Microwave reflection filter including a ladder network of resona

Details Microwave reflection filter including a ladder network of resona Microwave reflection filter including a ladder network of resona

1996-06-10

1998-07-14

Lee, Benny

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

H01P 120

Patent

active

057810842

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a microwave filter and more particularly to a variable Q reflection mode microwave filter.
2. State of the Art
All passive resonators have a finite unloaded Q factor. In narrow bandwidth applications, this resistive loss can lead to difficulties in the design process. In a bandpass application, designs which provide for both a good input and output match exhibit transfer characteristics with significant amplitude variation over the passband if mid band loss is minimised. This passband variation can only be reduced with given Q factors if the mid band loss is increased, possibly to an unacceptable level. Even in the case of a single resonator, filter problems occur due to the resistive loss which prevents a good input and output match being simultaneously achievable.
In the case of a narrow band bandstop application, the resistive loss of the resonators causes a roll-off of the insertion loss into the passband. A reduction in unloaded Q can quickly cause this loss to reach an unacceptable level, particularly where noise figure is important and the notch filter has been introduced to reject signals which would limit the dynamic range of the receiver. This requirement now exists in several countries where cellular telephone systems have multi-operator configurations.
In a conventional bandstop filter, resonators are coupled off from a main through-line with an electrical separation of an odd number of 90.degree.. Each resonator couples loss into the system and this can be further increased by additional loss in the through-line. To meet a typical requirement at 900 MHz, at least 20 dB rejection has to be provided over a band in excess of 1 MHz, whilst the loss at the 1.5 MHz bandwidth is less than 2 dB. To achieve this, unloaded Q's of greater than 20,000 are required, resulting in the necessity to use dielectric resonators for all of the cavities.
To meet rejection levels of 20 to 30 dB, an alternative method may be used based upon the use of a bandpass filter connected to a 3 dB hybrid, as set out in our UK patent application No. 9324149.5: this "hybrid notch filter" is more compact and provides a slightly lower loss, but still needs unloaded Q's of the order of 20,000 for all cavities. However, we have now devised a filter which acts as a true reflection mode filter and meets the above requirements with much lower unloaded Q's.


SUMMARY OF THE INVENTION

In accordance with this invention, there is provided a microwave reflection mode filter which comprises a three-port circulator device having one port terminated by a one-port filter.
The filter may be arranged, as explained herein, to provide a maximally flat response. Instead, the filter may be arranged, also as explained herein, to provide an equiripple (or quasi equiripple) response.
It will be appreciated that low loss circulators, suitable for use in the filter of this invention, are readily available on the market.
Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a filter in accordance with this invention;
FIG. 2 is a diagram for use in explaining principles on which the filter of FIG. 1 is based;
FIG. 3 is a schematic diagram of a lossy ladder network for use in the filter of FIG. 2;
FIG. 4 shows the theoretical quasi equiripple response (with 20 dB stopband level) of a filter having a 7th degree ladder network;
FIG. 5 is a diagram of a ladder network for a filter having a quasi equiripple response;
FIG. 6 shows the measured response of a 5th degree filter in accordance with the invention;
FIG. 7 shows the measured response of a 6th degree filter in accordance with the invention;
FIG. 8 shows the measured response of a 6th degree filter which includes cross-coupling; and
FIG. 9 shows the passband loss of the 6th degree filter with cross-coupling .


DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 of the

REFERENCES:
patent: 3760303 (1973-09-01), Seidel
patent: 4296389 (1981-10-01), Fuller et al.
patent: 4418324 (1983-11-01), Higgins
patent: 4538123 (1985-08-01), Mariani et al.
Akao et al.; "A Quarter-Wave Coupled Band-Rejection Filter with maximally Flat Pass-Band Characteristics"; Electronics and Communications in Japan, vol. 52-B, No. 8, Apr. 1969; pp. 94-102.
"A Method of Designing Microwave AFR and PFR Equalizers", Znamenskiy et al., Telecommunications and Radio Engineering, vol. 39/49 No.5, May 1985 pp. 108-111.
"Microwave Filters, Impedance-Matching Networks, and Coupling Structures" Matthaei et al., pp. 85-86, 1964, McGraw Hill Book Company.

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