devices has opened up a large and growing commercial market for GNSS services. For example, GNSS is now used in the agricultural market to calculate statistics for weather, soil conditions, and crop health to help farmers maximize their yields and profits. Such innovations have stimulated demand for components that support military, industrial and consumer applications.

Due to the geostationary nature of GNSS, the various constellations each serve specific geographic regions. The United States constellation known as GPS (Global Positioning System) was the first of many to follow. Because the GPS system hovers above North America, and the signal will only reach those in America, Canada and Mexico, additional GNSS systems were launched subsequently to serve other regions of the globe. Some of these additional systems include Galileo over Europe, GLONASS (Global Navigational Satellite System) over Russia, and BeiDou (Compass) over China.

The satellites in these systems each transmit signals over L-band frequencies (L1, L2 and L5) that are picked up by the receiver in LBS-capable devices on Earth. The receiver accepts the incoming carrier frequencies from four satellites, and takes the coordinates from each in order to determine its current position in relation to the programmed destination.

Design Requirements

The L1, L2 and L5 bands function at low frequencies in the 1-2 GHz range, which makes them accessible and relatively cheap to operate within. As a result, this region of the spectrum is also occupied by multiple application bands for other communication systems. The dense development of L-band frequencies creates an increased risk of interference between different system bands. In GNSS systems, that interference can result in an immediate data error that will present itself either as an absence of information or as incorrect information.

For these reasons, when selecting filters for GNSS applications, it is important to choose a filter that is capable of high rejection with high selectivity or “sharp skirts” on an insertion loss vs. frequency graph. Narrow bandwidths are ideal for L-band applications where spectrum allocations are crowded, and low insertion loss is important for receiving the relatively weak signals transmitted from satellites to receivers on Earth.

GPS, Galileo and GLONASS often share center frequencies in the L1, L2 and L5 bands allowing interoperability for LBS-enable devices between regions, although the bandwidths within each system vary. GPS generally has the widest bandwidth (approximately 15 MHz) compared to Galileo and GLONASS. Because each constellation is geographically separated from the others, there is no risk of interference between the different systems while occupying the same center frequency.

This interoperability is convenient both for filter designers and end users. A GPS filter meant to capture a frequency range in the L1 band, for example, will be able to encompass the narrower bandwidths of Galileo’s E1 band and GLONASS’s II-L1 band. This overlap allows one bandpass filter to be used in multiple GNSS systems and gives the user greater value and mobility with a single device.

 

Spectrum Designations for GNSS Applications
Spectrum Designations for GNSS Applications

Choosing the Right Filter Technology

Several filter technologies are potentially capable of meeting the requirements for GNSS applications. Ideally the filter needs to have a high Q value to prevent interference from adjacent signals and low insertion loss to preserve the integrity of relatively weak incoming signal. A cavity filter offers the highest Q value of practically any filter technology as well as low insertion loss. However, cavity filters are typically large and expensive. These practical limitations preclude their suitability for most real-world GNSS applications in which both size and cost are important factors.

L-C lumped element filters are generally smaller and more cost effective. In theory, they are capable of achieving deep rejection with very high selectivity, but this might require designing in nine or ten poles in the filter response. Although more poles will create high rejection and sharp skirts, this approach comes with a compromise on insertion loss on the order of roughly 5-7 dB which would degrade the sensitivity of the receiver.

Ceramic resonator filters, by contrast, can achieve high rejection and selectivity with much lower insertion loss (typically 1.5 dB or better) than L-C filters with comparable rejection performance. Although they are slightly larger than L-C filters, ceramic resonator filters are still an acceptable size and cost for GNSS application requirements. This combination of performance, size and cost makes ceramic resonator filters an ideal solution for use in GNSS applications.

Ceramic Resonator Filters and Diplexers for GNSS Applications

Mini-Circuits offers a broad range of ceramic resonator filters specifically designed for GNSS applications, including both off-the-shelf models and custom designs. Our ceramic resonator filters are designed with high Q resonators that provide narrow passbands with low insertion loss ranging from 0.9 dB to 3.0 dB. They boast excellent rejection and selectivity as well as low profile packaging for dense system layouts. They offer excellent temperature stability and rugged construction, making them suitable for critical applications in harsh operating conditions.

For systems with special requirements, Mini-Circuits’ applications department can advise on solutions to accommodate additional screening, qualification, and custom designs. For example, stopband rejection of standard models can be further extended by cascading LTCC filters in series. Mini-Circuits also has the design and production capability in house to make passband or bandwidth modifications of existing filters or any other customization on demand.

Below are just a few examples of standard, off-the-shelf ceramic resonator band pass filters recommended for GNSS applications:

  • CBP-1228C+ (1217-1238 MHz) is a narrow band filter with low insertion loss, 1.3 dB Typ. It works in the GPS L2 band (1215- 1239.6 MHz).
Offers excellent rejection and high-power handling, equipped with miniature, shielded SMT package.
Offers excellent rejection and high-power handling, equipped with miniature, shielded SMT package.
  • CSBP-D1228+ (1203-1253 MHz) operates with low pass band insertion loss of 0.9 dB (Typ.) and is suitable for GLONASS G2 band (1237-1254 MHz).
Offers excellent rejection and stable insertion loss vs. temperature at 0.3 dB Typ.
Offers excellent rejection and stable insertion loss vs. temperature at 0.3 dB Typ.
  • CBP-1555C+ (1525-1585 MHz) works in the GPS L1 band (1563-1587 MHz). It provides low insertion loss of 1.1 dB Typ.
Offers a narrow bandwidth with excellent rejection, high power handling, and low pass band VSWR.
Offers a narrow bandwidth with excellent rejection, high power handling, and low pass band VSWR.
  • CBP-1183A+ (1165-1201 MHz) is a high selectivity, low insertion loss filter that can be used at the GPS L5 band (1164-1189 MHz) and Galileo E5a band (1164-1189 MHz).
Offers pass bands up to 6000 MHz with excellent temperature stability, low profile and rugged construction to withstand harsh environmental conditions.
Offers pass bands up to 6000 MHz with excellent temperature stability, low profile and rugged construction to withstand harsh environmental conditions.

In addition to ceramic resonator filters, Mini-Circuits offers diplexers for GNSS applications that provide the same benefits and options for customization. The same low insertion loss and high selectivity observed in Mini-Circuits’ ceramic filters can be expected from Mini-Circuits’ diplexers in both surface mount and connectorized packages:

  • CDPL-1710+ is an SMT diplexer with channel-1 operating at 1176 MHz (GPS L5 band and Galileo E5a band) and channel-2 operating at 1590 MHz (Galileo E1 band).
This surface mount configuration offers low insertion loss, low pass band insertion loss, high rejection, and good return loss.
This surface mount configuration offers low insertion loss, low pass band insertion loss, high rejection, and good return loss.
  • ZCDP-1710A+ is CDPL-1710A+ in a connectorized package.
This connectorized diplexer offers low insertion loss, high rejection, and good return loss.
This connectorized diplexer offers low insertion loss, high rejection, and good return loss.

Listing of Mini-Circuits Ceramic Resonator Filters for GNSS Applications

Bandpass Connectorized Filters:

Model Number

Passband F1 (MHz)

Passband F2 (MHz)

Stopband F3 (MHz)

Rejection @ F3 (dB)

Stopband F4 (MHz)

Rejection @ F4 (dB)

ZX75BP-1450-S+

1320

1580

DC-1100

46

2000-2500

54

ZX75BP-A1060-S+

1015

1105

DC-880

25

1350-4000

30

ZX75BP-A1230-S+

1160

1300

DC-950

30

1670-3500

20

ZX75BP-B1230-S+

1120

1340

DC-940

25

1750-3500

20

ZX75BP-B1280-S+

1160

1400

DC-955

40

1700-2200

40

Bandpass Surface Mount Filters:

Model Number

Passband F1 (MHz)

Passband F2 (MHz)

Stopband F3 (MHz)

Rejection @ F3 (dB)

Stopband F4 (MHz)

Rejection @ F4 (dB)

CBP-1000F+

900

1100

DC-790

20

1260-1800

20

CBP-1023A+

1005

1041

DC-970

20

1075-2400

20

CBP-1034C+

978

1090

DC-790

20

1400-2000

20

CBP-1060Q+

1030

1090

500-930

20

1190-1400

20

CBP-1062C+

960

1164

DC-735

20

1620-1900

20

CBP-1090C+

1060

1120

DC-955

20

1255-2200

20

CBP-1120F+

1020

1220

DC-880

20

1420-2000

20

CBP-1170C+

1110

1230

DC-900

20

1560-2200

20

CBP-1183A+

1165

1201

DC-1130

20

1235-2800

20

CBP-1228C+

1217

1238

DC-1140

20

1330-3000

20

CBP-1250C+

1215

1285

DC-1055

20

1510-2500

20

CBP-1260C+

1200

1320

DC-1025

20

1640-2450

20

CBP-1280C+

1170

1390

DC-950+

20

1850-2450

20

CBP-1280F+

1160

1400

DC-1000

20

1570-2100

20

CBP-1300A+

1200

1400

DC-1040

20

1640-3100

20

CBP-1307C+

1215

1400

DC-1000+

20

1820-2500

20

CBP-1320Q+

1280

1360

900-1170

20

1490-20000

20

CBP-1350C+

1300

1400

DC-1125

20

1665-2700

20

CBP-1400E+

1320

1480

DC-1150+

20

1600-2400

20

CBP-1400F+

1300

1500

DC-1090+

20

1740-2450

20

CBP-1423AF+

1333

1513

DC-1113

60

1669-2600

55

CBP-1450F+

1320

1580

DC-1150

20

1775-2350

20

CBP-1475E+

1375

1575

DC-1230

40

1750-2600

40

CBP-1490A+

1465

1515

DC-1430

20

1550-3000

20

CBP-1538J+

1518

1559

DC-1390

20

1750-3000

20

CBP-1555C+

1525

1585

DC-1415

20

1700-3600

20

CBP-1598AF+

1505.5

1690.5

DC-1264

60

1888-2900

60

CBP-1630F+

1500

1760

DC-1320

20

1960-2600

20

CBP-1645J+

1616

1675

DC-1500

20

1850-3000

20

CBP-A1060C+

1015

1105

DC-865

20

1350-2250

20

CBP-A1230C+

1160

1300

DC-950

20

1670-2400

20

CBP-B1230C+

1120

1340

DC-980

20

1750-2350

20

CSBP-B1300-75+

1210

1390

DC-1080

20

1545-2500

20

CSBP-D1189+

1130

1246

DC-950

20

1550-2400

20

CSBP-D1228+

1203

1253

DC-1020

20

1425-2500

20

Connectorized Diplexers:

Model Number

Passband (MHz)

Passband IL (dB)

Rejection (dB)

Return Loss (dB)

Crossover Isolation (dB)

ZCDP-1710-S+

1590

0.8

40 @ 1176

11

ZCDP-1710-S+

1176

0.8

50 @ 1590

11

Surface Mount Diplexers:

Model Number

Passband (MHz)

Passband IL (dB)

Rejection (dB)

Return Loss (dB)

Crossover Isolation (dB)

CDPL-1710A+

1176

0.8

50.6 @ 1590

10.9

CDPL-1710A+

1590

0.8

39.7 @ 1176

10.9

View all Filters

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