cb-model-941swb 电路原理图.pdf

3I:p Oz 00 ITI. r-(JoI ON p 0 0 . .r- 0 0r,- . :e z < ,.,ITI . ;a m ,.(I m z - 0< m ,. ;a . 0 z J.s=t. a I . II . i 1 ! .,., <:L_. rl-.: L t ,. : .,: ;i.,:, : ( if ,).f , QI 1. j:; 4 ,w.,. , i I , .,1+ r ID .D t., -.I 8I- f ii ; I i; i . .: <: j. . . -. ., . 0 - MEASURING FREQUENCY - THE WAVEMETER Reg Motion At the birth of radio, interference between users was negligible but as interest in this novel means of communication quickened problems arose and there was a general call to regulate usage by specifying powers, frequencies, times and other relevant criteria.Radio transmissions do not recognise country boundaries thus the International Telecommunication Union (ITU) was asked to consider the problems and makes recommendation to countries in accordance with its mdings. Suitable and stable frequencies of transmission are one essential factor in avoiding mutual interference.Here the ITU recommends to countries the bands which should be used for types of transmissions and the relevant authority in each country allocates the bands for use in that country then assigns frequencies to users and monitors suchusage. Thus there is a requirement for both the user and the monitoring authority to measure frequency with sufficient accuracy. This has led to a continuing development of the means for such measurement. The earliest common method of frequency measurement was the simple wavemeter. A coil of known inductance resonates with a capacitor of known size at a frequency which can be calculated. The inductance of simple coils can be quite accurately calculated from their physical makeup ( number ofturns, length, diameter and wire size). More complicated coils can be measured by comparison (eg. bridge methods) with other precalculated inductances or capacitances used as standards. Similarly capacitance can be determined by calculation or measured by comparison, Thus the frequency of resonance may be determined. When coupled inductively to a source of RF power such as the output of a transmitter a current flows in the coil/capacitor combination. At resonance this current is at a maximum as also is the voltage across the capacitor/inductorterminals thus providing a means of indicatingresonance.Resonance can also be judged from the maxima in power absorbed from the transmitter as indicated by a slight rise in the plate current ofthe output stage. Actually, while such a device is called a wavemeter the name is a misnomer as it really measures frequency.This comes about because in early days it was almost universal practice to deal with wavelength rather than frequency consequently such instruments were calibrated in the equivalent freespace wavelength and the name given in early times has stuck with this type of instrument. Up until about 1924 wavemeters were the principal frequency detelmining instruments. An accuracy of 1% could be expected for the commonly available types while precision devices gave 0.1% with temperature correction.Thevariable capacitors and inductors in these precision instruments are examples of some of the mest mechanical work with a great amolU1tof thought going into maintaining the accuracy of calibrationover long periods of time. 15 RadioFans.CN