Bandwidth of RFIF transformers

Figure 2-31A shows a parallel-resouam RF/IF transformer and Fig- shows the usual construciion in which the two coils {/„] and arc wound at distance d apart on a common cylindrical form. The bandwidth of the KF'IF transformer is the difference between the frequencies where the signal voltage across the otil put winding falls off 6 dB from the value at the resonara frequency (F(i), us shown in Fig. 2-31C- If F\ arid F> are 6-dB (also called the 3-<iB jiohit when signal power is measured instead of voltage) frequencies, ihen the bandwidth is F*- F\. The sh;ipe of the frequency responso curve in Fig. 2-31(7 is said to represent critical coupling.

2-31 Moderate coupling IF transformer. (A) The --ir^iit-. (.[V, <>oi! sparing; (('") bandpass response

An example of a subcritical or umlertuupled RF/IF transformer is shnvrn m Fig. 2-32, As shown in Figs. 2-32A and 2-32B. the windings are farther apart than in the critically coupled case, so the bandwidth (Fig 12-32C) is much narrower than in the critically coupled case. The subcti tic ally coupled KtVlF transformer is ofieai used in .shortwave or t ommunieations receivers in ord<>r to allow the narrow bandwidth 10 discriminate between adjacent channels.

The overcritically coupled RF/IF transformer is shown in Fig. 2-33. Notice in Figs 2-33A and 2-33H that the windings arc closer together, so the bandwidth (Fig. 2-33C) is much header. In some radio schematics and senil e manuals (not to ft,

2-32 Loose coupling IF transformer. (A) The circuit; (B) coil sparing; (C) bandpass response.

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2-33 Over-critical couplhig IF transformer. (A) The circuit; fB) coil spacing; (C) bandpass response.

mention early textliooks). this form of coupling was sometimes called hiijh-fidchtit run pit HQ because it allowed more of the sidebands of the signal (which carry the audio modulation) to pass with less distortion ■ »f frequency response.

The bandwidth of the resonant tank circuit. or the RF'IF transformer, can be suiHmariMl in a figure of merit called Q The Q of the circuit is the ratio of the bandwidth to the resonant frequency as follows: Q = BW/FR. An overcri tit-ally coupled (circuit has a low Q, while a narrow bandwidth suhcriti rally coupled circuit has a high Q.

A resistance in the LC tank circuii will cause itto broaden; that is, to lower its Q. The loaded Q (i.e., Q when a resistance in present, as in Fig. 2-34A) is always less than the unloaded Q. Some radios use a a wile hod resistor (Fig. 2-34B) to allow the user to broaden or narrow the bandwidth. This switch might be labeled J'idettn/ or tcnte or something similar on radio receivers

2*34 Resistor loading broaden response i A) Parallel method; (Bj Jii-li switch

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