RADIO RECEIVING CIRCUITS

This invention relates to a radio receiving circuit and is more particularly concerned with a circuit adapted for the reception of radio signals over a wide bandwidth without the use of a conventional external conductive antenna element. Known in the art are radio receivers having a tunable front end stage fed by a conductive wire or rod element usually resonant at or about the received frequency. High gain in the front end stage is usually not important due to antenna background noise being the limiting factor, unless the bandwidth is exceedingly narrow. So-called active antenna systems are also known where an untuned high gain front end is used, possibly with band-pass filters, in conjunction with a relatively short length antenna. This system makes up for the reduced signal strength from a short length aerial but by using a high gain, high dynamic range untuned input stage or stages. The system nevertheless requires some form of pickup for the electromagnetic signal.

One object of this invention is to provide receiving circuit arrangement which avoids the requirement for a conductive antenna element but which is capable of

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providing a satisfactory signal output to feed the input of a tuned radio receiver stage. Such a circuit arrangement is hereinafter referred to as a "receptor".

According to this invention there is provided a receptor for radio frequency signals, the receptor comprising an amplifying device with the input thereof connected to a circuit means producing an electrostatic potential and a c rcuit means producing an electromagnetic current on passage of electromagnetic radiation through the said circuit means, means combining said potential and current with 90° phase shift producing an input signal feeding the amplifying device.

The amplifying device preferably comprises a high gain wide band device coupled to a cascaded amplifier feeding an output.

In an embodiment the said circuit means comprises a serial connected lumped capacitive element and a lumped inductive element connected across the active input of the amplifying device. The inductive element is preferably arranged to have a distributed capacitive reactance. The said elements could be considered as forming a non-resonant transmission line coupling the amplifying device to ground; the line is of very small physical dimension. The distributed capacitive reactance may be replaced partly or totally by a lumped capacitive

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element.

A broadly resonant circuit may be provided coupled to the circuit means to increase gain and signal reception at one or more selected frequency bands.

A further feature is the very low "Q" factor of the circuit means providing a circuit which is not sharply resonant at any particular frequency thus allowing high gain to be used without oscillation.

The accompanying drawings show embodiments of a construction by way of example. In the drawings:-

Figure 1 is a circuit diagram of an embodiment,

Figure 2 is a circuit board layout for the front end only of the circuit of Figure 1,

Figure 3 is a circuit diagram of a modified embodiment, and

Figure 4 is a block diagram of a version using dynamic noise limiting.

An embodiment in accordance with this invention is described and shown by way of example with reference to Figure 1 of the accompanying drawing which is a circuit diagram of a receptor for operation in the VHF band 88 to 108 MHz, the HF band 550 to 1700 KHz and the LF band 150 to 350 KHz.

It is important to appreciate that the described circuit is completely self-contained and in particular

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requires no connection to an external aerial. The circuit elements to the left hand side of TR1 , at least, will need to be in the field of influence of electromagnetic radiation, i.e. unscreened.

The radio frequency signal is developed across C3, L2, C4, and C1 which is the capacity of L2 to ground. A broadly resonant circuit is provided by L1 ,C2 which provides a higher "Q" over certain selected frequency bands. L1.C2 are coupled to the gate of TR1 by C5 which has a very small value. The signal voltage on the gate of TR1 is small and hence this stage may operate at maximum gain without the risk of overload.

The input signal to TR2 being amplified, may have a wide dynamic range and the following stages TR2, TR3, and TR4 form a cascode amplifier with high gain and good dynamic range. The output through C12 feeds a conventional radio receiver.

The operation of the receptor circuit is as follows: As electromagnetic radiation is intercepted and passes through C3, L2, and C4 a magnetic field is developed within L2 and also in the associated capacitors. As the RF cycle passes through zero the induced magnetic field within C3, L2 and C4 collapses and a small RF voltage is developed at each end of L2 with 90° phase difference. The high frequency roll off is

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determined by C4 and the lower frequencies reflect back from C4 through L2 and C3, the resultant signal at the junction of C5, C3 and R1 is in phase.

The function of L1 , C2 is to improve the "Q" of C3, L2 and C4 at certain selected frequencies. The output of L1 , C1 through C5 is in phase with the output of C3, L2,C4 and in this arrangement C1 controls the amount of grounding and therefore the phase difference at the end of L2 at selected frequencies.

A practical operating circuit has the following values:

L1=0.47 μH

C2=5.6 pF

C5=10 pF

C3=0.20 μF

C1=2.2 μF

C4=5.6 pF

L2=0.08 μH to 1.0 μH (typical) In a practical construction the values are sufficiently small to be integrated onto a substrate together with the active components. The substrate may then form a dielectric reducing the physical dimensions. The receptor according to this invention has proved successful in test carried out in a vehicle using a commercially available radio. The receptor can be

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installed at any convenient location where interception of electromagnetic signals will occur such as within a rear view mirror housing. Grounding to the vehicle chassis is not required at the installation position of the receptor.

The circuit stages following TR1 comprises a conventional cascode amplifier stage TR2.TR3 feeding an output TR4. The active elements all comprise FET devices selected for high gain at the frequencies of interest and stability. The output at C12 is connected by a screened feeder to the radio receiver.

There are many constructional methods that can be used to produce a receptor. Examples are: surface mount, hybrid, hybrid ceramic chip and conventional printed circuit board (through hole). The receptor can be wholly or partly manufactured in a custom silicon chip format.

Figure 2 shows a typical receptor using surface mount construction. The printed board, PB, can be made in almost any shape or size. C1 is formed by the proximity of L2 to the ground line G, C4 can follow the same construction method, L1 in this example is a small wound component, but L1 can be incorporated as a circuit track component as is L2. The following low noise amplifier is indicated by AMP.

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Figure 3 is a receptor with the inclusion of series and parallel tuned circuits, STC, for increasing the circuit Q at selected frequencies. The components CT, LT are for trapping out unwanted frequencies. Resistor RA is for damping the turned circuits and can be included to broaden the bandwidth of the circuit if required. Capacitor CA provides a loose coupling to the gate of TR1. The tuned circuits shown can be lumped or linear in construction.

For a receptor with dynamic noise limiting it has been found that there are suitable locations on a vehicle where the receptor provides satisfactory radio signals but the said radio signals are obliterated with engine and vehicle electronic noise. Figure 4 is a receptor with dynamic noise limiting.

In operation, the radio signal RF is sampled at the drain of TR1 and passed to the noise amplifier NA. This stage amplifies both the radio signal and the unwanted noise pulses. The pulse detector and shaping circuits PD separate the wanted radio signal from the unwanted noise pulses. The output from the pulse detector and shaping circuit are the interfering noise pulses only, the wanted radio signal is removed.

The noise pulses may be altered in shape to trigger a circuit providing precise reconstructed pulses for

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example using radar interference limiting and/or blanking techniques, or the shaping circuits may be simply used for improving the rise time of the pulses prior to feeding to the inverting and switching stages S . The inverting and switching is for inverting the noise pulses into a negative pulse stream NSP, with respect to the normal receptor output. The negative pulses can now be used in a suitable circuit to phase cancel the noise pulses or the negative pulses can trigger a switching transistor TR5 to short circuit the receptor output at the correct timing rates to be in phase with the noise pulses appearing at the receptor output.

The noise limiting circuit may not require a direct connection to the receptor for noise pick up. The noise limiter may incorporate a receptor front end and can be tuned to multiple noise source frequencies and can be remotely mounted close to a noise source. TR1 to TR4 are the receptor stages as in Figure 1.

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