GB2122439A - Heterodyne receiver for FSK signals - Google Patents

Abstract

A heterodyne radio receiver is provided suitable for demodulating a binary FSK signal. In the receiver a voltage controlled oscillator (VCO 16) generates a reference output nominally at the center frequency of the input signal. The reference signal is mixed in a mixer (10) with the input signal to generate an intermediate frequency which, after filtering (11) and limiting (13), is fed back to the VCO. The mark/ space ratio of the limited signal varies according to whether the upper sideband or lower sideband is being received. A mark/space detector (17) is thus coupled to the output of the limiter to produce demodulated FSK data. A divider (14) may optionally be included. <IMAGE>

Description

SPECIFICATION Direct loop sonic heterodyne receiver This invention relates generallyto the field of electronic circuits, and, in particular, to a heterodyne radio receiver.

The present invention concerns an improved heterodyne radio receiver. To reduce the manufacturing costs and the packaging size of radio receivers, it is desirable to provide as much of the receiver circuitry as possible in integrated form.

By providing a receiver circuit utilizing a single low (i.e. in the audio spectrum) frequency intermediate frequency (l.F.) channel, the present invention facilitates an integrable receiver. In the prior art, the use of high frequency l.F.'s, particularly when high gain is required, requires a degree of physical isolation of circuit elements which does not easily lend itself to integration.

Accordingly, it is an object ofthe present invention to provide an improved radio receiverwhich easily lends itself to integration.

In accordance with the present invention there is provided a radio receiver responsive to an il ut signal,the receiver comprising a mixer having a first input responsive to the input signal, a second input, and generating an output, means for limiting the outputofsaid mixerto produce a limited output signal, a controller oscillator responsive to the limited output signal and generating an outputwhich is coupled to the second input of the mixer, and means responsive to the limited output signal for detecting differences in the mark/space ratio ofthe limited output signal.

The invention is pointed out with particularity in the appended claims. However, otherfeatures of the invention will becomemoreapparentandtheinven- tion will be best understood by referring to the following detailed description in conjunction with the accompanying drawings in which: Fig. 1 shows a block diagram illustrating a preferred embodiment of the Direct Loop Sonic Heterodyne Receiver of the present invention.

Fig. 2 illustrates waveforms useful in understanding the operation of the present invention.

Referring nowto Fig. 1, a block diagram ofthe Direct Loop Sonic Heterodyne Receiver of the present invention is shown.

The preferred embodiment is particularly suited to demodulating frequency shift keyed (FSK) input signals. In an FSKtransmission binary O's may be represented, for example, by the presence of an upper sideband (USB) in relation to the input signal centre frequency, and binary l's represented, for example, by the presence ofthe lower sideband (LSB).

The circuitofthe present invention comprises a mixer 10 having a first input coupled to receive the incoming FSKsignal over antenna 5. Mixer 10 has a second input coupled to the output of voltage controlled oscillator(VCO) 16, and it has an intermedi ate frequency (l.F.) output coupled to the input of band pass filter 11.

VCO 16 may be assumed to be ofthetype in which a positive voltage input results in an increase in frequency output. Band pass filter 11 may also possess a low pass characteristic.

The output of band pass filter 11 is amplified by amplifier 12, whose output is coupled to the input of limiter 13. Limiter 13 generates a rectangular-shaped waveform in which the transitions correspond to the zero-crossing points of the l.F. signal.

The output of limiter 13, according to a preferred embodiment, is coupled to a dividercircuitl5. It will be understood by those skilled in the artthatthe divider circuit 15may be omitted if desired. Essentially the divider circuit 15 serves to reduce the bandwidth requirements of the band pass filter 11. The division factor N of divider circuit 15 should be such that the sampling limit of the receiver circuit is not contravened.

The output of divider circuit 15 is fed backto VCO 16 via lead 20, and it is also coupled via lead 21 to the input of a suitable markispace detector 17. Mark'space detector 17 produces demodulated data at its output.

The operation of the preferred embodiment will now be described with reference to Figs. 1 and 2.

Let us assume the incoming signal has a frequency higherthan that oftheVCO outputi.e. that the USB is being received. Mixer 10 will then produce a beat frequencywhich filtered by band passfilter 11, amplified by amplifier 12, and limited by limiter 13.

Let us referto the output of limiter 13 as being either + 1 or - 1 depending upon whether it is more positive or more negative, respectively, with respect to a reference potential.

When the I.F. signal inputtotheVCO is +1-i.e.

when the positive half-cycle ofthe limiter output signal is being fed to the VCO-the VCO will move to a slightly higherfrequency,thereby decreasing the l.F.

frequency. This results in a longerthan nominal duration ofthat half-cycle.

When the l.F. signal to the VCO is - 1 - i.e. the negative half-cycle is applied to the VCO, the reverse occurs. The l.F. increases, and the negative half cycle is shortened. This results in a longermarkthan space when the USB is being received. This is shown by waveform 30 in Fig. 2.

When the LSB is being received, the reverse occurs.

The positive half-cycle increases the l.F. and decreases its duration. The negative half-cycle decreases the l.F.

and increases its duration. The result is a smaller mark to space ratio, as shown by waveform32 in Fig. 2.

By providing a suitable mark/space detector, the data content of the input FSK signal can be retrieved.

It will be apparent to those skilled in the artthat the disclosed Direct Loop Sonic Heterodyne Receiver may be modified in numerous ways and may assume many embodiments otherthan the preferred form specifically set out and described above.

Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope ofthe invention.

What is claimed is:

Claims (9)

1. A radio receiver responsive to an input signal, said receiver comprising: a mixer (10) having a first input responsive to input signal, a second input, and generating an output; means (13) for limiting the output of said mixerto produce a limitedoutputsignal; a controlled oscillator responsive to said limited output signal and generating an outputwhich is coupled to said second input of said mixer; and means responsive to said limited output signal for detecting differences in the marldspace ratio of said limited output signal.

2. The radio receiver as recited in claim 1 and further comprising filtering means responsive to the output of said mixer for producing a filtered output; amplifying means responsive to said filtered output for amplifying said filtered output and coupling said amplified and filtered output to said limiter.

3. The radio receiver as recited in claim 2 and further comprising dividing means coupled between said Iimiterandsaid detecting means.

4. The radio receiver as recited in claim 2 wherein saidfiltering meanscomprisesa bandpassfilter.

5. The radio receiver as recited in claim 1 wherein said detecting means comprises a mark/space detector.

6. The radio receiver as recited in claim 1 wherein said controlled oscillator is a voltage controlled oscillator.

7. A radio receiver responsiveto an input signal, said receiver comprising: a mixer (10) having a first input responsive to said input signal, a second input, and generating an output; a band pass filter (11) coupled to said mixer output for providing afiltered output; amplifying means (12) coupled to said filtered output for providing an amplified output; limiting means (13) coupled to said amplified output for providing a limited output; a controlled oscillator responsive to said limited output and providing an output at said second input of said mixer; and a mark/space detector responsive to said limited signal for detecting differences in the mark/space ratio of said limited signal.

8. The radio receiver as recited in claim 7 and further comprising dividing means coupled between said limiter and said mark/space detector.

9. The radio receiver as recited in claim 7 wherein said controlled oscillator is a voltage controlled oscillator.