Thanks to the business acumen of the Federal Government, spectrum for PCS and similar applications is now worth approximately $45.00 per Hertz. This simple fact has had a most profound impact on the economics of the filter industry. No longer is the cost for filter hardware the most important of criteria. Although still a consideration, cost has been replaced by spectrum conservation as the most significant element when considering the suitability of a particular filter topology or construction technique. The bill that cannot be sent by the communication service provider (due to co-site or other interference) represents far more $$$ to the provider than the additional cost for an improved filter. It is becoming increasingly clear that today the filter world lives in an era of technology rebirth, performance driven and ideal for creative technologists...but an era in which the resultant hardware must be produced reliably, quickly and in a “reasonably” cost-effective format. One of the most vexing of filter design and production problems is found in the prediction and compensation of interaction between filters which are connected together within the same assembly. In this article, we will discuss a method for interconnection that enables the tools of simulation to be used to both predict and correct these interactions, and thus maximize the performance while minimizing the size and cost of the assemblies.
If one examines the spectrum from about 10 MHz through 3 GHz, there is not an unoccupied segment available for use. It is only due to a user being located in a geographical region physically isolated from another user that enables both to communicate without interference, when both intend to use the same frequency allocation. Frequently, similar systems compete for “air space” when located in the same geographical location. Some signal separation is obtained through the use of orthogonal coding schemes in the time and space domains. However, when maximum system dynamic range is important, some isolation in the frequency domain is required to prevent the system base noise level from rising and thus degrading the dynamic range. Such isolation can best be provided by filtering with high quality “brick wall” filters. An example of such filtering is provided by RS Microwave P/N 71321A-1, a notch filter which displays the following specifications:
Typical test data is shown in Fig. 1. Dimensions are as shown in Fig. 2.