CA1164053A - High speed data transmitter - Google Patents

Abstract

A B S T R A C T
Data transmitters are known which are essentially linear amplifiers designed for single side band suppressed carrier mode of operation and while these are suitable for voice and single tone keyed transmission, they are less satisfactory for multi-tone transmissions because the transmitter peak power handling capability limits the average output power per tone. The present invention enables high average power per tone in a multi-tone transmitting system. A plurality of means are provided for generating RF tones. The tone modulated RF signals are then combined to form a single resultant modulated RF
signal and the resultant RF signal is fed to an antenna.

Description

~ ~ ~4053 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 ~ ~ 64053 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 1 1 ~4053 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:

1 1 64~53 (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.

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A radio frequency system for transmitting a plurality of RF tones each tone being generated separately using common oscillators and independent modulators comprising a plurality of means for generating RF tones, means for passively combining the tone modulated RF carrier signals to form a single resultant modulated RF signal, and means feeding said resultant RF signal to an antenna.

2. A system as claimed in Claim 1 wherein each said RF tone generating means comprises an IF conversion stage having a tone input and feeding an RF
conversion stage which, in turn feeds a power amplifier.

3. A system as claimed in Claim 2 wherein the IF conversion stage feeds the RF conversion stage through a filter which removes one side band of fre-quencies.

4 A system as claimed in Claim 2 or 3 wherein the outputs of the power amplifiers are combined in pairs in combining baluns, the outputs of the combining baluns are combined in pairs in further combining baluns, and so on until a single output of a final combining balun is reached, the output of the final combining balun being fed through an antenna coupler to an antenna.