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MOTOROLA Technical Developments Volume a October 1988
FM PLL DEMODULATOR WITH LOOP BANDWIDTH CONTROLLED BY TRANSISTOR MATCHING
by Jerry Hallmark, Bob Higgins, and Bill Ooms
Conventional PLL FM demodulators require large capacitors in the loop filter in order to provide a con- trolled loop response. For an integrated demodulator, the large loop filter capacitors must be provided as exter- nal parts. This adds to the parts count of a radio and thus increases its cost and manufacturing time.
The proposed circuit uses a first order phase locked loop in place of a higher order loop typically used for FM demodulators. An example of such a loop is shown in Figure 1. This block diagram shows acurrentcontrolled, current-output phase detector and a current controlled oscillator (ICO). In this first order loop, there is no loop filter outside the inherent l/s response (due to the phase detector output connected to the frequency control input of the oscillator).
-0 OUT
FIG. I
Pl+lSE~&ECTOR
IN- N
El
ICO
F Nn
"lco "lco
.
L I
The loop bandwidth of a first order PLL is determined by its loop gain, and thus to control the loop band- width, one must simply have control over the loop gain. This invention controls the loop bandwidth by:
1. choosing a first order loop in which the bandwidth is only a function of loop gain,
2. using a current-controlled oscillator and a current-controlled, currentoutput phase detector, and
3. taking advantage of the ability to get very good transistor matching on an IC
Figure 2 shows a schematic of how transistor matching is used in the present invention to control the loop gain (and hence the loop bandwidth) of the PLL demodulator. In Figure 2, the phase detector had a current ouptut, I PD, whose magnitude is a functi...
