GB2279526A

GB2279526A - Quadrature FM detector

Info

Publication number: GB2279526A
Application number: GB9311299A
Authority: GB
Inventors: Michael William Dormer
Original assignee: VTech Communications Ltd
Current assignee: VTech Communications Ltd
Priority date: 1993-06-02
Filing date: 1993-06-02
Publication date: 1995-01-04
Also published as: GB9311299D0; CA2124879A1

Abstract

A quadrature FM detector apparatus for demodulating an FM signal so as to produce a baseband audio signal utilizes a delay circuit 103, which may comprise a filter or even a digital shift register, for delaying the FM signal by one-quarter of its frequency. The delayed FM signal is then mixed with the original signal to obtain a baseband audio signal. The filter may be a two stage, four pole Bessel filter. Other filters such as Gaussian, Butterworth, all-pass filters may be used instead. <IMAGE>

Description

Quadrature i Detector Apparatus The present invention relates in general to an FM demodulator apparatus and, in particular, to a quadrature FM detector which utilizes a delay to create the quadrature signal.

Various designs for a quadrature FM demodulators or detectors are well known in the radio art.

Typical quadrature detectors accept a frequency modulated signal, create a phase shifted version of the signal and mix the signal and its phase shifted version together to obtain baseband audio signal.

Phase shifting is typically accomplished through the use of a resonant LC network, inducing a phase shift which varies linearly with frequency. Thus, mixing the shifted and unshifted signals generates an output voltage linearly dependent on the frequency of the frequency-modulated signal -- at least over the linear range of the LC network.

Quadrature FM demodulators are often utilized in superheterodyne (or superhet) receivers. Superheterodyne receivers receive a transmissions signal and mix it with a local frequency, also known as the beat frequency, to convert the received signal to a lower intermediate frequency. Typically this intermediate frequency is on the order of 10 MHz. One reason for an intermediate frequency of this high a magnitude is that LC quadrature FM demodulators are useful, as stated above, only in linear range of their LC network. This linear range can be described in terms of the operation frequency and the network's quality factor, Q, as: operating frequency/Q. Given a wide-band frequency modulated system, and that Q is generally on the order of 20 the operation frequency must at least be in the 10 XHz range to be effective.

Due to this range, quadrature demodulation typi cally tend to be utilized only in narrowband frequency modulated systems. Narrowband FM has only one component on either side of the carrier frequency and thus usually are more able to obtain a linear response from an LC network.

However, wide-band FM tends to be favored over narrowband FM in low-power applications, because wide-band FM requires a smaller carrier amplitude.

In a low-power transmission and reception system, particularly one utilizing a lower intermediate frequency (on the order of 200 kHz), standard quadrature FM demodulators do not produce the desired audio signal.

It is thus an object of the present invention to provide a quadrature frequency demodulator apparatus, which can operate at all frequencies for both wide and narrow band FM transmission signals.

This and other objects will become apparent in light of the attached specification and drawings.

A preferred embodiment of the invention comprises a quadrature FM detector apparatus for demodulating an FM signal so as to produce a baseband audio signal. The quadrature FM detector apparatus includes a bessel filter which delays the FM signal by one-quarter of its period. The apparatus further includes mixer means for mixing the FM signal with the delayed FM signal such that baseband audio is produced.

Fig. 1 of the drawings is a block diagram of a prior art quadrature FM detector showing, in particular, a ninety degree phase shifter; Fig. 2 of the drawings is a block diagram of the present quadrature FM detector apparatus; Fig. 3 of the drawings is a schematic diagram of a bessel filter which may be utilized in the present quadrature FM detector apparatus; and Fig. 4 of the drawings is a block diagram of a digital shift register which may be utilized in the present quadrature FM detector apparatus.

While this invention is susceptible of embodiment in many different forms and in many different systems, there is shown in the drawings and will herein be described in detail, two specific embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated.

Fig. 1 of the drawings is a block diagram of prior art quadrature FM demodulator 10. Quadrature FM demodulator 10 accepts a "limited" FM signal from a limiting amplifier -- an amplifier operating at saturated amplitude. Quadrature FM demodulator 10 includes a mixer 11 and phase shifter 12. Phase shifter 12 in most cases comprises an LC network consisting of one inductor and two capacitors, chosen to provide a ninety degree phase shift, as is known in the art. The resulting output of quadrature FM demodulator 10 is baseband audio.

Fig. 2 is a block diagram of quadrature FM detector apparatus 100 of the present invention.

Quadrature FM detector apparatus 100 accepts FM signal 101, which may be at any frequency and either a narrow-band or wide-band FM signal, toward producing baseband audio signal 102. Quadrature FM detector apparatus 100 includes bessel filter 103a for delaying FM signal 101 by one-quarter of its period, the period of a signal being the inverse of its center frequency, thus producing delayed FM signal 104. FM signal 101 and delayed FM signal 104 are fed into mixer means 105 which mixes the signals together such that baseband audio 102 is produced.

Mixer means 105 may be a balanced mixer so as to reduce carrier leak-through, though it may also consist of a standard mixer.

Fig. 3 is a schematic diagram of bessel filter 103a, which is capable of delaying FM signal 101 by one-quarter of its period. Bessel filter 103a can be used because the required delay -- even at lower frequencies -- is rather short (on the order of 1 millisecond). Bessel filter 103a exhibits constant group delay "in band" having a cutout frequency much greater than the frequency of FM signal 101.

In a preferred embodiment, Bessel filter 103a has two stages and four poles, with taps between the second and fourth poles. Potentiometer 120 adjusts the delay of bessel filter 103a. Other filter types, such as, Gaussian, narrow Butterworth, and various all-pass filters are also within the scope of the present invention, however, these implementations while functional, do tend to present more difficult designs.

An alternate approach to using bessel filter 103b is to utilize digital shift register means 103b in its stead. The output of a limiting amplifier, FM signal 101, can be thought of as a digital signal (because of the very high gain) and FM detection as effectively nothing more than counting zero-crossings, such that digital shift register means 103b, shown in Fig. 4 may be utilized as delay means 103.

Ideally, digital shift register means 103b would be of infinite length and clocked infinitely quickly to minimize quantization errors. However, digital shift register means 103b has been found to induce tolerable quantization error and thus provide proper delay of FM signal 101 where a digital shift register sixteen bits long clocked at approximately 12.8 MHz is used -- where the frequency of FM signal 101 is approximately 200 kHz.

The foregoing description and drawings merely explain and illustrate the invention and the invention is not limited thereto, except insofar as the appended claims are so limited and as those skilled in the art who have the disclosure before them will be able to make modifications and variations therein without departing from the scope of the invention.

Claims

1. A quadrature FM detector apparatus for demodulating an FM signal so as to produce a baseband audio signal, said quadrature FM detector apparatus comprising: - a two stage, four pole bessel filter having a tap between the second and fourth poles which delays the FM signal by one-quarter of the period of the FM signal; and - mixer means for mixing the FM signal with said delayed FM signal such that baseband audio is produced.

2. A quadrature FM detector apparatus for demodulating an FM signal so as to produce a baseband audio signal, said quadrature FM detector apparatus comprising: - a digital shift register means which delays the FM signal by one-quarter of the period of the FM signal; and - mixer means for mixing the FM signal with said delayed FM signal such that baseband audio is produced.