1-4244-1468-7/07/$25.00 ©2007 IEEE 195

Active Hybrid Band-pass Filter for Microwave Telecommunication Systems Using High-order Modulations

Siniša P. Jovanović1, Aleksandar D. Nešić2

Abstract – This paper presents a concept, design and realization of an active hybrid band-pass filter that provides both amplifying and filtering function within a modern wideband converter capable to handle high-order modulated digital signal. This paper introduces a new type of a microstrip band-pass filter element that serves as passive parts of the active hybrid filter.

Keywords – Printed filters, group delay, folded quarter-wavelength microstrip lines, high capacity digital telecommunication systems.

I. INTRODUCTION Modern high capacity digital telecommunication systems

utilize complex modulation procedures such as 128 QAM or 256QAM with challenging requirements for employed transmitting subsystems. These requirements mostly call for high phase linearity, flat amplitude characteristics, low group delay and low level of intermodulation products. Filters that are used within these telecommunication systems strongly affect all mentioned characteristics except the level of the intermodulation products so they have to meet the same, or ever stronger requirements. In the same time, it is highly beneficial if the required filters are realised as printed structures due to their low cost, high repeatability, reliability and suitability for integration.

This paper features realization of a printed band-pass filter for integrated broadband microwave receiver operating in frequency range from 5.4 to 6 GHz. Besides fulfilling the mentioned pass-band requirements, the filter has to provide sufficient attenuation of signals from the symmetrical bandwidth (1.7 to 2.3 GHz - for designated LO frequency of 3.95GHz) that could, after conversion, interfere with regular signal having high-order modulation.

II. FILTER’S ELEMENTARY CELLS High order modulations such as 128QAM or 256QAM are

very sensitive to various distortions like group delay, amplitude variation within the bandwidth, as well as intermodulation products. Due to various signal post-processing and pre-distortions algorithms and techniques the specifications for acceptable distortions levels introduced by microwave receiver/converter are not uniquely specified. Because of that it is difficult to specify characteristics of microwave receivers’ components so we need to retreat to

1Siniša Jovanović is with the IMTEL Micro-Opt, Blvd M.Pupina 165B 11070 N. Belgrade, Serbia, E-mail: siki@insimtel.com

2Aleksandar Nešić is with the Institute IMTEL Communications, M.Pupina 165B 11070 N.Belgrade, Serbia, E-mail: aca@insimtel.com

some experience-based criteria. In that manner the overall converters amplitude flatness within the pass-band should be within ±0.5dB, while group delay variation should be lower than 2ns. Subsequently, assuming worst case error accumulation scenario, the tolerances for components, like filters, shouldn’t be more than a fraction of these overall values.

Coupled quarter-wavelength microstip lines shown on Fig.1 form a simple band-pass filter which main characteristics are shown on Fig. 2. Within the frequency range of interest (from 5.4 to 6GHz - between the markers M1 and M2 on Fig. 2) the filter has return loss better than -25dB, and very flat transmission characteristics (<0.05dB p-p) as well as group delay characteristics (<4ps p-p). The insertion loss over the symmetrical bandwidth (1.7 to 2.3GHz) is however very mild with values better than -12dB only. Also, for chosen dielectric substrates RO4003 (εr=3.38, h=0.2mm, tanδ=15×10-4) the required dimensions of the coupled line (Wa=60µm) as well as the gap between them (Sa=50µm) could be to small for effortless massive fabrication, especially for relatively lengthy couplers like this one with La=8.4 mm.

Fig. 1. Layout of a simple band-pass filter with coupled quarter-

wavelength microstrip lines

f [GHz] Fig. 2. S-parameter and group-delay frequency characteristics for the

filter from Fig.1

In order to overcome such technological limitations a different band-pass filter can be used like one introduced on Fig.3. The structure of the new filter is similar to the structure of the filters reported in [1, 2]. The filter consists of two mutually coupled folded quarter-wavelength microstip lines. As shown on Fig.3., each line is folded in the middle with one

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end connected to input/output microstrip line while the other end is grounded. The total unfolded length of each filter’s line would be close to half-wavelength. The total length of the filter (Lb=8.6 mm) is practically unchanged, but its critical dimensions (Wb=.1mm and Sb=.12mm) are significantly relaxed relative to the filter from Fig.1 making the new filter much suitable for fabrication. The new filter’s structure requires grounding via holes [3] and consequently is more complicated than the structure from Fig.1. However, this is not significant disadvantage for filters intended to be integrated with active circuits because grounding via holes are already required for proper functioning of active circuits. The main characteristics of such filter are shown on Fig. 4.

Fig. 3. Layout of a simple band-pass filter with folded quarter-

wavelength microstrip lines

3 4 5 6 7 82 9

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de

lay(2

,1)

m1

m2

dB(S

(2,1

))dB

(S(1

,1))

f [GHz] Fig. 4. S-parameter and group-delay frequency characteristics for the

filter from Fig.3

Within the frequency range of interest (5.4 to 6GHz - between the markers M1 and M2 on Fig. 4) the filter has return loss better than -30dB, and very flat transmission characteristics (<0.05dB p-p), while group delay variation (<27ps p-p) is bigger than for the previous filter, but still very low. This filter is more selective than the previous one having the insertion loss better than 17dB over the symmetrical bandwidth (1.7 to 2.3GHz), however, this value has to be significantly improved for practical application within a microwave converter/receiver.

III. FILTER CASCADING A simple cascading of the filter’s elementary cells in order

to improve its selectivity, like one shown at Fig. 5, is not a

good solution as it can be seen from Fig.6 showing S-parameter frequency characteristics of such cascade. The frequency characteristic of the cascade is distorted with two parasitic pass-bands at both sides of the regular pass-band. The unwanted pass-bands originate from a resonant circuit formed by two halves belonging to two adjacent filters together with a connecting transmission line which length affects the central frequency of the parasitic pass-band. The simplest way to prevent these unwanted resonances is to separate basic filter cells with ideal isolators as shown on Fig.7.

BPF4BPF3BPF2BPF1

IN OUT

Fig. 5. A simple cascade of four elementary cells

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dB

(S(2

,1))

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Fig. 6. S-parameter characteristics for the cascade from Fig.5

TE

RM

3

TE

RM

2

TE

RM

1 BPF4BPF3BPF2BPF1

CIR3CIR2CIR1IN OUT

Fig. 7. A cascade of basic filter cells separated with isolators

As shown on Fig.8 the cascade has promising characteristics. Within the frequency range of interest (between the markers M1 and M2 on Fig. 8) the cascade has return loss better than -23dB, and very flat transmission characteristics (<0.2dB p-p) as well as group delay variation (<125ps p-p). More importantly, the insertion loss over the symmetrical bandwidth (1.7 to 2.3GHz) is significantly improved to the value better than -60dB. However, the cascade from Fig.7 is too unpractical and expensive for mass-production application because of isolators’ high price. It can be overcome if isolators are replaced by amplifiers that can provide necessary reverse isolation between filters, and in other hand, are anyhow required to amplify a received signal before its conversion. Fig. 9 shows a cascade of basic filters separated with amplifiers having 9dB gain and isolation of -25dB, which is the worst case isolation for this class of

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amplifiers. The other characteristics of the amplifiers from Fig.9 are ideal.

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de

lay(2

,1)

m1m2

dB

(S(2

,1))

dB

(S(1

,1))

a f[GHz]

Fig. 8. S-parameter and group-delay frequency characteristics for the filter cascade from Fig.7

BPF4BPF3BPF2BPF1

IN OUTAMP1 AMP2 AMP3

Fig. 9. A cascade of basic filter cells separated with ideal amplifiers

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de

lay(2

,1)

m1m2

dB

(S(2

,1))

dB

(S(1

,1))

a f[GHz]

Fig. 10. S-parameter and group-delay frequency characteristics for the filter cascade from Fig.9

As shown on Fig 10. the characteristics of the new cascade is very similar with characteristics of the cascade with ideal isolators. The only difference is S21 frequency characteristic which is shifted for the total gain of the amplifiers from the cascade. Since the phase characteristics of the amplifiers are ideal the group delay has the same value like in the previous case.

IV. REALIZATION OF AN ACTIVE HYBRID BAND-PASS FILTER

A cascade like one from Fig. 9 is realized using Hittite’s amplifiers HMC318MS8G [4]. The HMC318MS8G are surface mount low cost C-band variable gain low noise amplifiers (VGLNA) that serve the full UNII and HiperLAN bands. The HMC318MS8 operates using a single positive supply that can be set between +3V or +5V. When a control voltage of 0V to +3V is applied, the gain of the amplifier will decrease while maintaining excellent return loss performance. A maximum gain of 9 dB is achieved when VCTL is set to 0V

and a minimum gain of -9 dB is achieved when Vctl is set to +3V. Also, the reverse isolation S12 of these amplifiers is better than -30dB, typically.

With amplifiers as isolation elements in filters’ cascade an hybrid structure consists of band-pass printed filters and active MMIC elements is formed. Fig. 11 shows simulated S-parameters results for such active filter for four different control voltage Vctl values at HMC318MS8 amplifiers. Fig.12 shows a photo of realized active filter model with four filter cells and three amplifier stages.

5.0 5.5 6.04.5 6.5

-20

-10

0

10

20

-30

30

m1 m2

S,S

,S[d

B]

1122

21

f[GHz]C Fig. 11. Simulated S-parameters of the active band-pass filter with

HMC318MS8 amplifiers for Vctl=0V, 1V, 1.4V and 1.8V

Fig. 12. A realized model of an active band-pass filter with four filter

cells separated with three variable-gain HMC318MS8G amplifiers

V. MEASURED RESULTS Fig 13 shows S-parameter characteristics of the realized

active hybrid band-pass filter model for control voltages values close to 0V. The realized active hybrid filter suppresses signals from symmetrical bandwidth (1.7 to 2.3GHz) for more than 70dB relative to signals within the bandwidth from 5.4 to 6GHz (between the markers 1 and 2 on Fig. 13). Also, within the bandwidth, a gain flatness of about 0.6dB is achieved, with return loss better than -14dB. Fig 14 shows group delay characteristics of the realized active hybrid band-pass filter. It can be seen that active elements from the cascade contributed very little to the overall group delay variation (<150ps p-p) within the pass-band region (between the markers 1 and 2 on

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Fig. 14). The identical active hybrid band-pass filter like presented above is integrated within the broadband microwave down-converter shown on Fig. 15 [5] with overall maximal conversion gain of 85dB±0.5dB and image rejection of 60dB.

Fig. 13. Measured S-parameters characteristics of the realized

model of an active hybrid band-pass filter

Fig. 14. Measured group delay characteristics of the realized

model of an active hybrid band-pass filter

VI. CONCLUSION This paper presents concept, analysis and realization of an

active hybrid distributed filter that in pass-band frequency range from 5.4 to 6 GHz fulfils a very strong requirements regarding group delay and amplitude flatness as well as intermodulation distortion. Also, a new elementary filter cell consisting of two mutually coupled folded quarter-wavelength microstip lines is introduced. The cascade of these filters separated with amplifiers with sufficient reverse isolation forms a filter with high insertion loss in the stop-bands that keeps all good characteristics in the pass-band. Within the pass-band the obtained active hybrid filter has amplitude

flatness of about 0.6dB and group delay variation lower than 150ps, while the signal suppression over the symmetrical bandwidth is better than 70dB.

The presented filter structure is applied for realization of an integrated wideband microwave down-converter having overall conversion gain up to 85dB. There is a very good agreement between all designed, simulated and measured characteristics.

Fig. 15. An active band-pass filter integrated within a microwave

down-converter

ACKNOWLEDGEMENT The authors would like to thank Ms. M. Marjanovic and

Mr. M. Tasic for their help in the realization of the experimental model. This work has been supported by the Ministry of Science of the Republic of Serbia.

REFERENCES [1] Chi-Yang Chang and all: “Folded quarter–wave resonator

filters with Chebyshev, flat group delay, or quasi elliptical function response”, IEEE, MTT-S, 2002, Vol.3, pp. 1609-1612.

[2] Chi-Yang Chang and Cheng-Chung Chen: “A Novel Coupling Structure Suitable for Cross-Coupled Filters With Folded Quarter-Wave Resonators”, IEEE Microwave and Wireless Components Letters, Vol.13, No. 12, December 2003., pp. 517-519.

[3] P. Watson, K. C. Gupta, “EM-ANN Models for Microstrip Vias and Interconnects”, IEEE Trans., Microwave Theory Tech.,vol. 44, no. 12, pp. 2395-2503, 1996.

[4] www.hittite.com [5] A. Nesic, S. Jovanovic, „Wideband Microwave Converter for

256QAM Modulation“ LI ETRAN Conference, Herceg Novi, 4-8 juna 2007, Conference Proceeding Vol. 2 (in Serbian)