GB2292278A - RF transmitter with means for suppressing harmonics - Google Patents

Abstract

Apparatus for the transmission of RF signals includes a filter circuit for suppressing a harmonic frequency of the fundamental frequency being transmitted. Where the signal is input at an input port and output at an output port, the filter circuit comprises a first parallel resonant circuit connected to the input port and a series resonant circuit connected across the input port, and a second parallel resonant circuit connected to the output port. The parallel resonant circuits are tuned to maximise power at a fundamental frequency and the series resonant circuit is tuned to minimize power at the harmonic frequency, as measured at the output port. Power levels at the second harmonic frequency of 50 dBm below that at the fundamental frequency can be attained. <IMAGE>

Description

RF TRANSMITTER FIELD OF INVENTION This invention relates to apparatus for the transmission of RF signals which includes circuit means for the suppression of unwanted frequencies. It is particularly described in regard to relatively inexpensive apparatus for the remote wireless control of devices, although it is not restricted thereto.

BACKGROUND OF INVENTION The remote control of devices using radio frequency wireless signals is well known. The bands allocated for transmission in connection with devices of the type contemplated herein are often in the UHF range, and increasingly stringent standards are being applied by regulatory authorities to limit interference. In accordance with one standard, the power output at the second harmonic of the fundamental frequency of the transmission is required to be 48dBm below the power output at the fundamental frequency.

It is possible to meet these standards by using multiple pole low-pass filter networks. However, these networks are complex, and they usually require a large number of sequential, iterative revisions before attaining a reasonably satisfactory operation. The complexity is compounded, given that the network must also provide an impedance match between the input and output ports, the values for which impedances are often unknown. Additionally, the multifflole networks are relatively inefficient in that it is difficult to suppress the second harmonic without adversely affecting the power output at the fundamental frequency.

The transmitters for the wireless control of devices are usually hand-held, compact, economically priced and mass produced. Where such transmitters are marketed in different countries throughout the world, they may be required to operate in different frequency bands. For example, in the United States of America and Canada, frequencies of about 433.92 MHz are allocated for the control of devices of the type particularly contemplated herein, whereas in the U.K., frequencies of about 418 MHz are allocated. It is desirable that any circuit means for suppressing the transmission of unwanted frequencies be usable without modification over a relatively wide band of fundamental frequencies. It is also desirable that the circuit means not necessitate the tuning of individual units, and that any measures taken to obviate the individual tuning of units do not increase manufacturing costs.

In a hand held wireless transmitter, it is preferred that the antenna be a whip type, so as to provide a moderately wide transmission band. Where the source of RF power for radiation by the antenna is double sided, it is necessary to convert this to single sided for input at the whip antenna. It is desirable that the circuit means provide such conversion without additional expense or bulk in a low cost, hand held unit.

It is an object of this Invention to provide in apparatus for the transmission of RF power a circuit means for the suppression of transmitted power at unwanted frequencies.

It is another object of this invention to provide such circuit means that is easily engineered.

It is yet another object of this invention to provide such circuit means that is suited for use where the apparatus is economically priced and hand held.

It is still another object of this invention to provide in such apparatus circuit means that is effective over a moderately broad band of fundamental frequencies so as to permit identically constructed means to be used in apparatus transmitting at different frequencies.

It is a further object of this invention to provide such circuit means that is suited for mass produced units which does not require tuning of individual units.

It is a still further object of this invention to provide such circuit means for low cost, compact hand held wireless transmitter units where the source of RF power is double sided.

SUMMARY OF THE INVENTION In accordance with a broad aspect of the invention, in an RF system comprising an RF source having an input port and a device into which RF power from the input port is to be output at an output port, there is provided a first parallel resonant circuit means connected to the input port and tuned to maximize power at the output port, and a series resonant circuit connected across the input port tuned to suppress a frequency at the output port that is a harmonic of the fundamental frequency. The series resonant circuit includes one of a pair of coupling elements; another of this pair of coupling elements connects the output port to another parallel resonant circuit tuned to maximize RF power at the fundamental frequency at the output port.

Where the source is double sided so as to provide a differential RF signal, the first parallel resonant circuit means comprises a parallel resonant circuit connected to each side of the input port, each of which is tuned to maximize the RF power available at the output port at the fundamental frequency. The coupling elements, together with the parallel resonant circuit connected to the output port, serve to provide a single ended input into the device, which is suited for use where the device is a whip antenna.

The output port device in the preferred embodiment comprises a whip antenna, and the parallel resonant circuits serve in part to match the impedance thereof with the impedance of the input port, and concomitantly as a band pass filter to generally reduce the band width of the signal reaching the antenna, thereby assisting in suppressing all frequencies other than the fundamental frequency.

The coupling element of the series resonant circuit together with its paired coupling element serves to provide a DC block between the input port and the output port. This pair of elements may be formed as overlapping, printed traces on opposed surfaces of the printed circuit board upon which the device as a whole is constructed, so as to be spaced apart by the thickness of the circuit board. Where the output port device is a whip antenna, this is conveniently formed by printing on the circuit board so as to connect directly to the coupling element.

The tuning of the system design is easily accomplished, particularly in comparison to the multiple pole filter network of the prior art.

Initial values for each of the inductor-capacitor pairs will suitably be selected so as to be reasonably close to the theoretical values and adjusted to maximize RF power output at the fundamental frequency and to minimize power output at the harmonic frequency to be suppressed. Power levels of the second harmonic frequency of 50dBm less than the power level at the fundamental frequency have been attained in the foregoing manner.

It should be understood that the terms "maximize" and "minimize" as used herein do not require that the resonant circuits be tuned so as to attain any absolute maximum or minimum values, a primary object of the tuning being to ensure that the power output at the second harmonic frequency, or such other harmonic frequency as is selected, has the requisite differential from the power output at the fundamental frequency.

It is found that the component values of the inductor-capacitor pairs are not highly critical, at least under circumstances that do not necessitate tuning of the filter circuit to absolute maximum or minimum values, which permits the use of medium tolerance pre-formed components and which obviates the tuning of individual transmitter units.

Where the RF system is intended for use over a moderate range of fundamental frequencies, eg. 410440 MHz, the system may conveniently be designed to pass a fundamental frequency in about mid-range and suppress the desired harmonic; if the fundamental frequency at this tuned system is changed to another value within this range, it is found that the suppression of the harmonic remains at satisfactory levels permitting the system to be used in different bands within the range without modification of the resonant circuits.

The foregoing objects and aspects of the invention, together with other objects, aspects and advantages thereof will be more apparent from a consideration of the following description of the preferred embodiment thereof taken in conjunction with the drawings annexed hereto.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. 1 is a schematic representation of an RF generating source and a filter network of the invention connecting the source to an antenna to form an RF transmitter; FIG. 2 is a positive image conductor pattern of one surface of the printed circuit board used in a compact hand-held implementation of the transmitter of Fig.

1; FIG. 3 is a negative image of the conductor pattern of the opposed surface of the printed circuit board of Fig.

2; FIG. 4 is a side edge view of the circuit board of FIG. 2; and FIG. 5 is a Spice simulator frequency response curve for a simulated filter circuit of the invention tuned to suppress the second harmonic frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings in detail, an RF transmitter for operation at a fundamental frequency of approximately 418 MHz comprises an RF Source 20, which conveniently comprises Silicon Chip 22 commercially available from Motorola, No. MC 13176. Operating details in regard to chip 22 are set out in MOTOROLA Semiconductor Technical Data Release MC 13175113176, the contents of which are incorporated herein by reference thereto as fully as if set forth at length herein. Typically chip 22 is operated with a supply voltage Vcc In the range of about 3 to about 5 volts. RF Source 20 further comprises a crystal oscillator 24 operating at the fundamental frequency divided by 32, to provide a reference signal which is connected externally to pins 8 and 9 of chip 22. A tank oscillator 26 connects externally to pins 1 and 4 of the chip 22.Chip 22 includes an interior current controlled oscillator (CCO) forming part of an internal phase lock loop circuit . The output from the COO is amplitude modulated by control signals connected to pin 16, and a differential RF signal is output at pins 13 and 14 of the chip, which pins may otherwise be identified as a double sided input port 30. RF source 20 includes a DC supply ground 28 to which VEE connects, and an RF system ground 34 to which Vcc connects.

A parallel resonant circuit comprising an inductor capacitor pair Ll,C1 and a similar resonant circuit L2,C2 connect to respective side of outlet port of RF ground 29. A series resonant circuit formed by inductor L3, capacitor C3 and a first RF coupling element 32 connects across input port 30. A second RF coupling element 34 is arranged in general opposition to first element 32. Second element 34 is connected at one end thereof to a third parallel resonant circuit L4,C4 and at the opposed end to antenna 40.

Where the fundamental frequency at which generator 20 operates is in the UHF range, parallel resonant circuit inductors L1, L2 and L4 will typically have an inductance having a value of the order of l0nH, and are conveniently formed, at least in a repetitively manufactured device, as printed microstrip inductors 42 on a double surface circuit board 44 on which chip 22 and other components of the device are mounted.

The coupling elements 32 and 34 are also formed as printed conductors on circuit board 44. Each of coupling elements 32,34 is formed as an open ended loop, the loops being respectively disposed on opposed surfaces of circuit board 44 in overiying relationship, spaced apart by the thickness of the board, of approximately imam. Antenna 40 is conveniently formed as a printed conductor on circuit board 44, and is formed in end to end relationship with second coupling element 34.

Antenna 40 is nominally a 114 wavelength antenna, but is somewhat truncated so as provide a more compact hand held unit.

Accordingly, for wavelength matching purposes, antenna 40 Is terminated with a third series resonant circuit formed In part by conductor L5 and an large open capacitor plate C5, which is formed by printing on a small circuit board 44A mounted In upstanding relationship to circuit board 44 across the end thereof.

The second capacitive plate of the second series resonant circuit formed by the DC supply ground 28.

The values of the inductor-capacitor pairs forming parallel resonant circuits L1,O1, L2,C2 and L4,C4 are, as a first approximation, selected, to be close to the theoretical values for providing a band-pass filter tuned to the fundamental frequency. Similarly, values selected for the series resonant circuit L3,C3 will be such as to present a low impedance to the particular frequency desired to be suppressed.The selected values are varied with reference to the power output at antenna 40 so as to maximize the output at the desired fundamental frequency and minimize the output at the frequency to be suppressed, and in practice the values of the components when the resonant units are tuned in this manner may vary appreciably for the theoretical, particularly where the impedance of outlet port 30 is somewhat different to the impedance of antenna 40, or of any other device which may form a part of an RF system.

An RF radio transmitter constructed and tuned in the foregoing manner for operation at a fundamental frequency of 418 MHz and to suppress the second harmonic of 836 MHz is found to have an output at the second harmonic of 54.5 dBm below the output at the fundamental frequency. When the fundamental frequency Is changed to 433.92 MHz, without varying the value of the foregoing parallel and series resonant coupling circuit components, the output at the second harmonic frequency remains substantially below that at the fundamental frequency.

The coupling elements 32,34 provide a DC isolation between the antenna 40 and the RF source 20. The particular loop form of coupling elements 32,34 was selected as providing a compact arrangement on circuit board 42, and other physical forms of these elements are possible.

The operation of the resonant coupling circuit described is simulated closely by a Spice simulator, considering the coupling elements 32,34 to be merely capacitive in their action, with the filters tuned to pass a fundamental frequency at 418 MHz and to suppress the second harmonic thereof. With reference to FIG. 5, such filter system shows a notch with a minimum value at the second harmonic frequency.

It will be apparent that many changes may be made to the illustrative embodiment while falling within the scope of the invention, and it is intended that all such changes be covered by the claims appended hereto.

Claims (21)

1. In an RF system comprising a source of RF power having an input port and a device into which said RF power is input at an output port a first parallel resonant circuit means connected to said input port tuned to maximize the RF power at said fundamental frequency at said output port; a series resonant circuit connected across said input port tuned to suppress a frequency at said output port that is a harmonic of said fundamental frequency; said series resonant circuit including one of a pair of coupling elements connected therein; the other of said pair of coupling elements connecting said output port to a parallel resonant circuit tuned to maximize the RF power at said output port at said fundamental frequency.

2. An RF system as defined in Claim 1 wherein said input port is double sided to provide a differential RF signal, and wherein said first parallel resonant circuit means comprises a parallel resonant circuit connected to each side of said input port.

3. An RF system as defined in Claim 2 wherein said coupling elements are printed elements on a circuit board.

4. An RF system as defined in Claim 3 wherein said coupling elements are respectively disposed on opposed surfaces of said circuit board.

5. An RF system as defined in Claim 4 wherein said coupling elements are arcuate and in overlaying relationship.

6. An RF system as defined in Claim 3 wherein each said parallel resonant circuit includes a printed microstrip inductor on said circuit board.

7. An RF system as defined in Claim 1 wherein said harmonic frequency is the second harmonic.

8. An RF system as defined in Claim 1 wherein said output port is connected to an antenna.

9. An RF system as defined in Claim 8 wherein said antenna is 1/4 wavelength whip antenna.

10. An RF system as defined in Claim 9 wherein said antenna is printed on a circuit board.

11. An RF system as defined in Claim 10 wherein said antenna is truncated and terminated with a second series resonant circuit tuned to maximize output at said antenna.

12. An RF system as defined in Claim 11 wherein said second series resonant circuit includes a printed circuit board capacitor plate disposed at right angles to said circuit board.

13. An RF transmitter adapted to radiate RF energy from an antenna at a fundamental frequency and suppress a harmonic of said frequency comprising: source means for generating a differential RF signal generally centered on said fundamental frequency at a double sided port; first and second parallel resonant circuits respectively connected to each side of said port tuned to maximize output at said fundamental frequency at said antenna; a series resonant circuit means connected across said port tuned to minimize the output at said harmonic frequency at said antenna; a first coupling element serially connected in said series resonant circuit means; a second coupling element associated with said first coupling element;; third parallel resonant circuit means connected to said second coupling element and said antenna tuned to maximize the output at said fundamental frequency at said antenna; said transmitter providing a power output at said antenna when measured at said harmonic frequency which has a value not greater than 50 dBw lower than the output measured at said fundamental frequency.

14. A transmitter as defined in Claim 13 wherein said second coupling element serves to connect said third resonant circuit to said antenna.

15. A transmitter as defined in Claim 14 wherein said first and second coupling elements are formed as generally identical overlaying conducting paths respectively disposed on opposite surfaces of a printed circuit board.

16. A transmitter as defined in Claim 14 wherein said conducting paths are open circular loops.

17. A transmitter as defined in Claim 13 wherein said fundamental frequency is in the range of 410 MHz to 440 MHz.

18. A transmitter as defined in Claim 17 wherein said harmonic frequency is the second harmonic of said fundamental frequency.

19. A transmitter as defined in claim 17 wherein said RF signal is amplitude modulated.

20. A transmitter as defined in claim 17 wherein said source means and said antenna are contained in a housing for hand-held use.

21. A hand-held wireless transmitter substantially as described herein with reference to and as shown in the accompanying drawings.