GB2200003A

GB2200003A - A coupling circuit for use between a modulator and a ceramic filter in amplitude modulation receivers

Info

Publication number: GB2200003A
Application number: GB08728199A
Authority: GB
Inventors: Luigi Anzini; Rinaldo Graziadei; Giorgio Rossi
Original assignee: SGS Microelettronica SpA
Current assignee: STMicroelectronics SRL
Priority date: 1986-12-31
Filing date: 1987-12-02
Publication date: 1988-07-20
Also published as: JPS63246930A; KR880008552A; IT8622911A0; GB8728199D0; DE3744338A1; FR2609221A1; IT1213447B

Description

v v 1 21^0 0 0 0 3 A Coupling Circuit for use Between a Modulator and a

Ceramic Filter in Amplitude Modulation Receivers Thi,s invention relates to a coupling circuit for use between a modulator and a ceramic filter in amplitude modulation receivers, of-a type intended for connection to the output of said modulator which would include an output stage consisting of a pair of differential cells connected in parallel to each other.

As is known, modulators find application to the field of analog communication systems, e.g. to radio receivers of amplitude modulated (AM) signals.

Modulators usually comprise frequency multiplier circuits to which'a signal pair are input and which output a signal that is Proportional to the product of the two input signals. More specifically, to a first input there is applied a constant amplitude oscillating signal, commonly referred to as the carrier, which is generated locally by an oscillator. incorporated to the receiver, and to a second input there is applied a modulating analog signal from the radio receiver antenna.

The modulator is essentially a frequency translation circuit and its function in radio receivers is basically that of generating the modulated amplitude signals. Further, in order to carry out the modulation with suppression of the carrier, so-called balanced modulators (mixers).are employed.

A typical balanced modulator would comprise an output stage in the form of a pair of differential cells interconnected in parallel to the inputs whereof the carrier signal is applied. Said pair are powered from respective collectors of a further differential cell-to whose inputs the modulating signal is applied.from the antenna. As a rule, the modulating signal from the antenna is within a frequency range of 560 to 1,600 kHz.

The moduLator output is connected, via a coupLing circuit, to a ceramic fifter for seLectiveLy picking up a moduLated signaL at a so-caLLed intermediate fixed (IF) frequency of 455 kHz from aLL those appearing at the moduLator output.

To impLement such a coupLing circuit, and to adapt the 1 moduLator output impedance to the input impedance of the ceramic fiLter, the prior.art proposes an approach discussed herein beLow.

The approach in question invoLves the use of an impedance transformer which has its primary winding terminaLs respectiveLy connected to a power suppLy poLe V S and one of the moduLator output t.erminaLs. In particuLar, the output terminaL wouLd be a coLLector of the differentiaL ceLL pair forming the moduLator output stage, the other coLLector being connected to the power suppLy poLe V S Between said primary winding terminaLs there is aLso connected a capacitor, the secondary winding being connected between ground and the above-noted ceramic-fiLter.- That prior coupLing circuit, whiLe performing substantiaLLy as expected, has the drawback of restricting the performance, as measured in decibeLs, of cross moduLation and fixed intermediate frequency (IF) rejection of the receiver.

Note shouLd be made of that cross moduLation is a measurement, expressed in decibeLs, of the extent to which a moduLated signaL, undesired and yet present at the output, moduLates the carrier of the desired signaL; and IF rejection refers to the ratio, aLso in decibeLs, of a moduLator abiLity to reject a signaL at an IF frequency of 455 kHz to the abiLity to accept a target signaL wi.thin the 560 to 1,600 kHz range.

i 51 f Z 1 k J The technical problem Underlying this invention is to provide a coupling circuit for use between a modulator and a ceramic filter in amplitude modulation receivers, which has such structural and operational features as to overcome the above-noted drawback affecting the prior art.

This problem is solved by a coupling circuit as indicated being characterized in that it comprises an impedance transformer having its primary winding terminals respectively connected to the terminals of said output stage.and the midpoint of said winding connected to a power supply pole V S 1 The features and advantages of the circuit according to theinvention will be more clearly understood from the following detailed description of an embodiment thereof,to be taken by way of example and not of limitation in conjunction with the accompanying drawing.

The drawing single figure shows in diagramatic form a coupling circuit according to this invention.

With reference to the drawing, the numeral 1 comprehensively designates a coupling circuit according to the invention and intended for connection between a modulator 2 and a ceramic fitter 3 in an amplitude. modulation radio receiver, not shown because conventional in design.

The modulator 2 is of a conventional so-called balanced (mixer) type and comprises an output stage 4 consisting of a pair of differential cells 7 and 8, parallel connected to each other. To the input terminals A and B of said stage 4, and.hence of said differential pair 7 and 8, there is applied a so-called carrier periodic signal.

Each differential cell 7, 8 in the output stage 4 is d 1 connected to and powered from a respective coLLector, Cl and C2, of a further differentiaL ceLL 5 whose input terminaLs H and K are appLied a moduLating signaL from an antenna, not shown, of the radio receiver.

The moduLator 2 is impLemented as an integrated circuit and has a pair of output terminaLs or pins 6a and 6b connected directLy to the respective interconnected outputs of the differentiaL ceLLs 7 and 8 of the output stage 4.

The coupLing circuit 1 comprises an impedance transformer or transLator 9 whose primary winding 10 has its input terminaLs connected to the respective output terminaLs 6a and 6b of the moduLator 2. Connected across said terminaLs is aLso a capacitor C.

AdvantageousLy in accordance with this invention, the midpoint 11 of said primary winding 10 is connected to a power suppLy poLe V S to power the coiLs in the primary winding 10 in a symmetricaL fashion.

More particuLarLy, said primary winding 10 comprises an equaL number of coiLs, nl and n2, between each terminaL and the midpoint 11 to provide a truLy symmetricaL and baLanced power suppLy-to said winding 10.

The secondary winding 12 of the transformer 9 has the terminaL pair, 13 and 14, respectiveLy connected to ground and said ceramic fiLter 3.

The signaL appearing at the output of the ceramic fiLter 3 is obviousLy suppLied to a demoduLator stage, not shown, for audio frequency broadcasting.

The baLanced moduLator 2 has the fundamentaL capabiLity of providi-ng moduLation with suppression of the carrier; that is, z ?0 f z J 1 the moduLator wiLL produce no o utput if one of the input signaLs is nuLL. However, prior art coupLing circuits mahe the moduLator unbaLanced owing to their non-Linearity and input/output characteristics, thereby th e abiLity to suppress the carrier deteriorates.

As is known, non-Linear phenomena introduce in the moduLator response socaLLed intermoduLation products consisting of harmonics at a frequency which is a muLtipLe of the carrier frequency, which products are objectionabLe.

By contrast, the coupLing circuit of this invention has a truLy symmetricaL and baLanced structure reLativeLy to the moduLator output. The primary winding 10 of the impedance transformer 9 provides a baLanced Load for the moduLator output; further, the circuit is SuppLied in a symmetricaL fashion with voLtage vaLes of the same ampLitude with respect to the midpoint 11 of the primary winding 10.

Thus, the inventive circuit affords the significant advantage of attenuating the non-Linear phenomena and of removing from the intermoduLation products the harmonics of the second order, as proved by actuaL Laboratory tests.

By eLiminating the intermoduLation products of the second order, performance in terms of decibeLs can be greatLy improved both in the respect of cross moduLation and of rejection at a fixed intermediate frequency IF.

1 z 1

Claims

1. A coupling circuit (1) for use between a modulator (2) and a ceramic filter (3) in amplitude modulation receivers, of a type intended for connection to the output of said modulator (2) which.would include an output stage (4) consisting of a pair of differential cells (7,8) connected in parallel to each other, characterized in that it-comprises an impedance transformer (9) having its primary winding (10) terminals respectively connected to the terminals (6a and 6b) of said output stage and the midpoint (11) of said winding (10) connected to a power supply pole (V S).

2. A coupling circuit according to Claim 1, characterized in that said primary winding (10) comprises an equal number of turns W and nD between each of said terminals and said midpoint (11).

Published 1988 at The Patent Office, State House. 8671 High Holborn, London WClR 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray. Orpington, Kent. BR5 3FLD. Printed by Multiplex techniques ltd. St Mw7 Cray, Rent. Con. 1/87.

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