диафрагмированные волноводные фильтры / 3dad5948-d90e-423c-8402-157a28ef5de8

Abstract

Characteristics are presented for narrow.band pass (~3%) filters designed with temperature compensated high permittivity dielectrics (barium nonatitanate). AtW band both duroid and Z cut quartz microstrip substrates have been investigated and the insertkn losses due to the various coupling fields are estimated to be ~ 2dB for 2 resonator filters and ~ 3.5 dB for 3 resonator filters.

Introduction

The concept of temperature compensating dielectric resonant devices is independent of frequency and this paper shows that millimetre wave systems requiring filters could possibly benefit from using high permittivity, temperature compensated dielectric resonators in low order modes, since they are inexpensive, small and

system requirements. Open microstrip circuits have been used at 600 GHz and are very attractive from the fabrication and cost point of view, so that data are presented for W band dielectric band

temperature coefficient of resonant frequency of 6 ppm;oC.

Microstrip Substrates and Dielectric Spacers

for W Band

AtW band the main microstrip substrates are 5880 duroid, water free fuzed quartz and Z cut crystal quartz which have respective permittivities

0.055 dB/mm, but, when only the dielectric filter is considered it is necessary to know the individual values of the conduction loss and the dielectric loss. The ease of circuit fabrication and the effects of temperatures must also be considered, as well as the technique for determining the insertion loss of the filter. Previous experience has shown that the determination of the errors in the value of the filter loss must take into account variations in microstrip losses, microstrip to waveguide transformer loss, and 'bolting up' losses. These last three losses are eliminated in the present work by fabricating the filter in situ.

In comparing substrates, the maximum thickness

mode. In the calculation of the conductor loss a correction must bemade for surface roughness and the skin depth of the conductor. The quartz circuits use gold metallizations which are 5 ~ thick and which for good adhesion require a surface finish of ~ 0 .15 ~ CLA and a chrome keying layer of

~0.04~. It is estimated that the finish of the

copper clad 5880 duroid is about 0.75 ~ CLA. Assuming the classical skin depths of chromium, gold and copper, which are respectively 0.26, 0.24

and 0.21 ~, the loss data of Table 1 were calculated.

Table 1. Theoretical estimation of conductor and dielectric losses for 50 n microstrip at 100 GHz for various 125 ~ thick substrates.

It maybe seen that the filter should have a minimum amount of conductor in its design. Such designs are given in Figures 1 and 3, where both microstrip and dielectric losses are minimised.

assembly from the test bench. Thus, after measuring the loss of the 50 n line assembly the section AB

directly3. Two sets of cuboid resonators were used

It was found that the smallness of the gap AB resulted in direct coupling (~-15 dB) between the

input and output microstrip. Figure 2 shows the

response of a filter using the 0.8 mm resonators

3.1 GHz (the ripple was due to the sweep oscillator

increase of band stop attenuation requires an increase in the distance AB,. and more resonators.

Three Resonator Filters

In the three resonator filter the question of s~pporting the central resonator must be considered,

s~nce, to cut down dielectric losses in the duroid it was decided to remove a section of the 50 n lin~,

and the underlying substrate and ground plane. The

Conclusion

Whilst the performance of these narrow band filters is not as good as metal E plane waveguide filters· they are small and directly compatible with millimetre wave integrated circuits both in technology and size. It would seem that they are comparable to coplanar filters 5

*As the resonator is raised above the microstrip line the coupling will change. In this filter the

T113.5

0.6 mm square resonators were used and the effect of spacer height on the absorption frequency and Q of the TEll mode is given below.

Acknowledgement

I wish to thank Plessey, Caswell, for the resonators and the use of their facilities.

References

1. MORGAN, 'Temperature compensated high permittivity dielectric resonators for millimetre wave systems', Int. J. of IR and MM Waves,~,Jan

1984.

2.SEASHORE and SINGH, 'mm wave component trade-

erature compensated dielectric filters for microwave

millimetre-wave fin line and metal insert filters'. Radio and Electronic Eng., 52,513-521, 1982.