Apparatus and method for radio channel selection

An improved apparatus and method for selecting a carrier frequency channel etween two radios are disclosed wherein one radio transmits a cyclic test signal sequentially on a set of frequencies for a period of time at each frequency sufficient to allow a second radio fo sweep through the entire set while pausing at each frequency for a period of time sufficient to receive a cycle of the test signal. The second radio utilizes the detected test signal to evaluate the transmission quality of the carrier frequency channel upon which it is received. After the second radio recognizes the test signal on a carrier frequency for a second time, it ceases evaluation and transmits an answer signal upon the carrier frequency with the highest transmission quality. The first radio is enabled to detect the answer signal by initiating reception at the end of each test signal transmission and sweeping through the entire set of frequencies at the rate of sweep of the second radio.

BACKGROUND OF THE INVENTION 
The present invention is related to a radio communication system and more 
particularly is related to a radio communication system capable of 
automatically evaluating carrier frequencies for transmission quality and 
selecting the frequency with the highest quality for operation. 
Broadcast communication involving mobile radios is more difficult than 
fixed radio communication. Along with the normal broadcast communication 
problems of propagation and interference, the mobile radio has the 
additional problems of low power limitation, antenna coupling losses, poor 
antenna patterns, and changing antenna losses and patterns. Full time 
mobile radio communication requires the availability of several carrier 
frequency channels spread across the broadcast spectrum of interest to 
accommodate short-term changes in ionospheric sky-wave propagation. Fixed 
radio communication circuits can take advantage of multiple frequency 
assignments and switch to frequencies which are propagating well and have 
low interference. A method of frequency selection is taught in U.S. Pat. 
No. 3,617,891, issued to D. H. Covill, entitled OPTIMUM FREQUENCY 
DETERMINING RADIO COMMUNICATION SYSTEM, filed May 26, 1969. 
The Covill patent discloses a structured frequency selection method 
suitable for fixed radio circuits. The method comprises a synchronous 
search mode which is entered first and an asynchronous search mode, 
entered only upon the failure to qualify a frequency channel during the 
synchronous search. The asynchronous search mode entails transmission of a 
test signal by a first radio on each frequency in a set of frequencies 
taken at a slow rate. The receiver of a second radio is tuned at a higher 
rate to each in the set of frequencies for detection of the test signal on 
the frequency of transmission. The asynchronous mode requires a degree of 
synchronism in that the second radio, upon detecting the test signal, must 
immediately retransmit it at the same frequency. The first radio must then 
detect the test signal, evaluate the signal-to-noise ratio at the carrier 
frequency, and transmit an answer signal to the second radio if the 
transmission quality of the frequency channel is acceptable. This method 
does not qualify every available carrier frequency channel in the set of 
assigned frequencies because it is interrupted when the first channel is 
qualified. Hence, the freqency channel with the highest transmission 
quality is not necessarily selected. Furthermore, the multiplicity of 
transmissions required limits the reliability of the method and extends 
the time required to evaluate the whole set of frequencies. 
Operators of battery operated mobile radios on the move in the field 
presently require a rapid, reliable means of determining the optimum 
frequency for use at a given time over a given path. An asynchronous 
selection method is preferred because the time required for antenna tuning 
before transmission is uncontrolled; it varies according to the constantly 
changing location of a mobile radio. The method must involve as few 
transmissions as possible in order to preserve battery power. Minimization 
of transmission is also important when the security of the circuit is at 
stake. Finally, a method is desired which will evaluate all available 
carrier frequencies for the purpose of selecting the best among them. 
SUMMARY OF THE INVENTION 
The present invention provides an improved apparatus and method for 
selecting an optimum radio carrier frequency for transmission between two 
radios. The unimproved apparatus is of the type where a test signal is 
transmitted from a first radio for a period of time on each of a set of 
carrier frequencies taken sequentially at one rate and a second radio is 
tuned to each in the set of carrier frequencies at a higher rate to search 
for the test signal. In the unimproved apparatus, the second radio detects 
the test signal asynchronously on a succession of carrier frequencies and 
retransmits the test signal after each detection to the first radio for 
evaluation of the transmission quality of the carrier frequency upon which 
the test signal has been received. The present invention provides an 
improvement to this apparatus by providing means to switch the first radio 
at the end of test signal transmission on a carrier frequency to a 
receiving mode and to sequentially tune the first radio at the higher rate 
to each frequency in the set to search for an answer signal from the 
second radio. Means are provided at the first radio to generate a 
characteristic test signal for transmission at each carrier frequency. At 
the second radio, when the test signal is received on a carrier frequency, 
the signal-to-noise ratio at that carrier frequency is computed by an 
evaluation means according to a method of the invention. After the second 
radio receives the test signal upon a carrier frequency for the second 
time, answer signal means tune it to the carrier frequency upon which the 
highest signal-to-noise ratio has been detected for transmitting an answer 
signal thereon. This improvement results in asynchronous evaluation of all 
available carrier frequency channels and selection of the channel having 
the highest signal-to-noise ratio. 
OBJECTS OF THE INVENTION 
An object of the present invention is to provide improvements in 
asynchronous radio channel selection apparatuses and methods. 
Another object is to provide an improved asynchronous radio channel 
selection apparatus by which all available carrier frequencies can be 
evaluated. 
Yet another object is to provide an improved asynchronous radio channel 
selection apparatus which reduces the number of transmissions required to 
select a channel, thereby reducing total power required to accomplish the 
selection and preserving the security of the circuit. 
A further object is to provide an apparatus and method for evaluating the 
signal-to-noise ratio of a carrier frequency radio channel. 
A still further object is to provide an improved asynchronous radio channel 
selection apparatus which ensures selection of the channel with the 
highest signal-to-noise ratio of a set of available channels. 
These and other objects of the invention will become more readily apparent 
from the ensuing description when taken together with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is illustrated a timing diagram for an 
improved method of radio carrier frequency channel selection. A set of 
candidate carrier frequency channels, three for example, can be 
established from which to select one for transmission between two radios. 
Under control of a channel selection apparatus, described in greater 
detail hereinbelow, a first radio can be caused to transmit a test signal 
on each frequency taken at a predetermined rate. For test signal 
transmission, the first radio operates in the transmit mode (XMIT), shown 
in timing diagram 10, during which it tunes to the first frequency in the 
set, channel 1 for example, and transmits a test signal, described in 
greater detail hereinbelow, for a period of time long enough to permit a 
second radio to receive for a period of time, for example 0.65 seconds, on 
each of the candidate channels during the transmission. This is shown in 
timing diagram 10 as TUNE1/XMIT1. After the first radio transmits the test 
signal on channel 1 for a period based upon the number of carrier 
frequencies in the set, it receives for a period of time, for example 0.65 
seconds, on each of the candidate channels in a search for an answer 
signal. After the search, shown as R1/R2/R3 in timing diagram 10, the 
channel selector then tunes the first radio for transmission of the test 
signal on the next candidate channel. This is shown as TUNE 2/XMIT 2 in 
timing diagram 10. The first radio repeats the transmit sequence until it 
recognizes an answer transmission from the second radio. Upon recognition 
of the answer transmission, the channel selection apparatus stops the 
transmit sequence while tuned to the recognized channel and switches the 
first radio to normal operation, permitting an operator to communicate. 
As shown in timing diagram 11 of FIG. 1, the second radio, also under 
control of a channel selection apparatus, in a receive (RCV) mode enters a 
continuous tuning sequence to receive for a period of time, for example 
0.65 seconds, on each of the candidate channels. When the first radio 
transmits on a channel to which the second station is tuned, assuming the 
channel propagates, the channel selection apparatus will recognize the 
transmission and assign a value to the quality of the received signal 
according to a method described in greater detail hereinbelow and to the 
frequency channel upon which it was propagated. 
Thus, in timing diagram 11, the second radio channel selection apparatus 
recognizes channel 1 and assigns a value to it. Channel 2 is also 
recognized and qualified. Channel 3 does not propagate, no recognition 
occurs and no value is assigned. 
When the selection apparatus in the second radio recognizes channel 1 for 
the second time, it tunes the radio to the channel having the highest 
observed signal quality, channel 2 in this case, and transmits an answer 
signal on the selected channel. This is shown as the TUNE 2/XMIT 2 
sequence at the end of timing diagram 11. This answer signal is 
transmitted for a period of time, 18 seconds for example, to ensure that 
the first radio will be tuned to and receiving on the selected channel at 
some time during the answer transmission. After the channel selection 
apparatus in the second radio ends the answer transmission, it stops the 
selection process and switches the radio to voice operation. 
The portable AN/PRC-104 radio set illustrated in FIG. 2 provides high 
frequency, two way communication between military personnel in a field 
environment. It is representive of the type of radio to which the method 
of the invention may be applied. The AN/PRC-104, represented in FIG. 2 as 
radio 35, comprises six major subassemblies. The control head 23 allows 
the operator to control radio 35 by providing channel selection, audio 
signal coupling, and mode selection. A modulator/demodulator 12 performs 
modulation and frequency conversion of audio signals during transmission 
to produce a transmit RF signal; during reception it performs frequency 
conversion, filtering and audio detection. Digitally controlled 
synthesizer 13 develops local oscillator (LO) signals and a beat frequency 
oscillator (BFO) signal used for conversion and detection. RF power 
amplifier 14 amplifies the transmit RF signal during transmission. A 
digitally controlled harmonic filter 15 removes spurious received signals 
and suppresses transmitter harmonics. A digitally controlled antenna tuner 
16 automatically matches antenna impedance to the radio set. 
During transmission, a voice signal from a handheld microphone, not shown, 
is routed through control head 23 to the modulator/demodulator 12 along 
signal path 20. Modulator/demodulator 12 utilizing the BFO and LO signals 
mixes the audio to produce a 2 to 30 MHz transmit RF signal. The specific 
frequency of transmit RF signal is determined by the LO signals developed 
in synthesizer 13. Production of the LO signals in synthesizer 13 is 
controlled by binary coded decimal (BCD) signals on signal bus 17. 
Manually controlled channel selection switches, not shown, on control head 
23, connected to signal bus 17, produce the BCD. 
The transmit RF output from the modulator/demodulator is routed to RF power 
amplifier 14, which utilizes transmission gain control (TGC) to equalize 
the power level of all transmitted signals, and then through harmonic 
filter 15 to suppress transmitter harmonics. Binary coded channel select 
signals from the channel selection switches on control head 23 are 
transmitted along signal bus 24 to harmonic filter 15. These signals 
energize specific filter select relays and connect the proper filter for 
the operating frequency period. The transmit RF output is routed from 
filter 15 through antenna tuner 16 to antenna 34. Antenna tuner 16 
responds to the BCD signals on bus 17 to automatically transform actual 
antenna impedance to a preset value, presenting a constant load to the 
radio set. 
The received RF signal follows the same path, in reverse, as the transmit 
RF with the exception that amplifier 14 is bypassed when switch 35 is 
activated by a signal on keyline 22. Spurious received signals are removed 
by harmonic filter 15 and the signal is applied directly to 
modulator/demodulator 12. The receive RF is then converted to audio in 
modulator/demodulator 12, which includes an amplifier utilizing automatic 
gain control (AGC) to equalize the power level of all received signals, 
and routed to the microphone along signal path 19 through control head 23. 
During operation of radio 35 with a handheld microphone, a press to talk 
(PTT) switch on the handset is depressed during transmission causing a 
grounding signal to be transmitted through control head 23 on signal path 
21 to modulator/demodulator 12 to control keyline 22. When the carrier 
frequency channel is changed for transmission, a pulse (.DELTA.f) is 
transmitted on signal path 18 to modulator/demodulator 12 where a 
tune-start command is generated to initiate a tune cycle at antenna tuner 
16. During an antenna tuning cycle, tuner 16 develops a tune in progress 
(TIP) signal. Modulator/demodulator 12 outputs an audio tone on path 19 
whenever the TIP signal is present. If local conditions do not permit 
tuning antenna 34 to the desired transmission frequency, 
modulator/demodulator 12 produces a beeping tone on path 19 when the TIP 
signal ends. 
Referring now to FIG. 2 and FIG. 3, wherein like reference numerals 
designate like or similar elements throughout the two views, there is 
illustrated in FIG. 3 a channel selector apparatus which operates 
according to the invention to control radio 35. The channel selector 
illustrated in FIG. 3 comprises microprocessor 42 controlled by selector 
control panel 46 and selector interface control 45. Digitally controlled 
display 47 provides control information relevant to the operation of the 
illustrated selector to an operator. Display panel 47 can comprise a 
digitally controlled liquid crystal display operated by binary coded 
decimal data interchanged with microprocessor 42 on display data bus 48. 
Multiplexer 43, connected to and under the control of switch panel 46, 
allows filter select data to be fed to harmonic filter 15 from either 
control head 23 or microprocessor 42 on signal bus 24. Multiplexer 44, 
also connected to and under the control of selector switch panel 46, 
allows channel frequency data to be fed to synthesizer 13 and antenna 
tuner 16 from either control head 23 or microprocessor 42 on signal bus 
17. 
When radio 35 is under the control of the channel selector illustrated in 
FIG. 3, the following circuits controlled by microprocessor 42 are 
activated: key circuit 50, TIP detector 53, receive signal evaluator 52, 
test tone generator 54, and .DELTA.f circuit 55. 
The .DELTA.f circuit 55, which can comprise a one-shot multivibrator, is 
connected to modulator/demodulator 12 through OR gate 56, spliced into 
signal path 18. OR gate 56 also has an another input the .DELTA.f signal 
supplied by control head 23 during normal operation of radio 35. Signal 
path 40 provides connection between an output of microprocessor 42 and the 
input of .DELTA.f circuit 55. 
Key circuit 50, which can comprise a gated transistor with its collector 
connected to signal path 21, its emitter to ground, and its base to 
microprocessor 42 on signal path 59, provides the push to talk signal to 
modulator/demodulator 12 when radio 35 is transmitting under the control 
of the channel selector. Tune in progress tone decoder 53 is connected 
between audio receive input line 19 and an input of microprocessor 42 on 
signal path 51. Also connected between receive audio signal path 19 and an 
input of microprocessor 42 on signal path 57 is receive signal evaluator 
52 described in more detail hereinbelow. Test tone generator 54, also 
described hereinbelow, is connected to microprocessor 42 on signal paths 
59 and 58 and to transmit audio signal path 20. 
The operation of the channel selector illustrated in FIG. 3 comprises three 
modes, LOAD, XMIT, and RCV, selected by operation of an appropriate mode 
select switch, not shown, on switch panel 46. 
In the LOAD mode radio 35 is controlled entirely from control head 23 and 
the selector becomes functionally transparent to radio 35. The LOAD mode 
is used to enter the set of carrier frequencies in BCD into microprocessor 
42 and to reject loaded carrier frequencies to which antenna tuner 16 
cannot tune. A carrier frequency loading sequence may, for example, be 
commenced when an appropriate reset switch, not shown, on switch panel 46 
is depressed to clear the appropriate memories in microprocessor 42. An 
appropriate step switch, not shown, on selector switch panel 46 which 
indexes the numeric display on display 47 is depressed until the numeral 1 
appears on display 47. The first assigned carrier frequency is selected on 
the channel selection switches located on control head 23. An appropriate 
channel load switch, not shown, on selector switch panel 46 is depressed 
thereby loading filter select data through multiplexer 43 and channel 
frequency data through multiplexer 44 together with the binary code for 
"1" from bus 48 into microprocessor 42. The next assigned carrier 
frequency is thereafter dialed into radio control head 23, the step switch 
on switch panel 46 is depressed, incrementing the channel display number 
to two, and the load/step sequence is repeated until the data for all 
assigned carrier frequencies has been loaded into microprocessor 42 
together with assigned binary codes which identify the channels. When the 
last carrier frequency has been loaded, the selector illustrated in FIG. 3 
is ready for operation in either the XMIT or RCV mode. In the XMIT mode 
the selector illustrated in FIG. 3 takes control of radio 35. The operator 
waits until the channel selection is made before attempting to 
communicate. When the mode switch on switch panel 46 is set to XMIT the 
selector interface control 45 initiates an appropriate XMIT program stored 
in microprocessor 42, switches multiplexer 43 and multiplexer 44 so that 
data on signal busses 24 and 17 will originate at microprocessor 42, and 
switches the display panel control to accept channel number data from 
microprocessor 42 on display data bus 48. Microprocessor 42 then tunes 
radio 35 to the first assigned carrier frequency through busses 24 and 17, 
causes .DELTA.f circuit 55 to output of .DELTA.f pulse, and keys radio 35 
through key circuit 50. When radio 35 is first keyed it causes antenna 
tuner 16 to tune to the assigned frequency. While antenna tuner 16 is 
tuning, the audio tone generated in response to the TIP signal is fed to 
microprocessor 42 through TIP decoder 53 which causes microprocessor 42 to 
idle while the tune is in progress. 
If antenna tuner 16 is unable to tune to a channel frequency the beeping 
tone following the TIP signal, audible to the operator through the 
handheld microphone, has no effect upon the XMIT mode. The second radio 
simply will not receive the test signal on the untuned channel, continuing 
unaffected in the RCV mode. The first radio, at the end of the test signal 
transmission period will proceed with the XMIT mode. 
When the TIP signal stops and the channel is successfully tuned, 
microprocessor 42 provides a test signal through test tone generator 54 to 
radio 35 for transmission at the selected frequency. This test signal is 
transmitted for a period based upon the number of candidate carrier 
frequencies loaded into the selector. After test signal transmission, 
microprocessor 42 causes radio 35 to be unkeyed and sequentially tuned for 
0.65 seconds to each of the candidate carrier frequencies. Following the 
reception sequence, the next assigned channel frequency is fed to radio 35 
through busses 24 and 17 from microprocessor 42 and the above described 
sequence is repeated for each of the candidate frequencies. During the 
receive sequence of the XMIT mode, an answer tone package from the second 
radio will eventually be recognized on one of the candidate frequencies by 
microprocessor 42 through receive signal evaluator 52. When an answer 
signal is recognized microprocessor 42 stops the selector with radio 35 
tuned to the selected frequency. At any time during the transmit mode the 
reset button on switch panel 46 may be pressed which will cause 
microprocessor 42 to begin the XMIT mode again. 
In the RCV mode microprocessor 42 provides filter and channel select data 
on paths 24 and 17 to radio 35 for periods of 0.65 seconds at each carrier 
frequency channel and simultaneously feeds the corresponding channel 
number to display panel 47 by way of display data bus 48. The selector 
cycles radio 35 continually through the channel frequencies. When radio 35 
is tuned to a frequency on which a test signal is being transmitted, 
signal evaluator 52 recognizes the test signal, assigns a value to its 
quality according to a method described hereinbelow, and passes the data 
to microprocessor 42. Microprocessor 42 acknowledges the recognition, 
determines if the assigned value is the highest observed, and, based on 
this evaluation, keeps track of the highest quality received frequency 
channel for use when a cycle recognition occurs. 
Cycle recognition occurs when any frequency channel has been recognized 
twice. Upon cycle recognition microprocessor 42 activates .DELTA.f circuit 
55, feeds filter and channel select data of the best received frequency 
channel to radio 35 through multiplexers 43 and 44, displays the channel 
number of the selected channel on display 47, keys radio 35 for 
transmission, and feeds the answer tone to radio 35 for 18 seconds of 
transmission. After transmission of the answer tone signal, microprocessor 
42 stops selector operation with radio 35 tuned to the selected frequency. 
If antenna tuner 16 cannot tune the frequency for transmission of the 
answer signal the beeping tone is heard, the operator switches to LOAD and 
presses an appropriate reject switch, not shown, on panel 46. An 
appropriate flag is entered in microprocessor 42 causing it to skip the 
rejected channel in further RCV operations, and RCV is reselected and 
resumed when the reset switch in panel 46 is depressed. 
If the reset switch on switch panel 46 is depressed at any other time in 
the RCV mode, microprocessor 42 clears its memory of all recognition and 
value data and restarts reception at channel 1. 
Channel quality evaluation is accomplished through detection of a test 
signal comprising four tone pairs transmitted with power levels which vary 
over an 18 db range. Each tone pair is assigned a value. In order to keep 
the total power of the test signal constant, a single drone tone at 2450 
Hz is included in the package and its power level is varied to compensate 
for the level variations of the tone pairs. FIG. 4a shows the composition 
of the test signal tone package as it progresses through an entire value 
sequence. FIG. 4b shows the spectra of the tone pairs over the range of 
values which microprocessor 42 may be programmed to assign upon 
recognition of the respective tone pair. Thus the tone pair comprising the 
770 Hz and 1633 Hz tones with the lowest total power is assigned the 
highest value of four, and so on down through the value of one for the 852 
Hz/1633 Hz pair. The final value assigned to a frequency channel is 
determined by an appropriate routine in microprocessor 42 at the end of 
the 0.65 second period during which radio 35 is tuned to a carrier 
frequency. This valid value is derived as shown in FIG. 4c and is stored 
by microprocessor 42 together with the channel identification until either 
a higher value is derived or the test signal is recognized on a carrier 
frequency for a second time. When a second recognition of a carrier 
frequency occurs, the identification of the carrier frequency having the 
highest signal-to-noise ratio is retrieved by microprocessor 42 and an 
answer signal comprising the 852 Hz/1633 tone pair is broadcast on that 
frequency. 
Receive signal evaluator 52 shown in FIG. 3A, has the capability to 
recognize a total of 16 tone pairs. During the RCV mode it is possible to 
detect not only the four acceptable tone pairs but also 12 other 
unacceptable tone pairs, thus providing a false alarm capability. Filter 
network 60, which can be Cermatek part nos. 1296 and 1297, is designed to 
strip off the drone tone while passing the upper and lower tone 
frequencies to tone decoder 61. The tones are decoded in decoder 61, which 
may be a Telaris 7516-1 touch tone decoder, and the values assigned with 
acceptable values appearing on pins 1 through 4, and unacceptable values 
on gates 5 through 16 being fed to diode OR network 62 where a false alarm 
signal is developed. 
The ability of signal evaluator 52 to recognize a given tone pair depends 
primarily upon the signal-to-noise ratio at the input of tone decoder 61, 
directly related to the channel signal-to-noise ratio. Acceptable tone 
pairs are transmitted at preset levels which range over 18 db as shown in 
FIG. 4B. The tone pair with the least power can be recognized by tone 
decoder 61 only if the noise is sufficiently below the level of these 
tones at its input. Therefore, a frequency channel on which the least 
power pair is recognized is likely to have the better signal-to-noise 
ratio than the frequency channel on which only the 852 Hz and 1633 Hz pair 
is recognized. 
Due to AGC and TGC performance characteristics of radio 35, the total power 
in the test signal is kept nearly constant by putting more power into the 
drone tone (see FIG. 4B) as the power in the tone pairs is reduced. 
Presence of the drone tone does not degrade the performance of signal 
evaluator 52. 
Tone pairs are generated by test tone generator 54 shown in FIG. 3B. 
Microprocessor 42 controls tone generation through control decoder 68, 
which can be an RCA analog multiplexer 4051 or equivalent, configured as a 
two-line to four-line decoder. Decoder 68 provides control signals to tone 
pair generator 69, which can comprise Motorola device MC14410 or 
equivalent. Tone generator 69 is configured to continually generate the 
1633 Hz tone whenever an additional tone is enabled by control decoder 68. 
If an additional tone is not enabled, as when decoder 68 is inhibited 
during a drone tone period for example, generator 69 will not produce the 
1633 Hz tone. Drone tone generator 65, which can be Motorola device 
MC14410 or equivalent, is configured to provide a continual drone tone at 
2450 Hz. 
Decoder 67, which can be an RCA analog decoder 4052 or equivalent, is 
connected to allow microprocessor 42 to reconfigure resistor array 66 in 
such a manner that the signal power levels of the tone pairs generated by 
pair generator 69 and the drone generator 65 can be shaped to produce the 
characteristics illustrated in FIG. 4B. Drone tone generation is maximized 
during the drone tone period when both decoders are inhibited. Inhibiting 
decoder 68 suppresses the tone pairs. Inhibiting decoder 69 removes 
resistor array 66 from the output of generator 65 thus preventing it from 
dividing drone tone power. Output buffer 70 can comprise an appropriately 
designed operational amplifier; it provides impedance matching to transmit 
audio line 20. Analog gate 71, which can comprise Microsystems part 7510, 
is controlled by the same control line 59 from microprocessor 42 used to 
activate key circuit 50. 
Microprocessor 42, in the preferred embodiment comprises RCA part CDP 1802, 
which, with reference to a manual published by RCA entitled "User Manual 
for the CDP1802 COSMAC Microprocessor", 1976, can be appropriately 
programmed to control and carry out a sequence of steps necessary to 
accomplish the method of the invention. General flow diagrams suitable for 
designing a control program for microprocessor 42 are illustrated in FIGS. 
5a and 5b. 
In FIG. 5a, subroutine 75 comprises the LOAD mode operations performed by 
microprocessor 42. In the LOAD step 76, a channel identification code is 
taken from bus 48 and stored together with frequency data from bus 17 and 
filter data from bus 24. The mode switch on panel 46 is sampled in the 
LOAD decision step 77. If the switch remains in the LOAD position, step 76 
is repeated for the next set of channel data. If the reset switch on panel 
76 is depressed during the LOAD mode microprocessor 42 will empty its 
memory of all channel data. 
If the result of step 77 is negative, the mode switch is sampled in 
decision step 78 to determine which of the remaining two modes have been 
selected. 
In the XMIT mode, shown in subroutine 80 of FIG. 5a, step 81 is executed 
first causing frequency and filter data associated with, for example, 
channel 1 to be retrieved from memory and placed on the appropriate signal 
busses. The .DELTA.f circuit 55 is pulsed and the expiration of the TIP 
signal is awaited. In step 82 key circuit 50 is activated, test tone 
generator 54 is enabled, the test signal is transmitted on the selected 
frequency, and the channel identification is placed in bus 48 to be 
displayed on panel 47. The length of the test signal transmission can be 
varied by an appropriate timing subroutine included in step 82. At the end 
of test signal transmission, an answer search subroutine comprising steps 
83, 84, 85, and 86 will cause radio 35 to be unkeyed and tuned 
successively to each in the set of selected frequency channels. When the 
end of the answer search is reached in decision step 86, the next channel 
in the set is tuned, step 87, and the next channel selected for test 
signal transmission. When the answer signal is recognized, the channel 
identification remains displayed on panel 47 and microprocessor 42 idles 
leaving radio 35 tuned for operation to the channel on which the answer 
signal was received. 
In FIG. 5b, subroutine 90 comprises the RCV mode operations. Step 91 causes 
radio 35 to be tuned to the first non-rejected frequency in the set; 
filter and frequency data are put out and the channel identification is 
displayed. An appropriate timing subroutine can be nested in step 91 to 
control the time spent tuned to any channel. If no test signal is 
received, decision step 92 is exited for step 93 wherein microprocessor 42 
notes the non-recognition. The reset function, described hereinabove, is 
performed in decision step 94 and step 96. If the reset switch has not 
been depressed, the next channel is tuned in step 95, its identification 
displayed and the answer signal searched for in step 92. This process is 
repeated until a test signal is detected when step 92 is exited and 
subroutine 100 entered. 
Subroutine 100, also illustrated in FIG. 5b, first tests the accumulated 
data for the channel on which the test signal is detected for a reject 
indication. If the channel has been rejected, decision step 101 is exited 
and subroutine 90 entered at point E. If no rejection has occurred, the 
false alarm decision block 102 is entered, causing microprocessor 42 to 
idle if a false alarm is indicated by receive signal evaluator 52. If no 
false alarm occurs, the accumulated data for the channel is investigated 
in step 103 to determine whether a prior recognition of a test signal has 
occurred in the channel. If not, the signal-to-noise ratio is computed by 
microprocessor 42 according to the method of the invention utilizing the 
outputs of evaluator 52. Microprocessor 42 also places a recognition 
indication at an appropriate location in memory where information 
regarding the respective channel is stored. Microprocessor 42 also 
compares the computed value against a computed value residing in an 
appropriate memory location to keep a record of the highest computed 
value. The higher value together with the identification of the channel on 
which it was calculated is kept. 
If a prior recognition has occurred, step 106 is executed to determine 
whether a non-recognition has intervened. This is necessary to prevent a 
termination of the RCV mode within a single test signal transmission. If a 
prior non-recognition has occurred, radio 35 is tuned in step 107 to 
transmit the answer signal. If radio 35 cannot be tuned, the beeping tone 
alerts the operator to perform the reject operation described above--steps 
108 and 109. After a reject operation, subroutine 90 is entered at point 
E. 
If radio 35 can be tuned, it is keyed, the answer tone is transmitted, the 
channel identification is displayed and microprocessor 42 idles leaving 
radio 35 tuned for operation to the channel on which the answer signal was 
transmitted. These operations are performed in steps 110 and 111. 
The foregoing description taken together with the appending claims 
constitutes a disclosure of a specific embodiment of the subject invention 
such as to enable one reasonably skilled in the electronics and radio 
engineering arts, and having the benefit of the teachings contained 
herein, to make and use the invention. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings, and, it is therefore understood 
that, within the scope of the disclosed inventive concept, the invention 
may be practiced otherwise than as specifically described.