In the radio telephone industry, research and development has for a long time been aimed at the production of smaller, less expensive and more efficient units and components. This direction has lead to efficient exploitation of the available space on circuit board surfaces, to an increase in the degree of component integration, and towards rationalization of functions.
High frequency components of radio telephones constitute an exception to this trend. High frequency components have to date been extremely complex in construction, comprising discrete components and strip constructions.
It is known in the art to employ in the transmitter part of a radio telephone an output control circuit for controlling the output level of the transmission. The operation of the output control circuit is based on indications of the output level of the transmitter and comparison with a reference value. An essential part of the output control circuit is formed by a directional coupler, placed immediately adjacent the separation filter of the reception circuit, with which a voltage comparable with the output of the transmitter is produced. The directional coupler is therefore used for measuring the transmission output power, whereby the output level is controlled with the output control circuit. The directional coupler is usually located after the transmitter or before the antenna.
The directional coupler is conventionally constructed using various coupled transmission line constructions and transformers.
In U.S. Pat. No. 5,230,093 (Rich et al.), a transmitter filter with integral directional coupler for cellular telephones is disclosed. The filter includes both a transmitter signal filter and a directional coupler on a ceramic block of one or more pieces. The directional coupler is realized by two coupled transmission lines provided by two holes on the left side of the ceramic block and the transmitter filter is realized by five short-circuited coupled transmission lines provided by five holes on the right side of the ceramic block. The ceramic block is enlarged in order to provide two additional holes functioning as transmission lines to implement a directional coupler integral with the filter. The drawback of this implementation is that the size of the ceramic block has grown as a result of providing it with a directional coupler. As was stated before the aim of the radio telephone industry has been to reduce the size of the components, such as filters used in a radio telephone, and not to increase the size.
As is well known in the art, duplex filters composed of two separation filters and based on resonators are used to separate the signals to be transmitted and received. The duplex filter is therefore usually composed of two separate bandpass filters, one of them being connected to the receiver branch, its mean frequency and bandwidth corresponding to the receiving band, and the other filter being connected to the transmitter branch, its mean frequency and bandwidth corresponding to the transmission band. The other ends of the filters have frequently been connected to a common antenna line via a transmission line matching the impedance. Even though the duplex filter has frequently been accommodated within a single housing provided with terminals for transmitter, receiver and antenna, in practice it is composed of two separate bandpass filters because the isolation between the filters must be made as great as possible so that the electromagnetic leakages therebetween should not impede the functioning of the filter.
The filters constructed using the helix technique are provided with a metal partition between both of the filters, with which the required isolation is efficiently implemented. The couplings in duplex filters to the resonators and the couplings between the resonators have commonly been implemented by means of matching couplings formed by various transmission line constructions and discrete components. The bandpass filters of a dielectric, e.g., ceramic duplex filter, are usually implemented by two different ceramic blocks or by one block whereby the two bandpass filters are separated by one short circuiting strip line provided on one side of the ceramic block such as explained in U.S. Pat. No. 5,239,279.
A widely used directional coupler, having low manufacturing costs, can be constructed using microstrips or strip lines mounted directly on a telephone circuit board. Drawbacks of the structure include the relatively large circuit board area required and the high dependence of the operation on the properties of the circuit board substrate material and variations therein. The substrate material affects the dielectric insulation material losses of the microstrip line, these being greatly dependent on the so-called loss tangent of the material, and it affects the line losses, which are affected by the smoothness of the surface of the material. One example of implementing a directional coupler using strip lines has been explained and shown in U.S. Pat. No. 5,111,165 (Oldfield), which is incorporated herein by reference.
In order to reduce the costs, an advantageous general purpose material is frequently selected for the substrate material in radio telephones. The use of such a material causes deterioration in the functioning of the directional coupler constructed thereon. The increased material losses of the substrate material and the tolerances of the properties can be directly detected in the insertion attenuation of the directional coupler and in the variation of said properties.
By using a directional coupler design which is based on using separate transmission lines (e.g., coaxial cables), on a transformer, or by constructing the switch on a separate, stable or low loss substrate, these losses can be reduced and the operation stabilized. However, a plate area of at least the same size is needed, and in addition, the use of a separate substrate increases manufacturing costs.