Figure 10-11.-Low-pass filter circuit.Figure 10-12.-Schematic diagram of a high-pass filter section.and C2 must withstand the line voltage. Therefore,filters are also rated as to maximum voltage.At frequencies immediately below cutoff, the filterlooks capacitive to both the generator and the load.Inductive reactance XL has very little influence, and nofiltering action takes place. However, at frequenciesabove cutoff, the series reactance of coil L becomesincreasingly higher. The series reactance of coil L islimited only by the resistance of the coil and itsdistributed capacitance.Coil L then functions as ahigh-frequency disconnect. The bypass values of bothC1 and C2 become increasingly higher, and are limitedonly by the inductance of the capacitors and their leads.As a result of these two actions, high-frequencyisolation between points A and B is achieved.High-Pass FiltersIn almost all radio transmitters operating at highfrequencies (HF) and above, the master oscillatorsignal is generated at a submultiple of the outputfrequency. By use of one or more frequencymultipliers, the basic oscillator frequency is raised tothe desired output frequency. At the input to theantenna, an overdriven output amplifier may outputthe output frequency and harmonics of the outputfrequency. A high-pass filter is very effective inpreventing the undesired harmonics from reaching theantenna and being radiated.High-pass filters are also useful for isolating ahigh-frequency receiver from the influence of energyof signals of lower frequencies. Figure 10-12 shows atypical high-pass filter being used to reduceradio-noise interference. In symmetrical high-passfilter sections (Z_{in }= Z_{out}), the series combination ofC1 and L should resonate at @ times the desiredcutoff frequency. The L/C ratio that is chosen shouldhave a square root equal to the terminal impedance.Bandpass FiltersBandpass filters provide a very high impedanceabove and below a desired band of frequencies withinthat band.Bandpass filters find their greatestapplication in the following reamer:Decoupling the receiver from shock andoverload by transmitters operating above andbelow the receiver bandMultiplexing and decoupling two or morereceivers or transmitters using the sameantennaA bandpass filter can be one of many forms andconfigurations, depending upon its application. Forfiltering antennas, a bandpass filter normally consists ofone or more high-pass filter sections, followed by one ormore low-pass filter sections. The configuration ofsections is normally selected so the upper limit of thepass band approaches or exceeds twice the frequency ofthe lower limit of the pass band. Figure 10-13 showstypical arrangements for bandpass filters.Figure 10-13.-Examples of bandpass filter circuits.10-13