18-08-2012, 10:42 AM
A useful & informative post, David: thanks.
However I feel that I must point out to some of our less-experienced readers that amongst all the passive components that one finds in electronics circuitry of any vintage, the 'R.F. choke' is the one item that is easy to mis-apply and whose behavior is generally not fully appreciated. The main reason (but not the only one) for this is simply that an 'R.F. choke' is far from being a 'pure inductance' - although a brief analysis of most designs where one is used seems to regard it as being such. Apart from its d.c. resistance, there is the all-important issue of its inherent capacitance: any R.F. choke, above a certain frequency, starts to behave more like a capacitor that an inductance! What can be even more disastrous to any given circuit is the frequency where that capacitance resonates with the choke's inductance - the last thing you want a 'choke' to do! Most of the chokes in the photograph show R.F. chokes wound in sections: this is an attempt to reduce that self-capacitance. So, in summary, when a cct. calls for a R.F. choke of so many mH, that specification is not really adequate: the manufacturer's reference ideally is also required so that the R.F. choke employed in your circuit will have the same R.F. characteristics as that used in the prototype design.
I have access to several inductance-measuring instruments, and many have been the occasion when, during circuit development, I have substituted on R.F. choke of a known inductance and d.c. resistance for another with those same parameters, only to find that the behavior of the cct. is then substantially different.
Al. / August 18th. //
However I feel that I must point out to some of our less-experienced readers that amongst all the passive components that one finds in electronics circuitry of any vintage, the 'R.F. choke' is the one item that is easy to mis-apply and whose behavior is generally not fully appreciated. The main reason (but not the only one) for this is simply that an 'R.F. choke' is far from being a 'pure inductance' - although a brief analysis of most designs where one is used seems to regard it as being such. Apart from its d.c. resistance, there is the all-important issue of its inherent capacitance: any R.F. choke, above a certain frequency, starts to behave more like a capacitor that an inductance! What can be even more disastrous to any given circuit is the frequency where that capacitance resonates with the choke's inductance - the last thing you want a 'choke' to do! Most of the chokes in the photograph show R.F. chokes wound in sections: this is an attempt to reduce that self-capacitance. So, in summary, when a cct. calls for a R.F. choke of so many mH, that specification is not really adequate: the manufacturer's reference ideally is also required so that the R.F. choke employed in your circuit will have the same R.F. characteristics as that used in the prototype design.
I have access to several inductance-measuring instruments, and many have been the occasion when, during circuit development, I have substituted on R.F. choke of a known inductance and d.c. resistance for another with those same parameters, only to find that the behavior of the cct. is then substantially different.
Al. / August 18th. //
