Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a radio frequency system for transmitting
a plurality of tones.
Data transmitters currently in use in the high frequency (~.F.) band
are single side band (SSB) equipments which have been pressed into service.
These are essentially linear amplifiers basically designed for SSB suppressed
carrier mode of operation. This basic design, while eminently suited to SSB
voice and single tone keyed transmission (CW) are unsuited to multi-tone trans-
missions such as Frequency Diversity Multiplex (teletype) and Kineplex data.
With these multi-toned transmission, the transmitter peak power handling cap-
ability limits the average output power per tone.
"Kineplex" is a registered trade mark of Collins Radio Co. and re-
fers to a multi-tone audio frequency phase modulated system. In this system,
a family of audio tones are individually modulated for fixed periods of time
called "frames". The rotation of a phase vector in increments of 90 between
successive frames produces bits of intelligence for each tone so modulated for
each frame,
Frequency Diversity Multiplex is defined as the transmission of
duplicate information at different frequencies. When these frequencies lie
within the bandwidth of a single transmitter, reference is made to "inband
frequency diversity".
While some degree of peak over-driving is acceptable in multiplex
frequency diversity teletype systems, Kineplex, which is a phase modulated
system, if far more critical of over-driving or clipping, which results in the
generation of spurious signals which in turn results in an increase of back-
ground noise on the transmitted signal. The Collins AN/SRC-23A(V) equipment
is a transmitter specifically designed for Kineplex data use and it has an
automatic load control (ALC) circuit, which controls the transmitter power RF
gain. This circuit has a typical attack time of 1.5 milliseconds and an
average decay time of 150 milliseconds. Thus this transmitter, to a large
degree, regulates itself to maintain unclipped operation.
The present invention provides a new approach to the problem of
providing high average power per tone in multi-tone systems. According to the
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invention, there is provided a radio frequency system for transmitting a
plurality of RF tones each tone being generated separately using common osci-
llators and independent modulators comprising a plurality of means for gener-
ating RF tones, passive means for combining the RF tones to form a single
resultant RF signal, and means feeding the resultant RF signal to an antenna.
The invention will now be further described in conjunction with the
accOmpanying block diagram of a system in accordance with the invention.
Referring to the drawing, it can be seen that this system is adapted
to utilize sixteen different tone inputs. However, it will be obvious that the
principles of the invention could be used with fewer or with more tone inputs
than shown in this exemplary embodiment.
A tone input, e.g. tone input No. 1, is fed to a mixer 10 over line
11. The mixer 10 is also fed a frequency, e.g. 580 kHz, over line 12 from a
first converter oscillator 22. The output of mixer 10 is thus a modulated IF
signal on line 13 which may be fed through a filter 14 to remove one side band,
e.g. the lower side band. The signal is then fed to another mixer 15 having an
RF signal fed to it over line 16 from RF translator oscillator 23. The output
of mixer 15 is fed over a line 20 to a power amplifier 21.
All of the IF mixers are fed by the first converter oscillator 22
and all of the RF mixers are fed by RF translator oscillator 23.
The other tone inputs 2 to 16 are handled in the same way as tone
input 1.
The outputs of power amplifiers 21 and 24, for tones 1 and 2, are
combined in a combining balun 25. Similarly, as can readily be seen from the
drawing, tones 3 to 16 are combined in combining baluns also, resulting in 8
outputs from the first group of baluns. The outputs of these baluns are again
combined in a further group of baluns generally indicated at 30 resulting in
four outputs. These four outputs are combined in baluns generally indicated
at 32 resulting in two outputs and these outputs are combined in a balun 33
resulting a final single resultant output at 34. The output 34 is fed to an
antenna coupler 35 and thence to an antenna 36.
The system according to the invention prevents the generation of
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intermodulation products by the simple expedient of amplifying only one tone
in each of a number of RF power amplifiers.
The amplifiers are broadband and each covers the entire section of
the spectrum in use. For example, HF would cover 1.6 - 30 MHz and UHF would
cover 225-400 MHz
The combining baluns are simply passive couplers which are known
in the art. Passive couplers are constant impedance devices which pass a sig-
nal from one port to a multiple number of output ports while providing isols-
tion between all the output ports. The insertion loss of each channel is very
small in the order of 1-3dB, and the inter-channel isolation relatively large;
typically 20-40dB. Being passive components, they transmit signals equally
well in both directions and thus may be used as combiners ~as here) as well as
splitters. A number of types of passive couplers hybrid units are available
from Hatfield Instruments Ltd., Burlington Way, Plymouth PL5 3LZ, Devon,
Great Britain.
Antenna couplers are, of course, well known. Antenna coupler 35 may
comprise a four-stage high-Q tuned coupler, for example, which will reject
harmonics.
With the system according to the invention, intermodulation frequen-
cies may be as much as 60dB down from the tone amplitudes, and are thus not
significant.
The audio tones l to 16 may lie between 300 and 3000 Hz. Transla-
ted to a final RF of 3.5 MHz, these would lie between 3,500,300 Hz and
3,503,000 Hz.
As shown in the drawing, transmitter excitation involves separate
mixing and IF/RF amplification for each of the channel input tones. Although
not shown, buffer amplifiers may be provided between the oscillators 22 and
23 and each of the mixing circuits such as 10 and 15.
The power amplifiers 21, 24, etc. may be up to 300 Watts peak enve-
lope power (PEP) linear amplifiers of integrated circuit construction.
The present invention provides a number of advantages over prior
art transmitting arrangements, such as the following:
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(1) high power per tone with minimal generation of intermodulation
products;
(2) capability of solid state construction resulting in small size;
(3) high efficiency;
(4) compatible with existing equipment when all RF amplifiers are
driven in parallel for voice or single channel FSK use;
(5) cheaper operation, in terms of power per tone, than conven-
tional designs;
(6) by combining various numbers of amplifiers, any size trans-
mitter can be produced.
Regarding item (1) above, when using only 300 Watt power amplifiermodules, the output power per tone for 16 tones is equivalent to what is pre-
sently accomplished by 50 kilowatt transmitters. With presently used trans-
mitters, the power per tone decreases as the number of tones used increases.
This is not a problem with the present invention.
The system shown in the drawing has been assumed to transmit the
upper side bands but a similar system can be used in conjunction with it to
handle the lower side bands.
