Abstract:
A method for radiofrequency (RF) transmission of digital information includes generating an RF signal using a voltage-controlled oscillator (VCO), stabilizing the RF signal from the VCO by providing an error signal from a phase-locked loop (PLL) to an input of the VCO, and combining the digital information with the error signal of the PLL input to the VCO, thereby causing variations in frequency of the RF signal from the VCO that represent the digital information. Apparatus for RF transmission of digital information includes a VCO, the VCO arranged to generate an RF signal, a PLL, the frequency input of the PLL coupled to the RF signal output of the VCO, an encoder, the encoder arranged to convert the digital information into a form where it has a data rate faster than a response time of the PLL, and a coupler, the coupler coupling both the error signal output of the PLL and the encoded digital information to an input of the VCO.

Description:
This application is a continuation of U.S. Ser. No. 09/127,087, filed Jul. 31, 1998 (now abandoned) and claims the benefit of U.S. Provisional Application No. 60/061,940, filed Oct. 14, 1997 which are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     This invention relates to wireless digital communication. 
     With ownership of home computers increasing, many households now have more than one computer. Purchasing separate peripherals (printers, scanners, modems, removable storage media) for each computer can be expensive and wasteful, when often only one of each peripheral may be needed. Also, households typically provide only one phone line for use by computer modems, requiring users of multiple computers to take turns accessing the on-line services. A home-based local area network (LAN) would allow the computers to share these peripherals, but most inexpensive networking hardware currently available requires cabling to physically connect the computers. As multiple-home-computer users often keep their computers in separate rooms, use of cable-based networking requires running cables through the house walls. This, in addition to the complications of administering network routers or hubs, has discouraged home use of current networking technology. 
     Some LAN technologies do not depend on re-wiring homes. Some use existing wiring, such as power lines or phone lines. To date, these technologies can experience interference and security problems (for example, power lines are typically shared with other nearby homes). In addition, existing wired LANs typically do not permit mobile access to the network by portable laptop computers. Wireless LANs (WLANs) have also been pursued for networking personal computers. For example, infrared-based networks have been developed, particularly in work-place environments for communication with mobile personal digital assistants (PDAs). 
     Digital radio communication has also been explored for WLANs. In 1985, the FCC established regulations to allow unlicensed use of certain bands if spread-spectrum (SS) techniques are used. In SS transmission, the energy radiated during transmission is spread across a wide spectrum of frequencies (or “channels”) and is therefore less likely to cause substantial interference with other radio communications. FCC SS regulations allow greater transmission of power to be used without special licensing, increasing the attainable range of communications for unlicensed systems. 
     Two principal SS transmission techniques include direct-sequence (DS) and frequency-hopping (FH). In DSSS, spreading is achieved through multiplication of the data by a binary pseudo-random sequence whose chipping rate is many times the data rate. In FHSS, the carrier frequency remains at a given channel for a duration of data transmission, and then “hops” to a new channel elsewhere within the spreading bandwidth. 
     DSSS allows for coherent demodulation, in which the receiver exploits knowledge of the carrier&#39;s phase reference to detect the encoded signals. With FHSS, however, phase coherence is difficult to maintain between hops. Non-coherent demodulation results in the advantage of reduced complexity, but at a cost of an increased probability of error. FHSS typically enables high data rates to be achieved without requiring the high-speed logic that an equivalent DSSS system would require. Also, a FHSS system can employ frequency diversity, which combats multipath fading by transmitting data at multiple frequencies, increasing the likelihood that data will be transmitted and received uncorrupted. The data requirements of digital communications equipment is increasing as the typical size of data files becomes larger. Complex modulation schemes have been used to handle higher bandwidth requirements and more efficiently use the bandwidth allocated to RF devices. However, existing modulation schemes can be either very logic-intensive or require very accurate and expensive clock references. An example of the former type of scheme is differential quadrature phase shift keying (DQPSK), in which complex (IQ) demodulation is required to recover the encoded data. An example of the latter type of scheme is frequency shift keying (FSK), in which small changes in frequency are used to represent the encoded data. The change in frequency in FSK is usually very small and the receiving system may have difficulty determining whether a particular shift in frequency it detects represents real data or an offset between transmitter and receiver reference clocks. 
     SUMMARY 
     In general, in one aspect, the invention features a method for radiofrequency (RF) transmission of digital information including generating an RF signal using a voltage-controlled oscillator (VCO), stabilizing the RF signal from the VCO by providing an error signal from a phase-locked loop (PLL) to an input of the VCO, and combining the digital information with the error signal of the PLL input to the VCO, thereby causing variations in frequency of the RF signal from the VCO that represent the digital information. 
     Embodiments of the invention may include one or more of the following features. The RF signal can be broadcast. The error signal of the PLL can be provided to the VCO by a loop filter. The rate of change of the digital information can be faster than a response time of the PLL. The digital information can be encoded such that it has a duty cycle which is substantially constant over the response time of the PLL. The digital information can be encoded according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard. A channel frequency of the RF signal can be changed according to a series of channel frequencies, wherein the series of channel frequencies is determined by generating a series of at least a first and a second channel-select signal, each channel-select signal comprising a frequency-valid indicator and a frequency-specification indicator, sending each channel-select signal in turn to the PLL, and changing the tuning frequency of the PLL according to the frequency specification of the first channel-select signal upon receiving the frequency-valid indicator of the second channel-select signal. The number of distinct channel frequencies in the series of channel frequencies can be a prime number. The series of channel frequencies can be repeated upon completion, and no channel can be used more than once in each repetition of the series of channel frequencies. The transmitted RF signal can be received, demodulated, a signal level threshold can be determined from the demodulated RF signal, and the demodulated RF signal can be compared to the signal level threshold to regenerate the digital information from the demodulated RF signal. The RF signal output of the VCO can be used as a local oscillator to demodulate the received RF signal. 
     In general, in another aspect, the invention features a method for RF transmission of digital information including generating an RF signal using a voltage-controlled oscillator (VCO), stabilizing the RF signal from the VCO by providing an error signal from a phase-locked loop (PLL) to an input of the VCO, encoding the digital information such that it has a duty cycle which is substantially constant over the response time of the PLL, forming a data packet containing the encoded digital information, and combining the data packet with the error signal of the PLL input to the VCO, thereby causing variations in frequency in the RF signal from the VCO that represent the data packets. 
     Embodiments of the invention may include one or more of the following features. The data packet can have a duty cycle which is substantially constant over the response time of the PLL. The digital information can be encoded and the packets can be formed according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard. 
     In general, in another aspect, the invention features a method for receiving digital information sent by an RF transmitter using the IrDA 4PPM data encoding standard including receiving a transmitted RF signal, demodulating the RF signal, determining a signal level threshold from the demodulated RF signal, comparing the demodulated RF signal to the determined signal level threshold to thereby regenerate IrDA-formatted data from the demodulated RF signal, and reproducing the digital information by decoding the IrDA-formatted data. 
     In general, in another aspect, the invention features a method of implementing communication between a plurality of host computers using spread-spectrum RF transmission, including exchanging digital information between a first host computer and a first RF transceiver, encoding the digital information using pulse position modulation (PPM) according to the IrDA 4PPM data encoding standard, forming a data packet from the encoded digital information according to the IrDA 4PPM data encoding standard, generating an RF signal using a first voltage-controlled oscillator (VCO), stabilizing the RF signal from the first VCO by providing an error signal from a first phase-locked loop (PLL) to an input of the VCO, combining the data packet with the error signal of the first PLL input to the VCO, thereby causing variations in frequency in the RF signal from the first VCO that represent the data packet, broadcasting the RF signal to a second RF transceiver, demodulating the RF signal at the second RF transceiver, determining a signal level threshold from the demodulated RF signal, comparing the demodulated RF signal to the determined signal level threshold to thereby regenerate the data packets from the demodulated RF signal, reproducing the digital information by decoding the data packet, and exchanging the digital information with a second host computer. 
     Embodiments of the invention may include one or more of the following features. Media access control (MAC) functions can be placed in host computer software at both the first and second host computers. The possibility that multiple transceivers will transmit simultaneously can be reduced by each transceiver waiting a pseudorandom time from the end of the last transmission or reception before transmitting. The pseudorandom time can be determined by a pseudorandom number generator, by clock drift of one host relative to another, or by interrupts generated by other operations of the host computers. 
     In general, in another aspect, the invention features an apparatus for RF transmission of digital information including a VCO, the VCO arranged to generate an RF signal, a PLL, the frequency input of the PLL coupled to the RF signal output of the VCO, an encoder, the encoder arranged to convert the digital information into a form where it has a data rate faster than a response time of the PLL, and a coupler, the coupler coupling both the error signal output of the PLL and the encoded digital information to an input of the VCO. 
     Embodiments of the invention may include one or more of the following features. An antenna can be coupled to the RF signal output of the VCO. The coupler can be a loop filter. The encoder can be arranged to convert the digital information into a form where it has a duty cycle which is substantially constant over the response time of the PLL. The encoder can convert the digital information according to the pulse position modulation,(PPM) scheme defined by the IrDA 4PPM data encoding standard. A channel frequency selector can change a channel frequency of the RF signal according to a series of channel frequencies, wherein the selector determines the series of channel frequencies by the selector generating a series of at least a first and a second channel-select signal, each channel-select signal comprising a frequency-valid indicator and a frequency-specification indicator, the selector sending each channel-select signal in turn to the PLL, and the PLL changing its tuning frequency according to the frequency specification of the first channel-select signal upon receiving the frequency-valid indicator of the second channel-select, signal. The number of distinct channel frequencies used by the selector in the series of channel frequencies can be a prime number. The selector can repeat the series of channel frequencies upon completion, and not use any channel more than once in each repetition of the series of channel frequencies. 
     In general, in another aspect, the invention features an apparatus for RF reception of IrDA 4PPM-encoded digital information including an antenna, the antenna receiving a transmitted RF signal containing the IrDA 4PPM-encoded digital information, a low-noise amplifier, the low-noise amplifier amplifying and filtering the received RF signal, a local RF oscillator, the local RF oscillator configured to produce a frequency in proximity to the channel frequency of the received RF signal, a mixer, the inputs of the mixer coupled to the output of the low-noise amplifier and the local RF oscillator, an FM demodulator, the input of the FM demodulator coupled to the output of the mixer, and a decoder, the decoder extracting the digital information from the demodulated RF output of the FM demodulator according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard. 
     In general, in another aspect, the invention features an apparatus for RF transmission and reception of digital information including a VCO, the VCO arranged to generate an RF signal, a PLL, the frequency input of the PLL coupled to the RF signal output of the VCO, an encoder, the encoder arranged to convert the digital information into a form where it has a data rate faster than a response time of the PLL, a coupler, the coupler coupling both the error signal output of the PLL and the encoded digital information to an input of the VCO, a switch, the switch configured to couple its input to at least one of its outputs, a first antenna coupled to a first output of the switch, the first antenna arranged to transmit the RF signal, an antenna arranged to receive a transmitted RF signal, a low-noise amplifier coupled to the attenna, the low-noise amplifier amplifying and filtering the received RF signal, a mixer, one input of the mixer coupled to the output of the low-noise amplifier and another input of the mixer coupled to a second output of the switch, an FM demodulator, the input of the FM demodulator coupled to the output of the mixer, and a decoder, the decoder reproducing the digital information from the demodulated RF output of the FM demodulator. 
     Embodiments of the invention may include one or more of the following features. The antenna arranged to receive the transmited RF signal can be the same antenna as the first antenna, and the apparatus can include a power amplifier, the input of the power amplifier coupled to the first output of the first switch, and a second switch, the second switch configured to couple the first antenna either to the input of the low-noise amplifier for reception, or to the output of the power amplifier for transmission. The decoder can reproduce the digital information from the demodulated RF output of the FM demodulator according to the pulse position modulation (PPM) scheme defined by the IrDA 4PPM data encoding standard. 
     Advantages of the invention may include one or more of the following. A 2.4 Ghz transceiver can be coupled to a number of personal computers to form a robust, low cost RF WLAN. The RF WLAN can support relatively high-bandwidth digital signal communication between computers or other devices. The RF transceiver does not require generating both a carrier signal and an information bearing signal that are then mixed, requiring an additional (relatively expensive) mixer and filter. The approximately 2.4 Ghz signal transmitted by the transceiver is directly generated by a relatively simple VCO and PLL feedback circuit whereby digital information is directly injected into the feedback circuit. By employing a particular encoding scheme used in infrared communications to encode the digital information injected into the feedback circuit, frequency drifts that would otherwise be expected by such direct injection can be avoided. More than one transceiver can be coupled to the same computer to increase data throughput by splitting data packets among more than one communications channel. By placing most Medium Access Control (MAC) features in software instead of hardware, relatively cheap and available CPU cycles can be used to perform MAC operations instead of additional MAC circuitry in the transceiver. 
     Other features and advantages of the invention will become apparent from the following description and from the claims. 
    
    
     DRAWINGS 
     FIG. 1 is a block diagram of a digital transceiver. 
     FIG. 2 is a block diagram of an RF WLAN employing digital transceivers. 
     FIG. 3 is a schematic diagram of a power supply used by the digital transceiver. 
     FIG. 4 is a schematic diagram of a phase-locked loop and a loop filter used by the digital transceiver. 
     FIG. 5 is a schematic diagram of a clock- and frequency-reference generator used by the digital transceiver. 
     FIG. 6 is a schematic diagram of a voltage-controlled oscillator and filtering elements used by the digital transceiver. 
     FIG. 7 is a schematic diagram of an RF switch, an RF power amplifier, and an antenna used by the digital transceiver. 
     FIG. 8 is a schematic diagram of an antenna and a low-noise amplifier used by the digital transceiver. 
     FIG. 9 is a schematic diagram of an RF mixer and filtering elements used by the digital transceiver. 
     FIGS. 10A and 10B are schematic diagrams of a wideband FM demodulator used by the digital transceiver. 
     FIGS. 11A and 11B are schematic diagrams of a computer interface to the digital transceiver. 
     FIG. 12 is a schematic diagram of a CIS ROM used in the digital transceiver. 
     FIG. 13 is a functional block diagram of a device driver for the digital transceiver. 
     FIG. 14 is a schematic diagram of an embodiment using a single antenna. 
    
    
     DESCRIPTION 
     FIG. 1 shows a digital radio-frequency transceiver capable of generating and receiving a radio-frequency signal carrying digital information, which includes transmitter  12 , receiver  14 , and power supply  16 . 
     Transmitter  12  includes local RF oscillator  20 , switch  22 , and transmitter power amplifier  24 . Local RF oscillator  20  includes a phase-locked loop (PLL)  30 , which tunes the carrier frequency of RF transceiver  10  (controlled by its control signals CLK  66 , DAT  68 , and LE  70  as described further below). Loop filter  32  couples the error signal of PLL  30  to voltage-controlled oscillator (VCO)  34 . The radio-frequency output  35  of VCO  34  is coupled to the frequency input of PLL  30 , and loop filter  32  serves to stabilize the resulting feedback loop between PLL  30  and VCO  34 . RF output  35  of VCO  34  is also coupled to switch  22  by band-pass filter  36 . Digital information to be carried on RF output  35  of VCO  34  is injected directly into loop filter  32  on line TX  64 . Switch  22  couples RF output  35  of VCO  34  to the input of transmitter power amplifier  24  when signal TRANSMIT  60  is asserted and signal RECEIVE  62  is not asserted. 
     Transmitter power amplifier  24  comprises filtering elements (capacitor C 84 , band-pass filter LC 2 , and capacitor C 5 ), RF power amplifier  38 , and transmission antenna  40 . Switch  22  is coupled by capacitor C 84  to band-pass filter LC 2 , which in turn is coupled to RF power amplifier  38  by capacitor C 5 . The output of RF power amplifier  38  is coupled to transmission antenna  40 . 
     Receiver  14  includes an RF receiver  26  and a demodulator  28 . RF receiver  26  includes a receiving antenna  42 , which receives a broadcast RF signal carrying digital information and supplies it to low-noise amplifier  44 . Low-noise amplifier  44  includes a band-pass filter that blocks frequencies other than those in the selected band of transmission channels. The output of low-noise amplifier  44  is coupled to demodulator  28 . Demodulator  28  includes mixer  46 , which accepts as its inputs both a carrier frequency input on line  18  and output  45  of low-noise amplifier  44 . Local RF oscillator  20  of transmitter  12 , coupled through switch  22 , supplies the current carrier frequency to mixer  46  on line  18 . In order to provide the reference carrier frequency, data transmission on line TX  64  ceases, the current transmission channel frequency is selected using control signals CLK  66 , DAT  68 , and LE  70  of PLL  30 , and switch  22  is then directed to send its input (the filtered output of VCO  34 ) to line  18 . The output of mixer  46  is coupled to low-pass filter  48 , the output of which is an intermediate frequency (IF) signal  49  encoded with the digital information. The output of low-pass filter  48  is coupled to wideband FM demodulator  50 . Wideband FM demodulator  50  asserts a carrier-detect signal CARRIER  76  when it has detected and is demodulating a signal. The baseband output of wideband FM demodulator  50  is AC-coupled to comparator  52  by coupling capacitor C 64 . Comparator  52  integrates the received and demodulated AC-coupled baseband signal to adaptively determine a signal-level threshold, and by comparing the AC-coupled baseband signal to the signal-level threshold, comparator  52  reproduces on line RX  74  the digital information that was encoded in the received RF signal. 
     Power supply  16  generates the various power voltages required by transmitter  12  and receiver  14 . From a single 5-Volt power supply VCC  80 , four analog power supply voltages are generated: AVCC  82 , VCCTXPA  84 , VCCLNA  86 , and VCCRX  88 . Different components of transceiver  10  are powered by different power supplies to conserve power, minimize noise, and improve stability. 
     FIG. 2 shows an RF Network  100  including two hosts  102 A and  102 B (e.g., two personal computers located in different rooms in a home), where each host  102  has a CPU  104 , a transceiver device driver  106 , and an interface  108  that couples Host  102  with an RF Transceiver  10 . (typically coupled to or within Host  102 ). 
     Transmission antenna  40 A of Host  102 A can send digital information via an RF transmission  101 A to receiving antenna  42 B of Host  102 B and vice versa. Transceiver device driver  106  handles all routing of data packets between applications running on CPU  104  and RF transceiver  10 . Transceiver device driver  106  controls this routing by sending and receiving control and data signals to and interface  108 , which then generates transceiver control lines  110 , described further below, which control the. operation of transceiver  10 . 
     FIG. 3 shows power supply circuit  16  in greater detail. Analog power circuit  120  creates 5-Volt AVCC  82  from VCC  80 , decoupled by capacitors C 82 , C 83  and C 105 . In one embodiment, resistors R 105  and R 107  can have positive resistances to couple C 82  and C 83  to “de-Q” any resonances inadvertently created. In the embodiment shown, resistors R 105  and R 107  are shown as shorts with zero resistance and can be omitted. 3-Volt analog power supply lines VCCTXPA  84 , VCCLNA  86 , and VCCRX  88  are created from AVCC  82  using low-noise voltage regulators U 3 , U 16 , and U 11 , respectively. Each 3-Volt analog power supply  122 ,  124 , and  126  powers separate portions of the transceiver circuitry, and separate voltage regulators U 3 , U 16 , and U 11  minimize noise between circuits. 
     Transmitter PA power circuit  122  creates VCCTXPA  84 , first by inputting analog power AVCC  82  into voltage regulator (Micrel MIC5215-3.0BMS) U 3 , which is bypassed by capacitors C 2  and C 102  to prevent instability. Voltage regulator U 3  is chosen to provide an output of 3 Volts to VCCTXPA  84 , and includes an enable/shutdown control input EN. Signal TRANSMIT  60  controls U 3 , such that when signal TRANSMIT  60  is not asserted, VCCTXPA  84  is disabled, and power is not supplied to transmitter power amplifier  24 , saving power when not transmitting RF signals. 
     Receiver low-noise amplifier power circuit  124  and receiver demodulator power circuit  126  are similar to transmitter. PA power circuit  122 , and each provides an output of 3 Volts. Low-noise amplifier power circuit  124  employs voltage regulator (Micrel MIC5205-3.0MBS) U 16 , and receiver demodulator power circuit  126  employs voltage regulator (Micrel MIC5205-3.0MBS) U 11 . Both voltage regulators U 11  and U 16  have an enable/shutdown control input EN. Signal RECEIVE  62  is coupled to the control inputs EN of U 16  and U 11 , such that when signal RECEIVE  62  is not asserted, both VCCLNA  86  and VCCRX  88  are disabled, and power is thereby not supplied to low-noise amplifier  44 , mixer  46 , or FM demodulator  50  of receiver  14 , conserving power when not receiving RF transmissions. Voltage regulators U 16  and U 11  are coupled to bypass capacitors C 91 , C 92 , and C 101 , and to C 37  and C 60 , respectively. 
     FIG. 4 shows in more detail circuits for PLL  30  and loop filter  32  (of local RF oscillator  20  in transmitter  12 ). PLL  30  employs digital PLL synthesizer (Motorola MC12210) U 19 . The tuning frequency of digital PLL U 19  can be selected clocking in a numerator and divisor through control lines CLK  66 , DAT  68 , and LE  70 . Digital PLL U 19  is powered by AVCC  82  and relies on a frequency reference supplied on line FREF  140 . Digital PLL U 19  accepts a frequency input on line FPLL  142 , provides an output error signal VT  144 , and provides a phase-comparator output signal LD  146  which is coupled to lock detect circuit  148 . Resistor R 32  and capacitor C 66  of lock detect circuit  148 , perform an integrating function and are coupled to resistors R 33  and R 34  through transistor Q 2 . Only when the digital PLL U 19  has acquired a lock on its frequency input on line FPLL  142  will phase-comparator output signal LD  146  be a nearly-constant HIGH, causing the (negative-true) signal LOCKED  72  to be asserted by lock detect circuit  148 . 
     Loop filter  32  includes resistor R 38  and capacitors C 71  and C 72 . Error signal VT  144 , the output of digital PLL U 19 , is filtered by loop filter  32 . Loop filter  32  is tied to ground by resistor R 37  and capacitor C 69 . By coupling line TX  64  to loop filter  32  through resistor R 39  and having the resistance of resistor R 37  be a small but positive value, the “ground” reference of loop filter  32  is shifted higher whenever line TX  64  is HIGH. Line TX  64  thus creates small deviations in error signal VT  144 , which directly modulates the RF frequency output of VCO  34  with any digital information provided on line TX  64 . 
     In one embodiment, modulation of the VCO output is accomplished by moving its input by 5.28 mV steps. This voltage differential is sufficient to cause a 750 kHz change in output frequency. If frequency hops are 1 MHZ, then a “1” is represented by an excursion of about ¾ of a hop from the current frequency (which represents a “0”). 
     FIG. 5 shows a circuit for producing frequency and clock references on lines FREF  140  and  2 NDLO  152 , respectively. A 20 MHZ crystal Y 3  is coupled to clock generator (ICS/Microclock ICS525) U 14 . Clock generator U 14  provides a buffered 20 MHZ clock output to frequency reference line FREF  140  and a buffered 120.588 MHZ clock output through resistor network  154  to line  2 NDLO  152 . Resistor network  154  includes resistors R 41 , R 42 , and R 43 , and in the embodiment shown, is designed to attenuate the clock output by 3 dB to meet the clock input requirements of wideband FM demodulator  50 . 
     FIG. 6 shows circuits for VCO  34  and band-pass filter  36  in more detail. VCO  34  employs VCO (Z-Communications SMV2385L) OSC 1 . VCO OSC 1  has a frequency range from 2285 to 2484.5 MHZ. PLL U 19  can control VCO OSC 1  over its entire range. The 3-Volt power supply for VCO OSC 1  is supplied by voltage regulator (Micrel MIC5205-3.0MBS) U 9 , which is bypassed by capacitors C 24 , C 25 , C 27 , and C 33  to prevent oscillations. The enable input EN of voltage regulator U 9  is coupled to AVCC  82  to always enable the power output of the voltage regulator U 9 , ensuring frequency stability. The RF output of VCO OSC 1  couples to resistors R 13 , R 14 , R 15 , and R 16 , forming a resistive power splitter  160  that minimizes the detrimental effects of load mismatch on the performance of VCO OSC 1 . Capacitor C 34  couples the RF signal from resistive power splitter  160  to line FPLL  142 , which then couples the output RF signal back to the frequency input FI of PLL  30  (FIG.  4 ), completing the feedback loop between PLL  30  and VCO  34 . Resistive power splitter  160  also supplies RF signal  35  to band-pass filter  36 . Band-pass filter  36  comprises low-pass filter (Toko LTF32161-F2R4G) LC 3 , inductor L 19 , and capacitor C 35 , where inductor L 19  and capacitor C 35  form a high-pass filter, such that band-pass filter  36  passes only those RF frequencies in the transmission band of channels to line FVCO  156 . 
     FIG. 7 shows switch  22  and transmitter power amplifier  24  in more detail. Switch  22  employs an L-Band SPDT GaAs MMIC switch (NEC uPG152TA) U 5 . Switch U 5  couples line FVCO  156  (carrying the filtered RF signal from VCO  34 ) to either mixer  46  or to transmitter power amplifier  24 , or to neither, depending upon the state of signals TRANSMIT  60  and RECEIVE  62 . When signal TRANSMIT  60  is asserted but RECEIVE  62  is not, line EFVCO  156  is coupled to capacitor C 84  of transmitter power amplifier  24 , and then to band-pass filter (Murata LFJ30-03B2442-BA84) LC 2 . When signal RECEIVE  62  is asserted but TRANSMIT  60  is not, line FVCO  156  is coupled to line  1 STLO  18 , which couples switch U 5  to mixer  46  of receiver  14 . If neither TRANSMIT  60  or RECEIVE  62  are asserted, or both are, switch U 5  does not couple line FVCO  156  to anything. Therefore, depending on which signals are asserted (TRANSMIT  60  or RECEIVE  62 ), filtered RF signal output FVCO  156  of VCO  34  is either passed to the next stage of transmitter  12  or sent to receiver  14 , but switch U 5  will not do both, an advantage discussed further below. 
     Transmitter power amplifier  24  includes power amplifier (HP MGA-83563) U 4 . Band-pass filter LC 2  couples to power amplifier U 4  through capacitor C 5  and inductor L 4 . 3-Volts power from VCCTXPA  84  is supplied to amplifier U 4  by inductors L 1  and L 5 , which are bypassed by capacitors C 7  and C 6 , respectively. Capacitor C 8  couples the output of power amplifier to transmission antenna  40 , thus transmitting the amplified (by about 20 dB) RF signal carrying digital information. 
     FIG. 8 shows RF receiver  26  in more detail. An RF signal carrying digital information is received by receiving antenna  42 , and is coupled to low-noise amplifier  44  through capacitor C 1  which forms a high-pass filter for the RF signal. Low-noise amplifier  44  comprises a two-stage low-noise amplifier, using two GaAs RFIC amplifiers (HP MGA-85563) U 6  and U 8 . The cascaded gain of the two amplifiers U 6  and U 8  is about 30 dB, giving about 5 dB of margin for an IF sensitivity of about −85 dBm. The two amplifiers U 6  and U 8  are coupled by band-pass filter (Toko TDF2A-2450T-10) LC 1 . Inductors L 6  and L 12  are used to supply amplifiers U 6  and U 8 , respectively, with 3 Volts from VCCLNA  86 . Bypass capacitors C 16 , C 31 , and C 106  filter VCCLNA  86 . The output of amplifier U 8  is coupled to mixer  46  by line RFIN  45 . 
     FIG. 9 shows mixer  46  in more detail. Down converter (HP IAM-91563) U 7  frequency-shifts incoming RF signal output RFIN  45  to an intermediate-frequency (IF) signal. The output of amplifier U 8  (of low-noise amplifier  44 ) on line RFIN  45  is coupled to input  3  of Down converter U 7  through capacitor C 30  and inductor L 21 . The current RF carrier frequency on line  1 STLO  18  is coupled to Down converter U 7  by RLC network  168 . RLC network  168  matches impedances between output  18  of local RF oscillator  20  and the input of Down converter U 7 , and includes capacitors C 22  and C 28 , resistors R 9 , R 10 , and R 11 , and inductor L 20 . On the intermediate-frequency output  6  of Down converter U 7 , capacitor C 29  and inductor L 10  form low-pass filter  48  to pass only the appropriate IF signal while blocking unwanted signals from Down converter U 7 . The IF signal is coupled to line  1 STIF  170  by capacitor C 26 . Down converter U 7  is powered by VCCRX  88  through inductors L 10  and L 11 , and capacitor C 20  is used to bypass VCCRX  88 . Capacitors C 23  and C 36  bypass the source of Down converter U 7 , and resistor R 100  is used to put Down converter U 7  into its high linearity mode for better performance. 
     FIG. 10, comprising FIGS. 10A and 10B, shows wideband FM demodulator  50  in greater detail. Wideband FM demodulator  50  comprises mixer FM IF system (Philips SA639DH) U 10 , powered by VCCRX  88  through resistor R 29 , and bypassed by capacitors C 51  and C 52 . Mixer FM IF system U 10  has as its inputs the output of mixer  46 , coupled by line  1 STIF  170 , and the 120.588 MHZ clock output generated by clock generator U 14 , coupled by line  2 NDLO  152 . The wideband data output DATA O of mixer FM IF system U 10  is coupled to capacitor C 64 . Mixer FM IF system U 10  includes an internal post-detection filter amplifier (not shown, but included in comparator  52 ) which receives the biased demodulated output of capacitor C 64  at POST I and outputs a filtered signal at POST O and SW O. Resistors R 30  and R 31  provide a constant bias voltage to post-detection filter amplifier input POST I of mixer FM IF system U 10 , driving the baseband signal into the clipping region of the internal post-detection filter amplifier. Bias resistors R 30  and R 31  obtain their power from VCCRX  88 . 
     Comparator  52  also includes amplifier U 12 A, configured as a comparator, and resistor R 23  and capacitor C 49 , which act as an integrator to adaptively determine an average signal level threshold from the demodulated signal output. The average signal level threshold is stored on capacitor C 49 . Amplifier U 12 A compares the demodulated signal output to the average signal level threshold and, coupled to pull-up resistor R 22 , reproduces the encoded digital information carried by the received radio-frequency signal on line RX  74 . Note that in the embodiment shown, comparator  52  is configured to make line RX  74  negative-true. 
     Mixer FM IF system U 10  also includes a Receive Signal Strength Indicator (RSSI) output  180 , which provides a voltage determined by the strength of the received IF frequency signal and feedback resistors R 27  and R 28 . RSSI output  180  is coupled to amplifier U 12 B, configured as a comparator. RSSI output  180  is compared to a voltage threshold determined by AVCC  82  and resistors R 24  and R 25 , and stored on capacitor C 50 . Amplifier U 12 B compares RSSI output  180  to the voltage threshold and, coupled to AVCC  82  by pull-up resistor R 26 , creates signal CARRIER  76 . Signal CARRIER  76  couples to LED D 1  through transistor Q 1  such that, when signal CARRIER  76  is asserted, LED D 1  illuminates to provide a visual indication that receiver  14  is receiving and demodulating a radio-frequency signal. LED D 1  is coupled to AVCC  82  by resistor R 19 . 
     FIG. 11, comprising FIGS. 11A and 11B, shows interface  108  between bus J 1  of a host computer  102  and digital RF transceiver  10  in greater detail. In the embodiment shown, interface  108  employs fast infrared controller (Texas Instruments TIR2000PAG) U 15 . Digital RF transceiver  10  is capable of being computer-controlled, through nine signals/lines TRANSMIT  60 , RECEIVE  62 , TX  64 , CLK  66 , DAT  68 , LE  70 , LOCKED  72 , RX  74 , and CARRIER  76  generated by controller U 15  under host control through bus J 1 . 
     Controller U 15  supports standard bus interfaces to host computer  102  and the Infrared Data Association (IrDA) 1.1 pulse position modulation (4PPM) data encoding format. In the embodiment shown, bus J 1  is a PCMCIA bus, but it could be appropriately configured to support ISA, PCI, RS-244, parallel, USB, or other bus architectures. 
     Controller U 15  follows the IrDA Serial Infrared Physical Layer Link Specifications (version 1.2) to encode every two bits of data to be transmitted as a 4PPM data symbol. The duration of each data symbol is subdivided into a set of equal time slices called “chips.” In the 4PPM scheme used by controller U 15 , the number of chips is set to four. Because there are four unique chip positions within each symbol in 4PPM, four independent symbols exist in which only one chip is asserted while the others are not. Table 1 defines the 4PPM data symbol representation of the four unique combinations of unencoded data bit pairs. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 4 PPM Symbol Encoding Definition 
               
             
          
           
               
                 Unencoded 
                   
               
               
                 Data Bit 
                   
               
               
                 Pair 
                 4 PPM Data Symbol 
               
               
                   
               
               
                 00 
                 1000 
               
               
                 01 
                 0100 
               
               
                 10 
                 0010 
               
               
                 11 
                 0001 
               
               
                   
               
             
          
         
       
     
     These four unique symbols are defined to be the only valid data symbols allowed in 4PPM. This allows easy error correction; if a data symbol is received which contains more than one asserted chip, the data is known to be corrupted. The position of the asserted chip within the data symbol indicates which possible combination of unencoded data bits is represented. As there are four valid data symbols, each data symbol represents two bits of unencoded data, so that a byte of unencoded data is represented by four data symbols in sequence. 
     Controller U 15  also follows the IrDA Serial Infrared Physical Layer Link Specifications (version 1.2) to define a complete 4PPM packet format for data transmission. Data to be transmitted is encoded according to the 4PPM scheme described. The 4PPM-encoded data in the packet is preceded by symbols defining a preamble field (PA) and a start flag (STA), and followed by symbols defining a frame check sequence field (FCS) and a stop flag (STO). PA can be used by the receiving decoder to establish bit synchronization. Once it has done so, it searches for STA to begin data symbol decoding. The receiving decoder continues to decode until it reads STO, which indicates the end of a 4PPM data packet. FCS contains a cyclic redundancy check (CRC) value which is first calculated using the IEEE 802 CRC32 algorithm from the unencoded data, and then encoded in the same 4PPM scheme and then is included in the data packet. PA, STA, and STO comprise symbols which are not one of the four valid data symbols described above, so that they are unambiguously distinguished from valid data symbols. 
     Controller U 15  accepts digital information intended for transmission on the data lines D 0 -D 7  of the computer bus J 1 . Controller U 15  then encodes the digital information into 4PPM data symbols, and assembles 4PPM data packets including PA, STA, the 4PPM data symbols, FCS, and STO. Controller U 15  then asserts signal TRANSMIT  60 , and sends the 4PPM data packet serially onto line TX  64  for transmission. 
     Referring to FIG. 6A above, the advantages of injecting digital information directly into loop filter  32  to directly vary RF output  35  of VCO  34  can now be understood. The IrDA data encoding scheme is used in digital RF transceiver  10  to encode data, because the encoded data and the additional packet information share important properties. In particular, PA, STA, and STO, as defined, share one characteristic with valid data symbols: in each, one chip is asserted for every three which are not. Thus a 4PPM packet always has a substantially constant duty cycle of 25 percent. Since the digital information has a substantially constant duty cycle, the digital signal on line TX  64  injected into loop filter  32  through resistor R 39  produces a substantially constant voltage offset when integrated and stored on capacitor C 69 . This substantially constant voltage offset is accommodated by capacitors C 71  and C 72  of loop filter  32 , where the values of capacitors C 71  and C 72  are chosen such that the frequency acquisition time of PLL  30  is longer than the rate at which digital information is transmitted on line TX  64 . Thus the transmission of digital information with a substantially constant duty cycle to VCO  34  through loop filter  32  does not disturb the lockon the carrier frequency of PLL  30 . This scheme allows direct generation of the transmission signal by VCO  34  without causing instability in the feedback loop between PLL  30  and VCO  34 . This scheme also eliminates the need for separately generating and then mixing both carrier and signal frequencies, allowing a simpler and relatively inexpensive design, without a separate (and relatively expensive) additional mixer and sideband filter. 
     When controller U 15  is instructed to receive data, it asserts signal RECEIVE  62 . upon receipt of a 4PPM data packet on line RX  74 , it will remove STA, check for illegal 4PPM symbols, and check for CRC and packet-length errors. Then it will signal the computer though bus J 1  that data has been received and decoded, and is ready to be read though bus J 1  on data lines D 0 -D 7 . 
     FIG. 12 shows a CIS ROM system  190  which includes nonvolatile PCMCIA attribute memory (ATMEL AT28C16-TC) U 17 , which can store the identity information for transceiver  10 , especially for Plug-and-Play identification during installation. 
     Digital RF transceiver  10  and its control by the circuitry of FIG. 11 constitutes the physical layer (PHY) of a wireless network interface. Other than the 4PPM encoding and packet formation, Media Access Control (MAC) functions are not present in the circuitry shown. Most MAC functions are located in transceiver device driver  106  of host computer  102  to simplify the design and reduce manufacturing costs. As communication through computer bus J 1  is often much more slower than that between a PHY and a hardware MAC, transceiver device driver  106  is configured to overcome a number of timing problems. 
     Where multiple devices share the same transmission medium, only one device is usually allowed to transmit at a time to prevent data corruption. Most MACs, such as those in standard Ethernet networks, use Carrier Sense, Multiple Access/Collision Detection (CSMA/CD) to share a single transmission medium among multiple devices. In CSMA/CD, any device that wishes to transmit data must first check that no other device is currently transmitting. While transmitting, the device must listen to detect the carriers of other devices. If it does so, it decides there was a collision, and attempts to resend the data. One problem results when other devices also try to resend their data, such that an infinite loop of collisions can occur. One solution to this problem is for each device to wait a random delay time after detecting a collision before attempting to retransmit its data. This procedure, called “random backoff”, makes it unlikely that more than one device will restart transmission at the same time. 
     In the present invention, implementing MAC functions in transceiver device driver software  106  of host computer  102  produces random backoff as a natural consequence. The clocks of each host computer  102  typically run at slightly different speeds from each other, and each host computer  102  can be interrupted by other components or devices which share bus J 1  with controller U 15 , and also by application interrupts. Thus, if no host is currently transmitting, any host can begin transmitting immediately, and it will be unlikely that more than one host will decide to begin at the same time. The only circumstance in which it will be likely is if there are more than two hosts active, and one ends a transmission. The other hosts may be waiting to transmit. In this case, each host must generate a random time, and wait for that time before transmitting. 
     In the present invention, each host begins to generate a random waiting time automatically when it has stopped receiving a transmission (or has just finished transmitting data), not just when the host wishes to transmit. Thus, even if a particular host coupled to the RF WLAN shown in FIG. 2 has no present need to send data, it has already begun its random backoff waiting period. If that period has expired when the host decides to send data, that host can immediately do so, without further delay. Thereby random backoff can be achieved without the loss of efficiency caused by having the host start a new random waiting period every time it desires to transmit. 
     The FCC mandates the use of spread spectrum techniques for the frequency band 2400-2483.5 MHZ. Digital RF transceiver  10  implements frequency-hopping spread spectrum (FHSS) transmission in this band. But as the frequency for transmission of encoded data is selected by control signals CLK  66 , DAT  68 , and LE  70  of PLL  30  (of digital RF transceiver  10 ), the frequency-hopping nature of digital RF transceiver  10  is entirely external to it. That is, transceiver  10  does not control which channels are used, the order in which they are used, and the chipping rate (how often the transmission frequency of transceiver  10  is changed, or “hopped”). In the embodiment shown in FIG. 11, it is transceiver device driver  106  of host  102  which determines the frequency-hopping algorithm. 
     One such frequency-hopping algorithm selects one of a number of pre-determined channel sequences for the WLAN. The hosts create the WLAN by synchronizing to a common phase on that sequence and thereafter hopping in unison through the given sequence. Each host attempts to establish synchronization with other hosts by traversing the channel sequence in reverse order, and waiting for a short period at each channel frequency to listen for transmissions from other hosts. If transmissions are detected, the selected hopping sequence and phase can be directly determined, and the host joins the WLAN. If a host does not detect transmissions from other hosts within a minimum period, it transmits a request for acknowledgement on its current channel. If another host acknowledges in response, the hopping sequence and phase can be determined directly. If there is no answer within a certain period, the host continues traversing the frequencies in reverse order. If a host traverses the channel sequence a number of times without detecting transmissions from any other hosts, the host concludes that it is currently the only active host, and selects an arbitrary channel within the sequence and establishes the WLAN itself. At this point the host begins traversing the channel sequence in a forward order. 
     The hosts on the WLAN keep in tight synchronization by occasionally broadcasting special data packets informing other hosts of their current local phase. All other hosts receiving a phase broadcast by one host can then adjust their phase if it is not synchronized with that of the broadcasting host. A host will also keep synchronized with other hosts by broadcasting a hop command on the current channel to informing the other hosts whenever it decides to hop to the next channel. A host that receives such a command will hop to the next channel, so that all hosts in the WLAN stay on the same channel. During normal operation, should a host ever fail to detect transmissions from other hosts for longer than a number of hop periods, it considers itself out of synchronization, and will once again begin the above-described procedure to establish synchronization. 
     A feature of the transceiver device driver  106  is that the time to hop from channel to channel has been reduced by preloading PLL  30  with the next channel frequency after enabling it to use the currently loaded channel frequency. PLL  30  is programmed by a pattern of bits presented on lines CLK  66 , DAT  68 , and LE  70 . The bit patterns represent the PLL frequency coefficients selecting particular channel frequencies. The preloading is accomplished by loading a bit pattern first having the latch enable bit (which instructs PLL  30  that already loaded and valid frequency are to be used) and concludes with the frequency coefficients for the next channel. Thus the frequency coefficients for the next channel have already been loaded into PLL  30  by the prior bit pattern that enabled the prior channel. When transceiver device driver  106  needs to change the channel frequency, the bit pattern it sends changes the channel frequency of PLL  30  immediately with the first bit of the bit pattern (the latch enable bit sent on line LE  70 ), rather than having to wait until the entire bit pattern is sent before changing the frequency. This enhances the ability of hosts to stay in synchronization. 
     Another feature of transceiver device driver  106  that reduces the time necessary to control PLL  30  is that the frequency coefficients are precompiled. That is, the bit patterns that need to be presented to PLL  30  in order to program it with the coefficients of the desired frequency are precalculated (one bit pattern for each frequency) and stored as strings within transceiver device driver  106  itself. Transceiver device driver  106  can then take advantage of string I/O instructions available in the instruction sets of many microprocessors, allowing transceiver device driver  106  to program PLL  30  with a single instruction, rather than needing one instruction for each byte or word of the bit pattern. This ensures that the output of the bit patterns to PLL  30  through controller  108  is done in the shortest possible time. 
     Multiple logically independent WLAN networks can be created by different sets of hosts using different predetermined channel sequences. Such a scheme makes more bandwidth available for data transmission while still meeting FCC requirements. Each set of hosts using the same predetermined channel sequence is referred to as a co-located network, or “CoNet”. There are two different methods for specifying the channel sequences for CoNets. The first is that all CoNets use the same channel sequence, but at any given time, all the hosts inma CoNet are on a different channel than those hosts in other CoNets. One advantage of this methodology is that two or more CoNets are, in principle, never broadcasting on the same channel at the same time. One problem with this methodology is that drift between the clocks that control the frequency hopping of the CoNets can cause the phase between CoNets to change, and when two CoNets come into phase, they will both be broadcasting on the same channel in a state of continual collision. Another methodology implemented in transceiver device driver  106  assigns a channel sequence to each CoNet such that any two CoNets, regardless of the phase relationship between them, will broadcast on the same channel only once in each channel sequence. For this to be true for the maximum number of possible channel sequences, the number of channels in each sequence should be a prime number. This allows the maximum number of CoNets with minimal collisions. 
     Collisions are handled by digital packet filtering employed by transceiver device driver  106 . Each packet of data transmitted is prepended with header information which includes both a number representing the current channel and a number representing the identity of the CoNet that the host belongs to. These numbers, which are transmitted as part of the 4PMM encoded data, allow a receiving host to determine whether a particular packet is intended for it. Hosts then can discard packets from other CoNets. 
     FIG. 13 shows a functional block diagram of transceiver device driver  106 . Transceiver device driver  106  couples to the Network Device Interface Standard (NDIS) Library and Miniport Wrapper 202 provided by the Microsoft Windows® operating system (OS). NDIS 202 provides the standard interface between applications coupled to the OS and transceiver device driver  106  (coupled to digital transceiver  10  via interface  108 ). When NDIS 202 sends a data packet to transceiver device driver  106  at its standard miniport, Miniport Send block  206  places the packet into Transmit Queue  208 , where each data packet waits to be sent in turn via transceiver  10 . When transceiver  10  signals transceiver device driver  106  (via a hardware interrupt sent by interface  108 ) that its internal FIFO buffer is getting low (having sent most of its data), Transmit ISR (Interrupt Service Routine) block  210  transfers the next block of data, from the currently pending packet in Transmit Queue  208 , to interface  108 . When the currently pending packet has been entirely transmitted, Transmit ISR  210  signals Transmit DPC (Deferred Procedure Call) block  212  that the empty packet should be retrieved and sent back to NDIS. 
     When data is received by transceiver  10 , it is first sent to Receive ISR block  214 , which places it into the currently pending received data packet in Receive Queue  216 . Once the currently pending received data packet is completed, Receiver ISR  214  signals Receive DPC  218  that the completed packet should be retrieved and forwarded to NDIS  202 . 
     Collision Avoidance Manager block  220  coordinates the random back-off procedures described above. Whenever transceiver  10  has finished transmitting or receiving a packet, Collision Avoidance Manager block  220  starts a random timer so that transceiver  10  can start to transmit a new data packet only when the random timer has expired, by controlling Transmit ISR block  210 . 
     Beacon Manager block  222  is responsible for sending a number of “beacons” or special signaling packets to all transceivers  10  coupled to an RF network. Beacon Manager  222  sends an “Idle” beacon periodically, to introduce some activity to the network, and help keep all transceivers in synch. Beacon Manager  222  sends a “Request” beacon during its hunting mode, when transceiver  10  attempts to find the current frequency of the network by stepping through the frequency hops backwards. When transceiver sends a “Request” beacon at the current frequency, the other RF-coupled hosts return an “Idle” beacon to acknowledge the correct current frequency. Beacon Manager  222  also sends a “Hop” beacon to alert all RF-coupled hosts to hop to the next frequency. Transceiver device driver  106  independently sends a DPC to NDIS 202 to “wake up” transceiver device driver  106  at the appropriate time for the next hop, which ensures that a hop will happen. The additional “Hop” beacon, however, can cause the hop in the RF WLAN to occur with a more predictable latency period, but can sometimes be interrupted if there is some temporary break in RF communications. 
     Hop Manager  224  coordinates the timing of hops, and queues up a hop beacon with Beacon Manager  222 , and sends a hop DPC to NDIS  202 . If Hop Manager  224  receives a “Hop” beacon (from another host), it checks to see if it is in fact a proper time to hop. If it is, then Hop Manager  224  calculates the next frequency channel (based upon a look-up table of frequency values), sends appropriate signals to interface  108  to have transceiver  10  hop, and then clears any queued “Hop” beacon of its own. 
     Synch Manager  226  coordinates the synchronization of transceiver  10  with all other transceivers  10  coupled to the RF WLAN. If transceiver  10  is fully in synch, Synch Manager  226  instructs appropriate Hop beacons to be sent. Synch Manager  226  determines if transceiver  10  suddenly falls out of synch for any reason, and then places the system in hunting mode, and uses Beacon Manager  222  to send “Request” beacons at channels traversed in backwards order (as described above). 
     Clock Manager block  228  places time stamps in beacon packets and retrieves time stamps placed in received beacon packets, and resets its clock based upon such time stamps, so that the entire RF WLAN can remain relatively synchronized in time. 
     FIG. 14 is a schematic diagram of an embodiment using a single antenna  250 . The antenna  250  arranged to receive the transmitted RF signal may be the same antenna as the transmitting antenna. The transceiver  10  may include the power amplifier  24  (without the antenna  40 ). The input of the power amplifier  24  may be coupled to the first output of the switch  22 . A second switch  252  may be configured to couple the antenna  250  either to the input of the low-noise amplifier  44  for reception or to the output of the power amplifier  24  for transmission. 
     Other embodiments are within the scope of the claims. For example, the MAC layer handled by transceiver device driver  106  can be incorporated in firmware or hardware along with transceiver  10 . The transceivers can be coupled to any device requiring wireless digital communication, including personal digital assistants, household appliances, televisions, telephones, and other electronic devices. Various other methods and devices for generating, encoding, receiving, and decoding digital messages can be employed. Various other radio-frequencies can be used, and other RF transmission schemes other than spread-spectrum frequency hopping can be employed.