US RE40620 E1
A monolithic AM transmitter is disclosed. An external antenna forms part of a resonance network so that the antenna resonance point is automatically tuned to the transmit frequency. This provides flexibility with no added cost to the transmitter. Additionally, components of the transmitter can be formed on a single monolithic integrated circuit.
1. A transmitter comprising:
a voltage-controlled oscillator having an operating frequency;
an antenna, the antenna forming part of a tuned circuit coupled to the voltage-controlled oscillator,
a differential structure of varactor diodes;
a varactor charge pump to provide a bias charge for varactor diodes in the differential structure of varactor diodes; and
a tuned circuit including the antenna and the differential structure of varactor diodes, wherein the differential structure of varactor diodes tunes the resonance point of the antenna to the frequency of the voltage-controlled oscillator.
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18. A transmitter having an automatic antenna tuning circuit, the transmitter comprising:
a transmit amplifier for being driven at a carrier frequency, the transmit amplifier for being connected to an antenna;
a variable capacitance, the variable capacitance and antenna forming at least part of a tuned circuit having a resonant frequency, wherein varying the variable capacitance tunes the resonance frequency of the tuned circuit; and
a tuning circuit connected to control the variable capacitance in response to a feedback control signal, the feedback control signal indicating whether the resonance frequency of the tuned circuit is matched to the carrier frequency, the tuning circuit for varying the variable capacitance based on the feedback control signal to cause the resonance frequency of the tuned circuit to match the carrier frequency,
at least the amplifier, variable capacitance, and tuning circuit being formed on a single integrated circuit.
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an oscillator providing a drive signal to the amplifier at the carrier frequency;
a phase difference circuit receiving a first signal corresponding to a phase of a signal generated by the amplifier and receiving a second signal corresponding to a phase of a signal generated by the oscillator, the phase difference circuit generating the feedback control signal, the feedback control signal being related to a difference in phase between the first signal and the second signal.
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37. A method for performing automatic tuning in a transmitter comprising:
driving a transmit amplifier at a carrier frequency, the amplifier being connected to an antenna;
varying a variable capacitance, the variable capacitance and antenna forming at least part of a tuned circuit having a resonant frequency, wherein varying the variable capacitance tunes the resonance frequency of the tuned circuit;
applying a feedback control signal to a tuning circuit connected to control the variable capacitance, the feedback control signal indicating whether the resonance frequency of the tuned circuit is matched to the carrier frequency; and
varying the variable capacitance by the tuning circuit based on the feedback control signal to cause the resonance frequency of the tuned circuit to match the carrier frequency,
at least the amplifier, variable capacitance, and tuning circuit being formed on a same integrated circuit.
38. The method of
providing a drive signal to the amplifier by an oscillator at the carrier frequency;
receiving by a phase difference circuit a first signal corresponding to a phase of a signal generated by the amplifier and receiving by the phase difference circuit a second signal corresponding to a phase of a signal generated by the oscillator; and
generating by the phase difference circuit the feedback control signal, the feedback control signal being related to a difference in phase between the first signal and the second signal.
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47. A transmitter comprising:
a voltage-controlled oscillator having an operating frequency;
an antenna, the antenna forming part of a tuned circuit coupled to the voltage-controlled oscillator,
a variable capacitance forming part of the tuned circuit; and
a tuning circuit connected to control the variable capacitance in response to a feedback control signal, the feedback control signal indicating whether the resonance frequency of the tuned circuit is matched to the carrier frequency, the tuning circuit varying the variable capacitance based on the feedback control signal to cause the resonance frequency of the tuned circuit to match the carrier frequency,
at least the oscillator, variable capacitance, and tuning circuit being formed on a same integrated circuit.
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This application is a continuation-in-part of application Ser. No. 09/074,997, filed on May 8, 1998, issued as U.S. Pat. No. 6,253,068, entitled “Fully Integrated All-CMOS AM Transmitter with Automatic Antenna Tuning,” which is based on the provisional application Ser. No. 60/046,128, filed May 9, 1997, entitled “Fully Integrated All-CMOS AM Transmitter With Automatic Antenna Tuning”, by J. Scott Elder, Joseph T. Yestrebsky, and Mohammed D. Islam.
This invention relates to transmitters and, in particular, to an integrated transmitter with automatic antenna tuning.
In the figures, elements having similar functionality are identically labeled.
In the embodiment of
Special Charge Pump circuitry 7b can be included within the embodiment to allow for applications which use a wide range of supply voltages. This makes the embodiment shown in
Reference Oscillator 1 generates a precise timing waveform based on a timing element or timing signal applied externally at the reference input pad 30 of the embodiment shown in FIG. 1a.
Phase Detector 2, Loop Filter 3, Filter Tune 3a, Differential Oscillator 5, Prescaler 5a and divide-by M circuit 6 collectively form a phase-locked loop (PLL) where the frequency of Differential Oscillator 5 is locked to that of Reference Oscillator 1.
Filter Tune 3a can itself be a phase-locked loop tuned to the Reference Oscillator 1. Filter Tune 3a generates a bias current Ibias for Loop Filter 3. This approach stabilizes the performance characteristics of Loop Filter 3 against semiconductor process variation during chip production and temperature variations during operation of the chip.
Differential Oscillator 5 serves as the source of the transmit frequency. It is important to note that oscillator 5 is within the phase-locked loop of phase detector 2, loop filter 3, prescaler 5a, and divide-by M circuit 6 and includes a voltage controlled oscillator (VCO). Oscillator tuning can be accomplished by means of a differential structure of diffusion type p-n diodes(varactor diodes) within differential oscillator 5. Some embodiments may alternatively include tuning diodes external to the integrated circuit and connected in a differential manner. Some embodiments may alternatively include a phase detector and up/down converter coupled to an array of tuning capacitors.
Differential oscillator 5 provides a differential output into a (differential) antenna L1, L2. The antenna formed by L1 and L2 is coarsely tuned by capacitor C1, and fine tuned to the frequency of Reference oscillator 1 via the tuning structure of differential oscillator 5, L1, L2, and C1 can be externally applied to the integrated circuit shown in
Addition of external resistor Rbias coupled to pad 33 in
In the embodiment shown in
In some embodiments, a charge pump 7b provides voltage multiplication, which allows the integrated circuit to operate with supply voltages below 3V. An externally applied pumping capacitor CFLY is supplied across pads 36 and 37. This supply voltage generation technique can either be open loop controlled or closed loop controlled. In a closed loop application, the operation of charge pump 7b ceases when the external supply voltage is sufficient for proper operation of the transmitter. In an open loop application, the voltage settles to an integer multiple of the input voltage source. The embodiment of
Track and hold circuit 4 holds the output voltage of loop filter 3 constant during transmit ‘spaces’, when differential oscillator 5 is disabled. This function requires an externally applied hold capacitor CH on pad 38. An alternative embodiment includes a hold capacitance CH integrated on the IC Chip (e.g., a DAC approach).
The operation of track and hold circuit 4 and differential oscillator 5 is controlled by the data input signal at pad 39. During “space” signals, differential oscillator 5 is off (i.e., not transmitting) and track and hold 4 holds the output voltage of loop filter 3 constant. During “mark” signals, track and hold 4 outputs the signal from loop filter 3 and differential oscillator 5 is on (i.e., transmitting). Some embodiments include a data encoder circuit 40 that inputs a data signal from pad 39 and encodes it for transmission. Transmission is accomplished by controlling the output power of differential oscillator 5. The output signal to antenna L1, L2 can be modified for any data transmission scheme.
A bandgap reference circuit 8 provides temperature and supply voltage compensated bias voltages and currents for critical nodes within the invention.
The embodiment of
Numerous physical embodiments of the invention exist, depending on the intended applications and system requirements. An embodiment of an integrated circuit is illustrated in
Having said this,
The embodiments of the invention shown in
Two Charge Pumps, pumps 7a and 7b, and appropriate control functions are included in the transmitter of
Reference Oscillator 1 develops the precision timing signal which is used by the phase-locked loop to set the transmit frequency. The phase-locked loop includes phase detector 2, loop filter 3, track and hold 4, differential oscillator 5, prescaler 5a, and divide-by-M circuit 6. In most embodiments, Reference Oscillator 1 is of the Colpitts variety, and is connected to a timing device external to the integrated circuit (IC). Typical timing devices include ceramic resonators, crystals, or (tuned) inductor-capacitor tank circuits. Phase lag capacitors generally associated with ceramic resonators are integrated onto the IC to lower cost.
In most embodiments, as shown in
Finally, reference oscillator 1 supports the application of an external (precision) timing signal rather than a timing element. The timing signal is applied at the same terminal as would receive an external timing element signal.
Phase detector 2 provides a measure of the frequency difference between the output signal from Reference oscillator 1 and the output signal from differential oscillator 5, frequency divided by scale factor M within Div_M circuit 6 and prescaled in prescaler 5a. Phase detector 2 needs only to discriminate frequency differences because phase error in the phase-locked loop (PLL) does not affect performance of the transmitter.
In most embodiments, phase detector 2 is a digital phase-frequency detector. Digital phase-frequency detectors are advantageous for at least two reasons: Firstly, the circuit output signal is differential in nature; secondly, when the phase-locked loop finally acquires frequency and phase lock, phase-frequency detector 2 outputs a series of very narrow pulses in the time domain, which is much easier to filter with an integrated loop filter than a phase detector that exhibits quadrature phase lock.
The output waveform of phase detector 2 contains both a dc and an ac term; the dc term represents the frequency error between the output signal of reference oscillator 1 and the output signal of differential oscillator 5 prescaled and divided by scale factor M. Only the dc term is of value; the ac term is removed by loop filter 3.
In many embodiments, loop filter 3 is a gm-c type filter, a gm-c type filter is advantageous for at least three reasons. Firstly, the variance in gm-c loop characteristics is almost completely related to the gm variances since integrated capacitors are well behaved. Secondly, a fairly high order filter may be constructed using only modest amounts of integrated capacitance, which reduces the cost of producing the integrated circuit chip. Thirdly, such filters are easily ‘tuned’ to stabilize their characteristics against semiconductor process and temperature variations. Such tuning is provided from a tuning phase-locked loop, filter tune 3a, ‘slaved’ from reference oscillator 1.
An embodiment of filter 3a is shown in FIG. 3b. Filter Tune 3a is itself a phase-locked loop, referenced to the frequency of the output signal from reference oscillator 1. The phased-locked loop of filter tune 3a may include a phase detector 300, current controlled oscillator 301, prescaler 302, and divided-by M circuit 303. The phase-locked loop of filter tune 3a adjusts the bias current of the current-controlled oscillator of filter tune 3a to force the current-controlled oscillator frequency to equal the frequency of the output signal from reference oscillator 1. This may be accomplished directly or indirectly through the use of frequency prescalers 302 or frequency dividers 303. The resultant bias current is mirrored in current mirror 304 to provide the bias current for loop filter 3.
During transmission of ‘spaces’, differential oscillator 5 may be disabled. This ‘breaks’ the phase-locked loop that includes phase detector 2 and differential oscillator 5. Track and hold 4 is used to hold the state of the phase-locked loop to that just prior to disabling differential oscillator 5, signifying a ‘space’ transmission. This is desirable for at least two reasons: (1) so that the phase-locked loop does not have to require frequency lock for each ‘mark’ transmission; and (2) to assure that differential oscillator 5 is not ‘chipped’ or swept in frequency as the Phase-locked loop settles out during a ‘mark’0 transmission.
The data signal input to track and hold 4, which is the output signal from data encoder 40, controls track vs. hold operation. During ‘space’ transmissions, track and hold 4 is placed in the hold mode.
Some applications may be better served (e.g., be of lower cost for the transmitters) by the use of an external hold capacitor (CH) for track and hold 4, as shown in FIG. 1a. However, the effective hold capacitance can be integrated onto the IC chip, with additional cost, and perhaps a modest compromise in the ‘droop’ of the hold voltage. For example, to support very low data rates, an external hold capacitor into the integrated circuit is preferred, but as the operational data rate increases, the impact of integrating this capacitor into the integrated circuit is less deleterious to cost and performance. So, an alternative embodiment is the integration of CH, which reduces the IC pin count of the resulting integrated circuit by one.
Differential oscillator 5 includes a voltage controlled oscillator (VCO). The voltage controlled oscillator can be based on two cross-coupled Field Effect Transistors (FET's) where antenna L1 and L2 serves as the load impedance for the FET's. The cross-coupled FET structure provides the feedback for oscillatory behavior. In the embodiment shown in
The differential oscillator frequency is adjustable via a dc control signal VAFC, which is the filtered output signal from phase detector 2. Two reverse-biased p-n diffused-junctions (varactors) are connected differentially across the cross-coupled FET's of differential circuit 5. As the dc level of VAFC varies, the capacitance of the reverse-biased p-n junctions varies inversely. This capacitance forms part of the resonant tank circuit which sets the frequency of oscillation. Thus, a differential varactor diode approach can be the method used to modify the resonant point of antenna L1, L2. An alternative varactor approach is to use the CV characteristics available in an MOS diode.
A bias current for differential oscillator 5 is set via addition of resistor Rbias, shown externally to the embodiment of
An alternative to providing Rbias externally is to integrate this onto the chip as a fixed, unmodifiable current. This will result in a fixed, unmodifiable transmit power level. If the on-chip Rbias resistor is replaced by a digital to analog converter, the transmit power level can be varied by communicating to the integrated circuit (via separate input pins) a data pattern that sets the desired transmit power level.
Input data is used to electronically steer the bias current to differential oscillator 5. During ‘mark’ transmissions, the bias current is connected to differential oscillator 5. Differential oscillator 5 then begins to oscillate, transmitting an RF carder during the ‘mark’ interval. Alternatively, during a ‘space’, the bias current is disconnected from differential oscillator 5, and so oscillation ceases, transmitting the equivalent of a ‘space’.
Special back-to-back trapped drain field effect transistor (FET) structures can be employed at critical high frequency nodes of differential oscillator 5 to minimize capacitance and improve bandwidth. The preferred embodiment is circular drain structures, although other structures (octagonal, hexagonal, etc.) are acceptable. This structure has important ramifications, since the result is a very small parasitic capacitance at the drain node of the FET and a larger capacitance at the source node of the FET. This ‘breaking-apart’ of the normally equal drain and source parasitic capacitances actually further improves the gain-bandwidth (GBW) of differential oscillator 5 beyond just the improvement anticipated by a lowering of the drain capacitance. Said another way, the extra source capacitance actually improves GBW.
A differential oscillator such as differential oscillator 5 is employed for a good reason, namely that such a structure driving a differential antenna results in reduced harmonic distortion of the transmitted signal. This is important in some critical applications, but is less important in other applications. Thus an alternative embodiment is the use of a non-differential oscillator where application requirements permit. This reduces the cost of the transmitter, as well as its pin count. It is anticipated that a differential scheme will be necessary to meet most international regulatory requirements similar to the United States FCC and Part 15 of the Code of Federal Regulations.
Differential oscillator 5 operates at a very high frequency, beyond the operating range of conventional CMOS logic dividers. This frequency is divided down by a high speed circuit, prescaler 5a, before further division by conventional logic can take place. One embodiment of prescaler 5a is composed of a concentration of high speed cross-coupled FET divide-by-two circuits.
Prescaler 5a is biased using a current which is tuned to reference oscillator 1. This guarantees that the bandwidth of prescaler 5a is temperature and process independent.
Special back-to-back trapped drain field effect transistor (FET) structures can be employed at critical high frequency nodes of prescaler 5a to minimize capacitance and improve bandwidth.
Once the frequency of the output signal from differential oscillator 5 is prescaled to an appropriate lower frequency, conventional CMOS logic dividers are used to further divide the frequency down to approximately the operating frequency of reference oscillator 1. This is accomplished by division function Div_M 6 (divide by M circuit).
The reverse-biased p-n junction capacitors (i.e., varactors) included in differential oscillator 5 require a large bias potential for proper operation. On the other hand, the varactors require very little bias current. The need for extra circuitry to construct the varactor bias voltage depends on the magnitude of the bias supply VDD externally supplied to the chip on pad 35.
For applications where the external bias supply is in the vicinity of 12V, extra bias generation circuitry is not required. However, for lower external bias voltages, a circuit is required to synthesize a larger varactor bias voltage VBB from the smaller external bias voltage VDD. This function is preferentially accomplished by a voltage multiplying charge pump 7b, since the load current is very small. Such a circuit in CMOS is easily integrated onto the chip without the need for additional external components. Thus, no additional pins are required to provide this function on the integrated circuit chip.
Applications for this transmitter chip often demand operation with external bias supply voltages ranging from 12V down to below 3V. However, operation at bias voltages much below about 5V in CMOS is difficult without special processing (to lower FET threshold voltages), and the use of sub-micron or deep-sub-micron technology in chip manufacturing technologies. To meet this requirement using inexpensive technology, a voltage multiplying charge pump 7b is included. Charge pump 7b provides supply voltages of sufficient magnitude (generally greater than approximately 4V) from externally applied voltages lower than this value. The output voltage from charge pump 7b, VBB is then used instead of the external bias voltage VDD to power-up other elements on the integrated circuit chip.
Charge pump 7b is different from varactor charge pump 7a. Firstly, charge pump 7b must supply much larger output currents than varactor charge pump 7a. Secondly, varactor charge pump 7b is required for the transmitter to operate except in applications where the external supply voltage is approximately 12V. Charge pump 7b, however, is only required for applications where the external supply voltage is below about 5V. Also, varactor charge pump 7a requires no external pins on the IC package. Charge pump 7b requires several package pins, (e.g., pads 36 and 37 for a pumping or ‘flying’ capacitor Cfly, and pad 34 for a filtering capacitor CF).
Thus, there are several physical embodiments of integrated circuits. It is perhaps most cost effective to include varactor charge pump 7a on any physical embodiment since this costs no package pins and only modest CMOS die area. However the inclusion of charge pump 7b is expensive due both to die area and required package pins, and thus may not be included on embodiments intended for applications with greater than about 5V external supply voltages. One further alternative is not to include charge pump 7a on the transmitter chip, but to provide this function externally, for sub-5V supply applications.
A bandgap reference circuit 8 is included on the chip of FIG. 1a. The purpose of this function is to generate reference voltages which are temperature and supply voltage stable. Stable reference voltages are required to bias various critical nodes within the transmitter chip.
Differential oscillator 5, as shown in the embodiment of transmitter 10 shown in
Differential oscillator 5 includes voltage controlled oscillator 11. Voltage controlled oscillator 11 forms a phase-locked loop with prescaler 5a, divide-by M circuit 6, phase detector 2, and loop filter 3, as has been discussed above. Track and hold 4 holds the output signal from loop filter 3 constant during transmit spaces, when the differential oscillator 5 is disabled from transmitting. Some embodiments of the invention omit track and hold 4, especially those with more complex modulation schemes. Due to the phase-locked loop, the output signal from voltage controlled oscillator 11 is at a frequency that is set by the frequency of the input signal at the reference-in pad 30 of the integrated circuit chip.
As was previously discussed, and shown in
As was previously discussed, filter tune 3a includes a phase-locked loop tuned to the frequency of reference oscillator 1. Filter tune 3a generates a bias current for loop filter 3 and stabilizes the performance of loop filter 3 against variations due to semiconductor processing and operating temperatures.
Voltage controlled oscillator 11 generates two differential signals, I and Q, which are quadrature in phase. The in-phase signal I is output from differential oscillator 5 to prescalar 5a. The in-phase signal I is also input, through buffer 12, to power amplifier 13. Power amplifier 13 provides the output signal that drives antenna 20.
The quadrature signal Q is provided, through buffer 14, to antenna tune circuit 15. Antenna tune circuit 15 adjusts the resonant frequency of the LC circuit formed by antenna 20 and tuning array 16 to correspond with the frequency of the output signals from voltage controlled oscillator 11. Tuning array 16 can, for example, include differential varactors 16a and 16b. Differential varactors 16a and 16b can be continuously variable varactors or discrete varactors which include a switched capacitor array. Varactors 16a and 16b provide a desired amount of capacittance to tune antenna 20 to the operating frequency of VCO 11.
Power amplifier 13 provides a differential output signal into antenna 20. The LC circuit, including antenna 20, is tuned such that a resonance frequency of the LC circuit matches the frequency of the differential output signal from power amplifier 13, which is the same as the operating frequency of voltage controlled oscillator 11, by antenna tune circuit 15. Antenna tune circuit 15 compares the phase of the differential output signal to antenna 20 with the quadrature output signal Q from voltage controlled oscillator 11 and varies the effective capacitance of tuning array 16.
In embodiments where tuning array 16 includes varactors 16a and 16b, varactors 16a and 16b can be operated in either full depletion mode or in enhancement mode, thereby creating high quality (Q) on-chip MOS capacitors. As a result of the high quality MOS capacitors, it is possible to tune the LC circuit including external antenna 20 without degrading the overall quality of the antenna network.
Power controller 17 provides a negative feedback closed loop built around antenna 20 in order to control and modulate the transmit power. In the embodiment shown in
Power controller 17 is controlled by an AM voltage signal input on pad 42. Data is input at pad 39. The data signal, shown here as an amplitude shift keyed (ASK) data sequence, is input to power controller 17. Power controller 17 modulates the output power of the transmitter in response to the data stream. In some embodiments, power controller 17 may include a data encoder 40 (FIG. 1a). Embodiments of the invention can utilize any data transmission scheme (e.g., XDSL). Further, Power controller 17 may use frequency modulation instead of amplitude modulation by, for example, modulating the control signal to varactors 16 or the control signal to VCO 11.
VCO 11 is also coupled, through buffer 14, to antenna tune 15. Antenna tune 15 controls capacitor tuning array 16 so that the resonant frequency of the circuit containing loop antenna 20 can be controlled by switching the capacitors of capacitor array 16 into the circuit. Antenna tune 15 includes phase detector 50 and up/down counter 51. Phase detector 50, compares the signals from VCO 11 with the signal from loop antenna 20 and adjusts the capacitance of capacitor array 16 by causing up/down counter 51 to be adjusted.
A circuit diagram of phase detector 50 is shown in FIG. 5h.
In general, all of the above discussed components can be produced on a single integrated circuit with CMOS technology.
Alternative Embodiments and Enhancements
By implementing on a CMOS integrated Chip, it is economical to include encoder functions on the same IC since most of the applications for this type of radio transmitter include an encoder before the transmitter. The encoder can be either fixed or variable (i.e., programmable, like a microprocessor).
Other embodiments include the addition of a shutdown mode for low-power duty-cycling of the invention.
An encoder such as encoder 40 of
The encoder on/off data stream can either be fixed or variable. An example of a fixed data stream would be one permanently programmed in an encoder circuit with either switches or jumper connections. An example of a variable encoder data stream is one similar to that used in rolling code encoder systems. Rolling code schemes are generally found in high security applications where the user is trying to evade capture of a valid encoder data stream for later broadcast to a receiving decoder. Generally, these rolling code encoder schemes embody erasable and reprogrammable memory circuits. In that embodiments of this invention now allow full integration of at least one embodiment of a transmitter function, all of the encoder circuit schemes can be integrated on one single silicon integrated circuit at a very low cost.
In that embodiments of this invention are built on a CMOS process technology, a microprocessor or similar computing circuit element can be combined on the same silicon integrated circuit with the invention since modern low-cost microprocessors are almost always constructed on a similar process.
The embodiments of the invention have certain performance limits at the present time that are expected to change in the future. One such limit is the achievable upper operating frequency limit. One skilled in the art will readily recognize that embodiments disclosed here can continue to be scaled as CMOS technology is scaled to continue applying the invention at higher and higher operating frequencies.
The United States agency that regulates unlicensed radio or RF transmitters, the FCC, has allocated certain frequency bands and emission types for which this type of device is intended. There are several regulating bodies in other parts of the world which in a similar manner control these same issues. In some instances, the World's other regulatory agencies are more restrictive in their operating rules than is the FCC. Embodiments of this invention can be tailored for operation under all of the various regulatory agency provisions.
The primary application of this invention, but certainly not the only application, is for automotive keyless entry, garage door openers, home keyless entry, security systems, and remote door bell ringers. Obviously, there are numerous other applications that combine a simple transmitter with a digital data stream generation circuit.
This invention's uniqueness is further exemplified by the observation that a complete RF radio transmitting system can be built with no other external RF components except an antenna.
While the embodiments specifically discussed here illustrate an AM type transmitter, one skilled in the art will recognize that an FM transmitter with virtually the same circuit techniques described herein can be built. The modifications that are necessary to build FM transmission entail changing the phase-locked loop division ratio or reference frequency by the requisite frequency deviation for the system. This will force the control system to tune the antenna between any of a number of frequencies determined by the phase-locked loop feedback divisor or reference frequency values. In the limit, these values approach infinity. A simpler FM modification is also possible with the invention, namely varying the varactor diode bias directly. This is a more traditional technique of FM modulation, although not previously implemented on CMOS silicon at the frequencies made possible by this invention.
As a result of the economy of scale associated with CMOS process technology, an extension of this invention is to build several transmitters on the same silicon die. This will allow one to build low-cost multi-channel systems which are either AM, FM, or otherwise. A multi channel-channel system enhances the achievable data rate in much the same manner that increasing a digital systems data bus or address bus width.
The embodiments described in this disclosure are examples only and are not intended to be limiting. One skilled in the art will recognize obvious variations, each of which are intended to be within the scope and spirit of this invention. As such, the invention is limited only by the following claims.