Automatic frequency control system and method for frequency-shift-key data transmission systems

The present invention provides a system and method of automatic frequency control (AFC) in a frequency-shift-key (FSK) data transmission system that allows a receiver to be used that has a bandwidth that approaches the requisite minimum bandwidth for a given data transmission rate and produces a substantial signal noise ratio (SNR) in the detected signal by the receiver. The invention includes a transmitter that outputs a SPACE signal and a MARK signal in a preamble that precedes the transmission data. The apparatus also includes a receiver that uses the SPACE and MARK signals to adjust the frequency of the signal output by a voltage-controlled-oscillator (VCO) to tune the receiver and thereby improve the signal-to-noise ratio (SNR) in the signals subsequently detected by the receiver.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention provides a system and method for performing automatic 
frequency control (AFC) in a Frequency-shift-key (FSK) data transmission 
system that allows a receiver to be used that has a bandwidth that 
approaches the minimum bandwidth for a given data transmission rate and, 
as a consequence, allows a substantial signal-to-noise ratio (SNR) to be 
realized in the signal detected by the receiver. 
2. Description of the Related Art 
A method that is widely used to transmit binary data is the 
frequency-shift-key (FSK) method. In the FSK method, an FSK transmitter 
modulates the frequency of a carrier signal between two predetermined 
frequencies according to the logical state, logical "0" or logical "1", of 
a binary data signal to produce an FSK signal. For convenience, the 
portions of the FSK signal corresponding to a logical "0" and a logical 
"1" in the binary data signal are hereinafter referred to as a SE and a 
MARK, respectively. 
As in all data transmission systems, a minimum bandwidth is required in an 
FSK data transmission system in order to accurately transmit binary data. 
The minimum bandwidth required in the receiver of an FSK data transmission 
system is defined by the following equation: 
EQU FSK.sub.min bw =B+.DELTA.f (1) 
where B is the baseband bandwidth, which is the data transmission rate or 
frequency of the binary data, and .DELTA.f is the frequency deviation. If 
the bandwidth of the receiver in an FSK data transmission system is less 
than the minimum bandwidth, then the receiver cannot reliably recover the 
binary data in the FSK signal output by the transmitter. If, on the other 
hand, the bandwidth in the receiver of an FSK data transmission system is 
greater than the minimum bandwidth, then noise can adversely affect the 
performance of the system by reducing the signal-to-noise ratio (SNR) of 
the binary data signal detected or recovered by the receiver. An FSK data 
transmission system where the bandwidth of the receiver is at or near the 
minimum bandwidth is less susceptible to noise and, as a consequence, the 
signal detected by the receiver has a higher SNR. Based on the foregoing, 
it can be seen that there is an optimum bandwidth for an FSK data 
transmission system, the minimum bandwidth set forth in equation (1), that 
is broad enough to adequately transmit binary data but narrow enough to 
substantially reduce or minimize the adverse effects of noise. 
The bandwidth of most FSK receivers is established in the design of the 
intermediate frequency (i.f.) filter or filters that process the signal 
output by a mixer, a device that converts the frequency of the signal 
received by the receiver to a lower frequency. Consequently, in designing 
an FSK data transmission system, the required data transmission rate is 
determined, the minimum necessary bandwidth is calculated using equation 
(1), and an i.f. filter or series of filters is designed that has at least 
the minimum bandwidth. Conventionally, the midpoint of the i.f. filter 
bandwidth is termed the i.f. frequency. 
The signal output by the mixer is termed the i.f. signal and has a 
frequency that is equal to the difference between the frequency of the 
received signal and the frequency of the signal output by a local voltage 
controlled oscillator (VCO). To avoid confusion between the frequency of 
the i.f. signal output by the mixer and the i.f. frequency of the i.f. 
filter, the i.f. frequency of the i.f. filter is hereinafter referred to 
as the center frequency of the i.f. filter. The frequency of the i.f. 
signal can be determined by the following equation: 
EQU f.sub.IF =f.sub.TX -f.sub.VCO ( 2) 
where f.sub.IF is the frequency of the i.f. signal output by the mixer, 
f.sub.TX is the frequency of the signal output by the transmitter, and 
f.sub.VCO is the frequency of the signal output by the VCO. 
To recover the binary data in an FSK data transmission system, the receiver 
must be tuned to the transmitter by adjusting the frequency of the signal 
output by the VCO so that the frequency spectrum of the i.f. signal output 
by the mixer is substantially symmetrical about the center frequency of 
the i.f. filter. If the receiver is tuned, then the SNR of the signal 
detected by the receiver will increase as the bandwidth of the receiver 
approaches the minimum bandwidth necessary for a given data transmission 
rate. 
A high SNR is extremely desirable in situations where data must be 
transmitted and/or received in environments where the transmitted signal 
is subjected to a high degree of noise. To achieve a high SNR, the 
receiver must have a bandwidth at or near the minimum bandwidth and the 
receiver must be precisely tuned. This allows the receiver to recover the 
binary data while at the same time substantially reducing the adverse 
effects of noise. Unfortunately, the frequency of the carrier signal 
output by the transmitter, the frequency of the signal output by the VCO, 
or both are likely to drift due to changes in temperature and the like. To 
compensate for drift in the transmitter carrier frequency, the frequency 
of the signal output by the VCO, or both, the receiver is typically 
equipped with automatic frequency control (AFC) circuitry that 
automatically tunes the receiver to the transmitter, i.e., adjusts the 
frequency of the signal output by the VCO such that the spectrum of the 
signal output by the mixer is substantially symmetrical about the center 
frequency of the i.f. filter. 
One method of achieving automatic frequency control is to continuously tune 
the receiver using a feedback signal that reflects the difference in 
frequency between the signal being output by the transmitter and the 
frequency to which the receiver is tuned. More specifically, this method 
involves continuously comparing the frequency of the signal being output 
by the transmitter with the frequency to which the receiver is tuned to 
generate a difference signal, continuously averaging the difference signal 
over a short period of time, and then tuning the receiver such that the 
average of the difference signal, which is typically obtained at the 
output of the receiver's detector, tends toward zero. This method works 
well with an analog data signal, like speech or music, where the average 
value of the data signal is close to zero and, as a consequence, the 
average frequency of the signal output by the transmitter is at or near 
the carrier frequency. Since the average frequency of the signal output by 
the transmitter is at or near the carrier frequency, the frequency of the 
signal output by the VCO can be adjusted by the AFC so that the spectrum 
of the i.f. signal is substantially symmetrical about the center frequency 
of the i.f. filter and, as a consequence, substantially all of the data 
can be recovered. However, this method does not work well with binary data 
signals that often exhibit a dc component, due to a number of consecutive 
logical "1"'s or logical "0"'s over a defined time period, because the 
average frequency of the signal output by the transmitter over the defined 
time period is greater than or less than the carrier frequency by an 
amount that reflects the dc component of the binary signal. Due to the dc 
component, the AFC adjusts the frequency of the signal output by the VCO 
such that the spectrum of the i.f. signal output by the mixer is not 
substantially symmetrical about the center frequency of the i.f. filter 
and, as a consequence, adversely affects the recovery of the modulation 
signal by the receiver. Typically, this problem is addressed by adding 
"balancing" SEs or MARKs to eliminate the dc component in the binary 
data signal. This solution, however, reduces the transmission rate of the 
meaningful data and in so doing substantially nullifies the benefits of 
using a receiver with a bandwidth at or near the minimum bandwidth, i.e., 
a high SNR. 
Another method of accomplishing automatic frequency control that is used in 
packet FSK data transmission systems where data is generally transmitted 
in bursts or packets of predetermined lengths involves transmitting an 
unmodulated carrier signal at the beginning of each packet in what is 
typically known as a preamble. The receiver, upon receiving the preamble, 
activates a feedback loop that utilizes the difference in the frequency of 
the carrier signal output by the transmitter and the frequency to which 
the receiver is tuned to generate a difference signal that is used to tune 
the receiver during the transmission of the data contained in the 
remainder of the packet. An example of this method of obtaining automatic 
frequency control in a phase-shift-key data transmission system is shown 
in U.S. Pat. No. 4,651,104, which issued on Mar. 17, 1987 to Miyo for a 
"Frequency Converter with Automatic Frequency Control". While this method 
of automatic frequency control allows a receiver to be utilized that has a 
bandwidth that approaches the minimum necessary bandwidth for a given data 
transmission rate and achieve a substantial signal to noise ratio, it has 
several drawbacks. Among the drawbacks, this method requires that the 
transmitter incorporate additional circuitry to generate the unmodulated 
carrier frequency that is transmitted during the preamble of a packet. 
Typically, FSK data transmission systems only incorporate the circuitry 
required to generate the SE signal and the MARK signal. Consequently, 
the need to include circuitry for transmitting the unmodulated carrier 
signal significantly adds to the cost of FSK data transmission systems 
that employ this method of achieving automatic frequency control. Another 
drawback associated with this method of achieving automatic frequency 
control in FSK data transmission systems is that the transmitter must be 
aligned such that the frequency of the SE signal and the frequency of 
the MARK signal are symmetrical about the frequency of the carrier signal. 
Otherwise, this method of AFC may detune the receiver and, in so doing, 
reduce the reliability of the FSK data transmission system. Moreover, the 
need to accurately align the transmitter adds significant manufacturing 
costs to an FSK data transmission system. 
Based on the foregoing, there is a need for a system and method of 
achieving automatic frequency control in an FSK data transmission system 
that allows a receiver with a bandwidth that approaches the minimum 
bandwidth for a given data transmission rate to be utilized, realizes a 
high SNR, and also addresses the failings in the known art discussed 
hereinabove. Specifically, there is a need for a system and method of 
providing AFC in an FSK data transmission system that allows a receiver 
with a bandwidth that approaches the minimum required bandwidth for a 
given data transmission rate to be utilized and produces a high SNR but 
does not require any circuitry in the transmitter to generate an 
unmodulated carrier signal. Moreover, there is a need for a system and 
method of providing automatic frequency control in an FSK data 
transmission system that allows a receiver with a bandwidth that comes 
near to the minimum bandwidth required for a defined data transmission 
rate to be used and produces a high SNR but does not require the 
frequencies associated with the SE and MARK portions of the signal 
output by the transmitter to be symmetrical about the frequency of the 
carrier signal. Additionally, there is a need for an automatic frequency 
control system and method in an FSK data transmission system that permits 
a receiver with a bandwidth that tends toward the minimum bandwidth 
required for the data transmission rate to be employed and provides a high 
SNR without reducing the data transmission rate of the meaningful data by 
using "balancing" SES and MARKS to compensate for the dc component 
typically associated with the binary data signals employed in an FSK data 
transmission system. 
SUMMARY OF THE INVENTION 
The present invention provides an automatic frequency control system and 
method for an FSK data transmission system that allows a receiver to be 
utilized that has a bandwidth that approaches the minimum bandwidth 
required for a particular data transmission rate and produces a high SNR. 
Due to the high SNR, an FSK data transmission system that incorporates the 
automatic frequency control system and method of the present invention can 
be used to reliably transmit data in high noise environments. For example, 
the United States government is in the process of requiring all gasoline 
stations to monitor their buried gasoline tanks for leaks that may allow 
gasoline or gasoline vapors to escape and contaminate the surrounding 
ground and/or ground water. Reliable transmission of data using an FSK 
data transmission system in the gasoline station environment requires a 
substantial signal to noise ratio due to the high level of noise generated 
by car engines, pumps, radios, and the like. An FSK data transmission 
system that includes the automatic frequency control of the present 
invention is capable of achieving the SNR required by this application. 
The AFC apparatus of the present invention includes a transmitter that 
broadcasts a preamble having a SE portion and a MARK portion. The AFC 
apparatus also includes a receiver that uses the SE and MARK portions 
of the preamble output by the transmitter to tune the receiver to the 
transmitter. More specifically, in a preferred FSK embodiment of the AFC 
apparatus, the SE and MARK portions of the preamble are used to adjust 
the frequency of a signal output by a voltage controlled oscillator so 
that when signal output by the voltage controlled oscillator is mixed with 
the signal received by the receiver the spectrum of the resulting i.f. 
signal is symmetrical about the center frequency of the receiver, which 
preferably has a bandwidth that approaches the minimum necessary bandwidth 
for a defined data transmission rate so that a high SNR can be achieved. 
In other words the receiver is tuned to the transmitter. The receiver can 
now operate under substantially optimum bandwidth conditions, i.e., 
substantially all of the binary data can be recovered while also 
substantially reducing the adverse effects of noise. Moreover, the 
receiver is tuned regardless of whether the transmitter, VCO, or both are 
initially detuned. Tuning also occurs regardless of whether or not the 
transmitter is misaligned, i.e., the frequencies associated with the SE 
and MARK are not symmetrical about the carrier frequency. 
Operation of the aforementioned preferred FSK embodiment of the invention 
commences with the transmitter outputting the SE and MARK portions of 
the preamble. The receiver, upon detecting the preamble, "down-mixes" the 
received signal using the signal being output by the voltage controlled 
oscillator. At this point, the signal output by the voltage controlled 
oscillator has not been adjusted to tune the receiver to the transmitter. 
The "down-mixed" signal is then processed by a detector to recover the 
logical "0" and logical "1" binary data signals that correspond to the 
SE signal and the MARK signal associated with preamble broadcast by the 
transmitter. The level of the logical "0" signal is related to the 
difference in frequency between the SE signal of the preamble broadcast 
by the transmitter and the frequency of the signal output by the voltage 
controlled oscillator. Similarly, the level of the logical "1" signal is 
related to the difference in frequency between the MARK signal of the 
preamble broadcast by the transmitter and the signal output by the voltage 
controlled oscillator. The logical "0" and logical "1" signals are used to 
tune the receiver such that the spectrum of the i.f. signal of the 
subsequently received data is substantially symmetrical about the center 
frequency of the receiver. The AFC circuitry in the receiver accomplishes 
this by sampling-and-holding the levels of the aforementioned logical "0" 
and logical "1" recovered by the detector, averaging the sampled-and-held 
signals, scaling the resulting average signal, and applying the resulting 
signal to the voltage controlled oscillator to adjust the frequency of the 
signal output by the voltage controlled oscillator. The signal applied to 
the VCO reflects the difference in frequency between the signal output by 
the VCO and the frequency midway between the frequencies associated with 
the SE and MARK portions of the preamble. This difference, in turn, 
reflects the difference between the center frequency of the receiver and 
the frequency midway between the frequencies of the SE and MARK 
portions of the preamble following the mixing operation. Consequently, the 
signal applied to the VCO changes the frequency of the signal output by 
the VCO such that the difference between the center frequency of the 
receiver and the frequency midway between the frequencies of the SE and 
MARK portions of the preamble following mixing approaches zero. This, in 
effect, adjusts the frequency of the signal output by the VCO such that 
the spectrum of the i.f. signal is substantially symmetrical about the 
center frequency of the receiver and thereby allows data to be reliably 
recovered while at the same time substantially reducing the adverse 
effects of noise. In the preferred embodiment of the invention, the 
automatic frequency control system does not operate to adjust the 
frequency of the signal output by the voltage controlled oscillator unless 
a predetermined signal strength is detected by the receiver. 
Based on the foregoing, the present invention provides an AFC system and 
method that allows a receiver with a bandwidth that tends toward the 
minimum bandwidth for a given data transmission rate to be utilized and a 
high SNR achieved while also exhibiting several advantages over the known 
art. Namely, the automatic frequency control system and method of the 
present invention can be used with an FSK data transmission system without 
having to add "balancing" SES and MARKS to compensate for a dc 
component in the binary signal. Moreover, the AFC system and method of the 
present invention does not require the transmitter to include any special 
circuitry for generating signals other than the SE and MARK signals 
normally output by an FSK system. Additionally, the AFC system and method 
of the present invention does not require the transmitter to be 
constructed such that the frequencies of the SE and MARK signals are 
substantially symmetrical about the frequency of the transmitter's carrier 
signal. Stated another way, the AFC system and method of the present 
invention is insensitive to the relationship between the frequencies of 
the SE signal, MARK signal and carrier signal used by the transmitter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention provides an automatic frequency control system and 
method for an FSK data transmission system that allows a receiver to be 
utilized with a bandwidth that approaches the minimum bandwidth required 
for a given data transmission rate and also achieve a high SNR. The 
present invention includes a transmitter for outputting a SE signal and 
a MARK signal in a preamble that precedes the transmission of user data. 
In addition to the transmitter, the invention includes a receiver that 
uses the SE and MARK signals output by the transmitter to tune the 
receiver and thereby reliably recover the binary data signals that are 
output by the transmitter following the transmission of the preamble 
containing the SE and the MARK and also substantially reducing or 
minimizing the effects of noise. 
With reference to FIG. 1, a preferred embodiment of the invention includes 
an FSK transmitter 10, hereinafter transmitter 10, for outputting a packet 
FSK signal where the frequency of a carrier signal is modulated between 
two predetermined frequencies according to the logical state, logical "0" 
or logical "1", of a packet modulation signal that includes a preamble 
portion and a user data portion. 
The FSK transmitter 10 includes a controller 12 for generating the preamble 
portion, receiving the user data portion from a device that outputs a 
binary data signal, and combining the preamble portion with the user data 
portion to generate the packet modulation signal that is used to modulate 
the carrier signal. The controller 12 also controls the enablement of 
other components that comprise the transmitter 10. Preferably, functions 
such as the generation of the preamble portion of the packet modulation 
signal and combining the preamble portion with the user data portion are 
accomplished using software located in the controller 12. 
Also included in the transmitter 10 is a linear frequency modulated 
oscillator 14 that outputs a carrier signal whose frequency is modulated 
between two predetermined frequencies according to the logical state of 
the packet modulation signal output by the controller 12. Operation of the 
linear frequency modulated oscillator 14 is enabled by the transmitter 
controller 12. 
The transmitter 10 also includes a frequency multiplier 16 for increasing 
the frequency of the FSK modulated carrier signal output by the linear 
frequency modulated oscillator 14 to a suitable radio transmission 
frequency. A variety of radio frequency bands are feasible. Like the 
linear frequency modulated oscillator 14, operation of the frequency 
multiplier is enabled by the controller 12. 
Also included in the transmitter 10 is a power amplifier 18 which amplifies 
that FSK packet radio signal output by the frequency multiplier 16 for 
broadcasting by a transmitting antenna 20. The operation of the power 
amplifier 18 is enabled by the controller 12. 
With reference to FIG. 2, the preferred embodiment of the packet modulation 
signal output by the controller 12 is illustrated. The packet modulation 
signal includes a preamble portion and a user data portion. The preamble 
portion includes a SE byte and a MARK byte. The SE byte includes a 
start bit, eight bits at a logical "0" level, and a stop bit. Similarly, 
the MARK byte includes a start bit, eight bits at a logical "1" level, and 
a stop bit. The user data portion is generally comprised of a known number 
of data bytes where each data byte includes a start bit, eight data bits, 
and a stop bit. By convention, the aforementioned start and stop bits are 
at logical "0" and "1" levels, respectively. Similarly, a SE bit or 
byte conventionally refers to a logical "0" level signal and a MARK bit or 
byte conventionally refers to a logical "1" level signal. The 
aforementioned conventions can, of course, be altered or modified 
according to the requirements of the particular application in which the 
invention is utilized. Moreover, the aforementioned bytes can be replaced 
with data structures of other lengths, like nibbles and words. 
With reference to FIG. 3, an FSK receiver 24, hereinafter receiver 24, is 
illustrated. The receiver 24 includes a receiving antenna 26 and a RF 
amplifier 28 for receiving the FSK packet radio signal broadcast by the 
transmitter 10 and amplifying the received signal, respectively. 
Further included in the receiver 24 is a mixer 30 for combining the FSK 
packet radio signal with a signal from a voltage controlled oscillator to 
convert the FSK oscillator radio packet signal to an i.f. signal. The FSK 
receiver 24 also includes an intermediate frequency (i.f.) filter 32. 
Preferably, the i.f. filter 32 has a bandwidth that approaches the minimum 
necessary bandwidth as determined by the data transmission rate of the 
packet modulation signal and set forth in equation (1). The bandwidth of 
the i.f. filter 32 is also symmetrical about a defined center frequency, 
455 kH.sub.z in a preferred embodiment of the invention. The i.f. filter 
32 operates to filter out substantially all of the signals outside of its 
bandwidth while passing the signal output by the mixer 30. If the i.f. 
signal output by the mixer 30 does not have a frequency spectrum that is 
symmetrical about the center frequency of the i.f. filter, then the 
ability of the receiver to recover the packet modulation signal is 
adversely affected. Also included in the receiver 24 is an intermediate 
(i.f.) amplifier/limiter for amplifying the signal output by the i.f. 
filter 32 until a defined amplitude limit is reached. 
A FM detector 36 is also included in the receiver 24 to recover the packet 
modulation signal by converting the signal output by the i.f. 
amplifier/limiter 34 to a voltage that is proportional to the 
instantaneous frequency of the signal output by the i.f. amplifier/limiter 
34, which is the difference in frequency between the signal output by the 
transmitter and the frequency of the signal output by a VCO. 
An interface circuit 38 is used to condition the recovered packet 
modulation signal output by the FM detector 36 for use by other processing 
hardware. Generally, the interface circuit 38 converts the output of the 
FM detector to a form that has the appropriate voltage levels for digital 
processing. 
The receiver 24 further includes automatic frequency control circuitry 40, 
hereinafter referred to as AFC circuitry 40, that utilizes the preamble 
portion of the recovered packet modulation signal output by the FM 
detector 36 to adjust the frequency of the signal output by a voltage 
controlled oscillator 42, hereinafter referred to as VCO 42, such that the 
spectrum of the i.f. signal output by the mixer 30 is substantially 
symmetrical about the center frequency of the i.f. filter 32, which 
preferably has a bandwidth that approaches the minimum necessary bandwidth 
required for the data transmission rate of the packet modulation signal, 
to allow the recovery of the packet modulation signal while also 
substantially reducing the adverse effects of noise and thereby achieving 
a high signal to noise ratio when the user data portion is recovered by 
the FM detector 36. 
A signal strength detector 44 is used to activate the AFC circuitry 40 when 
a preamble having an adequate amplitude for the receiver 24 to function in 
an acceptable manner is received. The signal strength detector 44 operates 
based upon a signal output by the i.f. amplifier/limiter 34. 
FIG. 4 provides a more detailed diagram of the AFC circuitry 40. The AFC 
circuitry 40 includes a SE sample pulse generator 46a and a MARK sample 
pulse generator 46b that generate a SE sample pulse and a MARK sample 
pulse, respectively, in response to the detection of a preamble of 
adequate signal strength by the signal strength detector 44. The signal 
output by the signal strength detector 44 that is used to activate the AFC 
circuitry 40 is timed to coincide with the recovery of the preamble 
portion of the packet modulation signal by the FM detector 36. In 
operation, the SE sample pulse generator 46a produces the SE sample 
pulse at approximately the center of the time during which the SE byte 
of the packet modulation signal is being recovered by the detector 36. 
Similarly, the MARK sample pulse generator 46b generates the MARK sample 
pulse at approximately the center of the time during which the MARK byte 
of the packet modulation signal is being recovered by the FM detector 36. 
Stated another way, the SE sample pulse generator 46a and the MARK 
sample pulse generator 46 b produce their respective sample pulses to 
coincide with the times when the FM detector 36 is recovering the SE 
byte and the MARK byte, respectively, of the preamble portion. 
The AFC circuitry 40 also includes a SE sample-and-hold circuit 48a and 
a MARK sample-and-hold circuit 48b that sample and hold, for the duration 
of the packet, the voltage level of the 8-bit portions of the SE byte 
and MARK byte, respectively, as they are output by the FM detector 36. An 
adder circuit 50 and a divide-by-two circuit 52 are used to determine the 
average voltage level of the signals held by the SE sample-and-hold 48a 
and the MARK sample-and-hold 48b. A scaling amplifier 54 scales the 
average signal output by the divide-by-two circuit 52 for application to 
the VCO 42. 
The AFC circuitry 40 further includes an analog gate 56 for use in 
preventing the scaled, average signal produced by the scaling amplifier 54 
from being applied to the VCO 42 until the signal output by the signal 
strength detector 44 has been detected and both the SE byte and the 
MARK byte of the preamble portion have been sampled. A tuning correction 
gate 58 is included in the AFC circuit 40 for controlling the analog gate 
56. In operation, the tuning correction gate 58 opens the analog gate 56 
and allows the scaled, average signal at the output of the scaling 
amplifier 54 to be applied to the voltage controlled oscillator 42 only 
after detecting the signal output by the signal strength detector 44 
followed by the SE sample pulse and the MARK sample pulse produced by 
the SE sample pulse generator 46a and the MARK sample pulse generator 
46b, respectively. If the tuning correction gate 58 has not received all 
of the aforementioned signals, then it inhibits the scaled, average signal 
at the output of the scaling amplifier 54 from being applied to the 
voltage controlled oscillator 42 by keeping the analog gate 56 closed. 
The scaled, average signal at the output of the scaling amplifier 54 that 
is applied to the VCO 42 is the voltage necessary to adjust the frequency 
of the signal output by the VCO 42 such that the receiver 24 is tuned, 
i.e., the spectrum of the i.f. signal output by the mixer 30 is 
substantially symmetrical about the center frequency of the filter 34 to 
allow for the recovery of the user data portion of the packet modulation 
signal while also substantially reducing the adverse effects of noise. The 
scaled, average signal that is applied to the VCO 42 is typically called 
the VCO correction voltage and can be mathematically represented as 
follows: 
EQU Vc=(K.sub.1 K.sub.2 (f.sub.m -f.sub.VCO)+K.sub.1 K.sub.2 (f.sub.s 
-f.sub.VCO))/2 (3) 
EQU V.sub.c =K.sub.1 K.sub.2 ((f.sub.m +f.sub.s)/2-f.sub.VCO) (4) 
where V.sub.c is the VCO correction voltage, f.sub.s is the frequency of 
the SE signal in the preamble, f.sub.m is the frequency of the MARK 
signal in the preamble, f.sub.VCO is the frequency of the signal output by 
the VCO 42, K.sub.1 is the frequency to voltage conversion constant of the 
detector 36, and K.sub.2 is the scaling constant of the scaling amplifier 
54. 
Operation of the illustrated embodiment of the invention commences when 
user binary data is applied to the controller 12 of the transmitter 10. 
The controller 12, in response to the application of user binary data, 
assembles the packet modulation signal. More specifically, the controller 
generates, preferably in software, the preamble portion of the packet 
modulation signal and adds the required start and stop bits of the user 
binary data to establish the user data portion of the packet modulation 
signal. In addition, the controller 12 enables the linear frequency 
modulated oscillator 14, frequency multiplier 16, and power amplifier 18. 
Once the aforementioned components are enabled the controller applies the 
packet modulation signal to the linear frequency modulated oscillator 14. 
The output of the linear frequency modulated oscillator is then processed 
by the frequency multiplier 16 and power amplifier 18 to produce an FSK 
packet radio signal that is broadcast via the transmitting antenna 20. 
The FSK packet radio signal is received by the reception antenna of the 
receiver 24 and amplified by the r.f. amplifier 28. 
Initially, the frequencies of the SE and MARK portions of the preamble 
portion of the FSK packet radio signal are each converted to a lower 
frequency using the mixer 30, which combines the SE and MARK portions 
with the signal being output by the VCO 42. The down-converted preamble 
portion of the FSK packet radio signal output by the mixer 30 is then 
applied to the i.f. filter 32 and amplifier/limiter 34 which amplifies the 
preamble portion of the down-converted signal up to a defined limit. If 
the preamble portion of the down-converted signal is of a sufficient 
strength the signal strength detector 44 activates the AFC 40. 
Once the AFC 40 is activated, the SE sample pulse generator 46A and the 
MARK sample pulse generator 46B cause the level of the SE and MARK 
bytes of the preamble being recovered by the FM detector 36 to be sampled 
and held by the SE sample-and-hold 48A and the MARK sample-and-hold 
48B, respectively. The level of the signal held by the SE 
sample-and-hold 48A reflects the difference between the frequency 
associated with the SE portion of the FSK packet radio signal and the 
frequency of the signal being output by the VCO 42. Similarly, the signal 
held by the MARK sample-and-hold 48A reflects the difference between the 
frequency of the MARK portion of the FSK packet radio signal and the 
frequency of the signal being output by the VCO 42. The signals held by 
the SE sample-and-hold 48A and MARK sample-and-hold 48B are averaged by 
the adder 50 and divide-by-2 circuit 52. The resulting signal is then 
scaled by the scaling amplifier 54. The voltage signal at the output of 
the scaling amplifier 54 reflects the difference between the frequency 
midway between the frequencies associated with SE and MARK portions of 
the preamble portion of the FSK packet radio signal and the frequency of 
the signal being output by the VCO 42 as described in equation 4. This in 
turn, reflects the extent to which the frequency spectrum of the i.f. 
signal output by the mixer 30 is not symmetrical about the center 
frequency of the i.f. filter. 
Once the tuning correction gate 58 has received the aforementioned signal 
from the signal strength detector 44 and confirmed that the SE and MARK 
bytes of the preamble have been sampled, it allows the VCO correction 
signal existing at the output of the scaling amplifier 54 to be applied to 
the VCO 42 by opening the analog gate 56. Otherwise the tuning correction 
gate 58 inhibits application of the scaled average signal to the VCO 42 by 
keeping the analog gate 56 closed. In response to the VCO correction 
signal from the analog gate 52, the VCO 42 adjusts the frequency of its 
output signal so that the frequency spectrum of the i.f. signal output by 
the mixer 30 when it mixes the signal output by the VCO 42 with the 
received signal is symmetrical about the center frequency of the i.f. 
filter 32. By adjusting the frequency of the signal output by the VCO in 
this manner, the receiver 24 can recover the packet modulation signal 
while also substantially reducing the adverse effects of noise and thereby 
produce a high SNR. 
The foregoing description of the invention has been presented for purposes 
of illustration and description. Further, the description is not intended 
to limit the invention to the forms disclosed herein. Consequently, 
variations and modifications commensurate with the above teachings and the 
skill or knowledge in the relevant art are within the scope of the present 
invention. The preferred embodiment described hereinabove is further 
intended to explain the best mode known of practicing the invention and to 
enable others skilled in the art to utilize the invention in various 
embodiments and with the various modifications required by their 
particular applications or uses of the invention. It is intended that the 
appended claims be construed to include alternate embodiments to the 
extent permitted by the prior art.