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AM STEREO MONITOR DECODER

IP.com Disclosure Number: IPCOM000005677D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2001-Oct-25
Document File: 2 page(s) / 92K

Publishing Venue

Motorola

Related People

Lawrence M. Ecklund: AUTHOR

Abstract

A Broadcast Monitor is employed to evaluate the signal quality of the over the air product that a radio sta- tion is producing, as well as to produce sufficiently accurate measurements to allow re-alignment of the station due to any changing of transmitter parameters over time. In order to accurately accomplish these goals, a preci- sion, wide bandwrdth AM Stereo decoding instrument is needed.

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MOTOROLA Technical Developments Volume 7 October 1987

AM STEREO MONITOR DECODER

by Lawrence M. Ecklund

   A Broadcast Monitor is employed to evaluate the signal quality of the over the air product that a radio sta- tion is producing, as well as to produce sufficiently accurate measurements to allow re-alignment of the station due to any changing of transmitter parameters over time. In order to accurately accomplish these goals, a preci-

sion, wide bandwrdth AM Stereo decoding instrument is needed.

The design approach generally used to decode AM Stereo is shown in the block diagram in Figure 1.

1 +L+R

I

IF INPUT

In this approach, as will be shown below, the term cos 6 is generated implicitly. The form of the trans- mitted signal for a C-Quama AM Stereo signal is:

Signal = \/[ (I + L + R)' + (L - R)'] (COS (WJ + 6 ) 1 [cos 6 I,

This equation can be rewritten as:

Signal = [I + L + R] [cos (WJ + &J ) 1.

   Therefore, the term on the envelope detector is 1 + L + R. The depicted feedback loop (designated with solid black lines in the drawing) forces the output of the I detector to be 1 + L + R, which means that the input to the I and Q multipliers has to be:

r!/ [ (I + L + R)' + (L - WI [ cos (W + 4 ) 1 bJs k+ 11 ,

[COSbl

or

I/ [ (I + L + I?)' + (L - R)'] [cos (W,l + # ) 1.

0 Motorola, Inc. 1987

58

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MOlVROLA Technical Developments Volume 7 October 1987

Therefore, the output of the Q detector is L - R.

   This design approach has two drawbacks. The first, and most important, is that the output distortion is a function of the gain of the feedback loop. Therefore, the loop must have relatively high gain (50 db) in the region of interest (20 Hz to 15 kHz), but must also have very good rejection of the carrierand harmonic frequen- cies, w cI 2w.z 3 w o etc. T...