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.

rlhr r

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

Was this article helpful?

0 0
The Ultimate Computer Repair Guide

The Ultimate Computer Repair Guide

Read how to maintain and repair any desktop and laptop computer. This Ebook has articles with photos and videos that show detailed step by step pc repair and maintenance procedures. There are many links to online videos that explain how you can build, maintain, speed up, clean, and repair your computer yourself. Put the money that you were going to pay the PC Tech in your own pocket.

Get My Free Ebook


Post a comment