GB2318696A - Radio transmitter package with combined power and modulation I/P pin - Google Patents

Abstract

A miniature radio transmitter package suitable for on-off signalling (in a key job for example) comprises an LC resonant circuit (L1, C1, C2, C3) coupled to a frequency-controlling resonant circuit (Q1, SAW1, R2) and having a capacitor (C1) connected across the output terminals (10, 11) to reduce RF leakage to the encoding circuitry (2). One of the terminals (10) functions as i) a power supply terminal ii) a data input terminal and iii) an RF output terminal.

Description

RADIO FREQUENCY TRANSMITTER The present invention relates to a radio frequency transmitter, a radio frequency transmitter arrangement and a radio frequency transmitter package.

A transmitter module normally incorporates the circuits required to generate radio frequency power at an appropriate frequency, and to vary the characteristics of the output in accordance with an extemal signal. A typical transmitter circuit employing a transmitter module is commonly used in applications such as radio keys, remote control, and wireless telemetry. An important characteristic of transmitter modules is that they do not produce excessive output at frequencies outside their allocated operating band.

Such outputs normally arise from harmonics generated in the oscillator in the module.

Transmitter modules currently available are too bulky for-use in miniature radio keys and their power supply requirements do not readily suit them to the variety of power sources commonly used. it is desirable to miniaturise such radio transmitters to the greatest possible extent and miniature radio transmitters having a size of about 13 mm x 13 mm and having four extemal terminals (data in, RF out and two power supply terminals) are known.

An object of the present invention is to reduce the number of external terminals (pins) to three or, most desirably. two and to enable further simplification.

In one aspect the invention provides a radio frequency transmitter comprising an RF oscillator circuit, two input terminals coupled to the oscillator circuit, and an antenna means coupled to an output of the oscillator circuit, one of said input terminals functioning as both a modulation input terminal and a power supply terminal.

Preferably the antenna means is coupled to said one input terminal. This feature enables the number of pins to be reduced to just two. Such a radio frequency transmitter can be used to transmit digital information by gating the power supply to its pins with an information-carrying signal. Altematively the power supply voltage or current can be modulated with an analogue signal to generate an information-canying amplitudemodulated radio signal.

Preferably the oscillator circuit comprises an LC resonant circuit and an amplifying device the LC resonant circuit including a capacitor connected directly across said input terminals. The impedance of the capacitor is preferably low compared to the total capacitive impedance of the LC resonant circuit. This feature reduces the RF impedance across the terminals and thereby reduces the leakage of RF back to the driving circuitry which is used to modulate the power supply voltage or current.

In a variant, an inductor is connected in series with said one terminal, coupled to the LC resonant circuit. The other end of the series inductor is decoupled to the other terminal of the module by means of a capacitor with a very low RF impedance. The valve of the series inductor and the degree of coupling are arranged to produce a low RF impedance across the terminals.

Preferably the LC resonant circuit includes an inductor connected between an output of the amplifying device and the capacitor, thereby filtering the output of the amplifying device. In this manner, spurious emissions caused by switching the oscillator and also by steady state harmonic distortion in the oscillator are reduced.

Preferably the oscillator circuit comprises a frequency-stabilising resonator device, such as a quartz crystal, Surface Acoustic Wave (SAW) resonator, cavity resonator or dielectric resonator for example.

Other preferred features are defined in the dependent Claims.

In another aspect the invention provides a radio frequency transmitter arrangement comprising a radio transmitter module, a modulator circuit arranged to modulate a DC power supply input connected across two input terminals of the radio transmitter module whereby one of the said input terminals functions as both a data input terminal and a power supply terminal, and an RF output coupled to the radio transmitter module.

Preferably said RF output is coupled to said one input terminal. This feature enables the number of connections to the transmitter module to be reduced to just two.

Preferably an RF impedance (e.g. an inductor or a resistor) is connected between an output of said modulator circuit and said one terminal. Particularly when a capacitor is connected across said two input terminals, this features reduced RF leakage back into the modulator circuit.

Further preferred features of this aspect of the invention are defined in the dependent Claims.

In a further aspect the invention provides a radio frequency transmitter package comprising an RF oscillator circuit, two input terminals coupled to the oscillator circuit, an RF output terminal (which is optionally also one of said input terminals) coupled to an output of the oscillator circuit, one of said input terminals functioning as both a data input terminal and a power supply terminal and the oscillator circuit being arranged to gerierate a modulated RF oscillation in response to a modulated energising signal applied across said input terminals. The package can for example comprise surface-mounted components on a miniature circuit board, the assembly being encapsulated, or the transmitter can be formed as an integrated circuit for example.

Preferred embodiments are described below by way of example only with reference to Figures 1 to 9 of the accompanying drawings, wherein: Figure 1 is a block diagram of an RF transmitter arrangement in accordance with the invention; Figure 2 is a block diagram of another RF transmitter arrangement in accordance with the invention; Figure 3 shows a tuned loop antenna for use with the transmitter arrangement of the invention; Figure 4 shows a whip antenna arrangement for use with the transmitter arrangement of the invention; Figure 5 shows a circuit diagram of a transmitter package for use in the arrangements of Figures 1 and 2; Figure 6 is an elevation of the transmitter package of Figure 5' Figure 7 is a circuit diagram of a variant of the transmitter package of Figure 5; Figure 8 is a circuit diagram of a further variant of the transmitter package of Figure 5; and Figure 9 is a circuit diagram of a further variant of the transmitter package of Figure 5.

Referring to Figure 1, the arrangement comprises a DC power supply 1 which energises an encoding circuit 2 which superimposes an infonnation-carrying signal independence upon an input signal received on input line 20, pulsed digital signal 7 on the DC across RF ground 5 and a positive supply line 8. A transmitter module 3 is connected at its positive power supply terminal 10 to supply line 8 via a dropper resistor RD and the junction of this positive terminal and resistor RD is also connected to a miniature antenna 4 (which may for example be a short whip antenna). Hence terminal 10 has the three fold function of: - i! receiving input date; ii) receiving DC power and iii) outputting RF to antenna 4.

Dropper resistor RD serves both to set the power supply voltage to transmitter module 3 (enabling this module to be used with different encoding circuits having different DC output voltages) and to reduce RF leakage to encoding circuit 2. An optional decoupling capacitor C10 connected between RF ground 5 and the output of encoding circuit 2 provides further RF filtering.

The encoding circuit 2 switches the module 3 at a suitable baud rate, an upper limit being set by the time taken for oscillations to start and stop when the power to the module is switched on and off. The practical frequency of oscillation of the transmitter module is determined by the type of resonator: 30 kHz to 200 MHz for a crystal, 200 to 900 MHz for a SAW resonator, and up to several GHz for cavity and dielectric resonators. Typically, a frequency of 418 or 433.92 MHz and a baud rate of up to 1200 would be used for a SAW stabilised transmitter module for use in the UK. Antenna 4 is suitably a tuned loop much shorter than 112 a wavelength, e.g. 0.2R or less, preferably 0 or less which presents a iow RF impedance on the order of a few ohms. The antenna therefore provides an approximate matching impedance to the low RF impedance of the module, so optimising the transfer of RF power to the antenna. The low RF impedance minimises the power loss in the dropper resistor RD.

Figure 2 shows a variant of Figure 1 in which modulation is applied to negative power supply terminal 11 of transmitter module 3, which is also coupled to antenna 4.

Figure 3 shows a loop antenna 4' tuned to resonance by a 1 to 5 pF variable capacitor C4.

Figure 4 shows a different antenna 4", which is a whip antenna typically less than 1/4 of a wavelength long, combined with matching components L2 and C5. The antenna presents a high and largely capacitive impedance. CS and the antenna capacitance resonate with L2 to form a series resonant circuit, thus presenting a low RF impedance to the module, and producing a much higher RF voltage at the antenna than across the module. This allows more RF current to be driven into the high impedance of the antenna, and thus causes it to radiate more RF energy.

Both the arrangements of Figures 3 and 4 are suitable for use with the arrangements of Figures 1 and 2 and are optimised for a carrier frequency of about 418 MHz.

It should be noted that in principle, either the drive current or the drive voltage to the transmitter module 3 could be modulated in either analogue or digital fashion to generate a modulated RF output. The antenna 4 could in principle be remote from module 3 and could be coupled thereto by a transmission line. Also the encoding circuit 2 could be remote from module 3 and could be connected thereto by a suitable (e.g twisted pair) cable.

Referring to Figure 5, an LC resonant circuit comprising series-connected capacitors C1,C2 and C3 and inductor L1 (which is suitably an etched track in, e.g. a miniature printed circuit board) is connected to a driver transistor Q1 in a Colpitts configuration, positive feedback to the emitter of Q1 being taken from the junction of C1 and C2. It should be noted that series inducator L1 and parallel capacitor C1 filter out unwanted harmonics of the carrier frequency.

The preferred component types and volumes are as follows: For operation at 418 MHz, L1 is preferably 20 to 40 nH, and the series combination of C1, C2 and C3, plus any strays provide a capacitance that resonates with L1 at approximately 418 MHz. For operation at other frequencies, these values are scaled approximately in inverse proportion to the frequency. The ratio of capacitors C1,C2,C3 is preferably 10:1:5. Q1 is preferably a bipolar transistor with adequate radio frequency performance BFS17 is a suitable type for us at 418 MHz. The SAW resonator or other frequency-stabilising means behaves as a very sharply tuned series resonant circuit, which presents a low impedance at the base of Q1 only at resonance, so allowing positive feedback to occur only at the resonant frequency of said resonator. Where frequency stabilisation is not required, the resonator SAW1 may be replaced with a capacitor which presents a low impedance at the base of Q1. A bias resistor R1 determines the DC operating current of Q1 at a given applied voltage applied across the module.

The above resonant circuit has a resonant frequency of about 418 MHz and is coupled to a further, frequency-controlling, resonant circuit comprising SAW resonator SAW1.

resistor R2 and the base emitter junction of Q1 which is also tuned to a frequency of 418 MHz A conventional bias resistor R1 is connected across the base and collector of Q1.

In a less prefened three-terminal variant (shown in phantom), a third, RF output terminal 12 is provided and is connected in series with an inductor L3 coupled to L1 and connected to negative power supply terminal 11.

The above circuitry preferably utilises surface-mounted components on a miniature printed circuit board, or may even comprise a single integrated circuit on a silicon chip, and is preferably encapsulated in encapsulation 9, leaving two (or in the less preferred embodiment, three) leads 10,11 (and 12) which are the sole connections to the module 3.

In the variant shown in Figure 7, an inductor L3 (shown in phantom) connects between C1 and the positive terminal 10, and is mutually coupled to L1, such that the RF voltage across L3 is approximately 1/10 of the voltage across L1. In a further variant, an inductor L4 (shown in phantom) connects in series with the negative terminal, and is coupled as previously described. L3 or L4 can be omitted and the inductors are all suitably formed from a PCB track. C1 is a decoupling capacitor, suitably 220pF for 418 MHz operation This embodiment has the advantage over the circuit of Figure 5 of coupling more RF power into the antenna (not shown) which can be connected as shown in any of Figures 1 to4.

In the circuit of Figure 8, the terminal 10 is connected to a tapping point part way up inductor L1. C1 is a coupling capacitor with a very low RF impedance. The RF voltage across the small section of L1 between the terminal 10 and Ci is approximately 1/10 of the voltage across L1 as a whole. As in the variant of Figure 7, this arrangement has the advantage over the circuit of Figure 5 that more RF power is coupled into the antenna (not shown) for a given amount of RF leakage back into the modulation circuit (not shown).

In the circuit of Figure 9, the inductor L1 is replaced by two inductors L1 and L5. The ratio of the inductances of L1:L5 is approximately 10:1 and similarly improves the transfer of RF power into the antenna.

The output impedance between terminals 10 and 11 at the canier frequency is preferably 10 ohms or less in all the embodiments.

It should be noted that the invention is not limited to transmitter arrangements coupled to antennae, but is for example in principle applicable to other communications systems operating at radio frequencies but not employing broadcast radio signals.

Claims (1)

1. A radio frequency transmitter comprising an RF oscillator circuit, two input terminals coupled to the oscillator circuit, and an antenna means coupled to an output of the oscillator circuit, one of said input terminals functioning as both a modulation input terminal and a power supply terminal.

2. A radio frequency transmitter according to Claim 1, wherein the antenna means is coupled to said one input terminal.

3. A radio frequency transmitter according to Claim 1 or Claim 2, wherein said oscillator circuit comprises an LC resonant circuit and amplifying means, the LC resonant circuit comprising a capacitor connected directly across said input terminals.

4. A radio frequency transmitter according to Claim 1 or Claim 2, wherein said oscillator circuit comprises an LC resonant circuit inductively coupled to an inductor in series with the antenna.

5. A radio frequency transmitter according to Claim 3, wherein said LC resonant circuit includes an inductor connected to one terminal of said capacitor and to the output of the amplifying means, said inductor and capacitor being arranged to filter the RF output to the antenna.

6. A radio frequency transmitter according to any preceding Claim, wherein the oscillator circuit comprises a frequency-stabilising resonator device.

7. A radio frequency transmitter according to Claim 6, wherein said resonator device is a SAW resonator.

8. A radio frequency transmitter according to Claim 6 or Claim 7, wherein the oscillator circuit comprises a transistor arranged to receive a positive feedback signal from an LC resonant circuit via said frequency-stabilising resonator.

8. A radio frequency transmitter according to Claim 3 or Claim 8, wherein positive feedback is taken from the junction of two capacitors in said LC resonant circuit.

10. A radio frequency transmitter arrangement comprising a radio transmitter module a modulator circuit arranged to modulate a DC power supply input connected across two input terminals of the radio transmitter module whereby one of the said input terminals functions as both a modulation input terminal and a power supply terminal. and an RF output coupled to the radio transmitter module.

í1. A radio frequency transmitter arrangement according to Claim 10, wherein said RF output is coupled to said one input terminal.

12. A radio frequency transmitter arrangement according to Claim 11, wherein an RF impedance is connected between an output of said modulator circuit and said one termina' 13. A radio frequency transmitter arrangement according to Claim 12, wherein said RF impedance is a resistor.

14. A radio frequency transmitter arrangement according to Claim 12 or Claim 13, wherein a capacitor is connected across said two input terminals.

15. A radio frequency transmitter arrangement according to any of Claims 10 to 14, wherein said modulator circuit is arranged to gate power to the radio transmitter module to generate a pulsed RF output.

16. A radio frequency transmitter according to any of Claims 10 to 15, wherein the radio transmitter module comprises an RF oscillator circuit as defined in any of Claims 3 to 9.

17. A radio frequency transmitter as claimed in any of Claims 1 to 9, which is in the form of a twoterminal package or a three-terminal package wherein the third terminal if present is an RF output terminal.

18. A radio frequency transmitter package comprising an RF oscillator circuit, two input terminals coupled to the oscillator circuit, an RF output terminal (which is optionally also one of said input terminals) coupled to an output of the oscillator circuit, one of said input terminals functioning as both a data input terminal and a power supply terminal and the oscillator circuit being arranged to generate a modulated RF oscillation in response to a modulated energising signal applied across said input terminals.

19. A radio frequency transmitter substantially as described hereinabove with reference to Figures 1 to 9 of the accompanying drawings.

20. A radio frequency transmitter arrangement substantially as described hereinabove with reference to Figures 1 to 9 of the accompanying drawings.

21. A radio frequency transmitter package substantially as described hereinbefore with reference to Figures 5 and 8, 6 and 7, or 6 and 8 or 6 and 9 of the accompanying drawings.