CA1079361A - Transmitter processing apparatus (articulation enhancer and noise excluder for single-sideband transmitter) - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Transmitter processing apparatus for improving transmission of signals for a transmitter including a balanced modulator and a succeeding first stage of amplification.
Circuit means is interposed between said balanced modulator and the first stage of amplification and includes a first emitter-follower stage having an output connected through crystal filter means to a linear integrated circuit means, the output of the linear integrated circuit means being connected through transformer means to a second emitter follower stage whose output is connected to the input of a digital inegrated circuit Schmitt trigger circuit having an output connected to the input of said first stage of amplification.

Description

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This invention relate~ to tran~mitt~r processing apparatus.
Transmitters used in radio cor~unica~ion for voice and data transmission are oEten of the single-sideband (SSB) type. It has been found that, particularly in a noisy channel, the articulation or information content is limited by the peak power handling capacity of the transmitter. For voice transmission, the peak content is typically about fourteen deci-bels above the average power, and contributes relatively little to the articulation. For multi-tone voice frequency telegraphy, a "cresting" phenomenon causes the peak power of the trans-mitter to be six decibels above the average (or single tone~
output when two tones are present simultaneously, the situation getting worse with the introduction of additional tones. In both of these cases, the average power is llmited to that which -does not cause the peak content to exceed the peak power handling capacity. It will thus be apparent that reduction of the peak-to-average power ratio will increase the articul-I ation of the communication channel for a given peak power ~20 output~
In order to achieve an increase in the articulation of the communication channel by a reduction of the peak-to-average power ratio, previous equipment has utilized processing at audio frequencies and/or a type of ,~
automatic gain control so as to limit the gain of the trans-mitter output stages. However, it has been found that audio processing is relatively ineffective and undesirable because of the in-band distortion which is produced, whilst the auto-matic gain control reduces the average power output. Further-3~ more, thi~ ocaurs afker the transmitter has already been over-.
driven.

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In more recent equipment use has been made of a clip-ping technique and bandwidth restoration at the inter-mediate frequency of the transmitter. Intermediate frçquency tIF) clipping of the signal is effective and the articulation enhancement increases with the clipping level.
~lowever, it has been ~ound that the level of clipping that can be employed is limited by the noise of the system (e.g. hum, background noise, power-supply ripple, hiss etc.) which increases in direct relationship to the level of the ~10 clipping which is employed. It will be appreciated that this noise can be annoying and also can be rather comprom-ising on the system, depending on the application of the circuit.
It is an object of the present invention to provide an improved apparatus which will give improved articulation and will not be as limited as the above system due to the noise of the system.
Accordingly/ the present invention provides signal producing apparatus adapted to produce an output signal consisting of pulses of constant amplitude but of varying duration, having a balanced modulator and a succeeding first stage of amplification including circuit means interposed between said balanced modulatox and said first stage of amplification, said circuit means comprises a first emitter-~ollower stage having an outpu-t connected through crystal filter means to a linear integrated cixcuit means, -the output o the linear integrated circui~ means being connec-ted through transformer means to a second7emitter-follower stage whose output is connected to the input of a digital integrated circuit Schmitt trigger circuit having an output connected to the input of said first stage of amplification.

An embodiment of the present invention will now be described, by way o~ e~ample, with reference to ~he accom-

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panying drawings in which:-Figure 1 is a diagramma-tic representation of a transmitter processing apparatus according to an embodiment of.the in~ention, Fig~lre 2 is a graphical representation of voltage waveforms at different po:i.nts in the circuit shown in Figure 1, and Figure 3 is a diagrammatic representation oE an alternate arrangement for par.t of the circuit shown in Figure 1.
Referring to the Figures, the transmitter's balanced modulator (not shown) provides its output through a transformer 2 from whence it is fed through the transmitter processing apparatus 4 which provides an output to the transmitter's first stage of ampli~ication 6 It will be noted ~hat one side of the secondary winding of transformer 2 is connected to ground whilst the other side is connected to the input of a first emitter-follower stage 8~ The stage 8 includes an ~PN type transistor and its output is fed through a line 10 to the input of a crystal filter sta~e 12 which is a Heathkit*
filter number 404-283. Due to transistor stage 8 it is matched to transformer 2 and the output of the filter is fed through a linear integrated circuit 14 which is a Fairchild* type 703 and provides extra gain to increase . the signal level.
The output o~ the linear integrated circuit 14 is fed through an intermediate frequency (IF) transformer 16, :~ which is a Heathkit* type number 52-7~, to a second emitter-*Trademark .
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follower stage 18 incorporating an NPN type transistor.
The emitter-follower stage 18 acts as an impedance-matching device and to act as an ,effective voltage source for the next stage. Its output is -Eed to an input of a digital integrated circuit 20 which is effectively func~ioning as a 1.
Schmitt trigger circuit or clipper circuit.
The digita~ integrated circuit is one half of a type 7400. It will be observed thak a potentiometer 22 is provided at the input of the second emitter-follower stage 18 and this is utilized to determine ox set the operating point of the second emitter-follower stage 18 which is directly coupled to the digital integrated circuit 20 connected as a Schmitt trigger whose output is fed through a potentiometer 24 to the input of the transmitter's first stage of amplification through a connecting line 26. The potentiometer 24 is utilized to set the drive level to the succeeding transmitter stage 6.
~ In use, the transmitter processing apparatus shown ;: in Figure 1 may be inserted in a single sideband transmitter circuit ahead of, or behind, a filter unit therein. The ac~ual 2~. position is really dictated by circuit convenience and the signal levels available and re~uired at various points so long as the processing apparatus is ahead of the AGC stages of the : transmitter.
In operation,the illustrated circuit operated to limit the peak excursions of the signal it received from the transformer 2 at khe output of the balanced modulator stage of ;:~:: ' .
: the transmitter. The operation can best be explained with reference: to Figure 2 in which a typical input signal to the : Schmitt trigge3- circuit ~O ~Figure 1) is shown as the top wave-~; 30 ~ form (a) of Figure 2. The Schmitt trigger circuit does not - respond to noise which occurs but as soon as the input signal , :; ~

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exceeds a voltage level B due to a voice signal, then the Schmitt trigger circuit is switched, at time tl, to produce an output pulse 40 as sho~ in waveform (b) of Figure 2.
The pulse 40 terminates when time t2 is reached as the input signal drops below the bottom decision threshhold level A.
The top decision threshhold level B is also indicated in Figure 2 in whi~h the waveform ~b~ is a representation of the output of the Schmitt trigger circuit. The waveform (c) in Figure 2 is a representation of the output with the D,C. component removed. It is to be noted that this waveform corresponds to the waveform (a) in Figure 2 but with constant amplitude and with the noise removed after being "top-limited" and "bottom limited".
It will be appreciated that for optimum fidelity, the threshh~ld levels A and B in Figure 2 should be as close together as possible and this is why it is advantageous to use a Schmitt trigger aircuit synthesized from one half of an integrated circuit type 7400. The threshholds should be set as I close to the noise level as possible without allowing noise to ¦ 2~ trigger the system more frequently than is considered reasonabla fox a given application. This can be selected by adjusting potenkiometer 22(Figure 1).
The circuit illustrated in Figure 1 was bullt into a single-sideband transceiver, t~pe Heathkit*SB-10~, and was found to function satisfactorily. It will be ` appreciated that the first emitter-follower stage B acts asan impedance transformer between the balanced molulator stage ~not shown) and the crystal filter stage 12 which was identical , to the one used for sele~tion of the desired sideband. The : 3a Fairchild*linear integrated cixcuit 703 provided symmetrical limiting characteristics and high gain. In use, it was found that the output of the transmitter consisted of either a stream ,-1 ; l *Txadema~k -5-75~36:~L

of constant c~mplitude pulses or zero po~er, which respectively represented a signal aondition and a no signal condition~
It will be appreciated that variations of the cirauit illustrated may well function according to the present inventi~n and the digital integrated circuit 20 may not actually be a Schmitt trigger circuit but may be an electronic circuit identical in fun~tion to a Schmitt trigger. It will be understood that the expression "Schmitt trigger means" used herein i~ intended to be interpreted broadly so as to refer to a device functioning in a similar manner to a true Schmitt trigger circuit to provide limi~ing.
By using the illustrated electronic circuit iden-tical in function to a Schmitt trigger, it was found that one could achieve infinite clipping and noise exclusion. Thus, there was no output from the transmitter until a signal greater than a preset -threshhold was present,at which point the out-put jumped to maximum power. Thus, as mentioned, the ou-tput of the trar.smitter consisted of either a stream of constant amplitude pulses or ~ero power, representing respectively signal 2~ and no signal conditions. It will be underskood that infinite ~; clipping may also be called zero crossing detection, in the absence o~ noise. It was found that for a given channel signal ~; to noise ratio, infinite clipping will give better articulation than other methods of processing. The noise-excluding ~eature eLiminates substantially all background noise as ~ell as any system noise ~e.g. hu~, ripple, hiss) generated before the processing apparatus 4 in Figure 1. The apparatus was simple to construct and relatively inexpensive. Instead of using symmetrical clipping (usually two solid-state diodes ~: 3a. . ~ with a common threshhold bias) as in pr0vious circuits, the constructed embodiment of the present inven~ion used the : ~

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. , 7400 series digital integrated circuit as a Schmitt trigger.
It was found that when the DC component of the Schmitt trigger was removed, a symmetrical waveform was obtained and thus if the Schmitt threshhold is set above the system noise level, then the noise does not appear in the output signal, i.e. this is bottom limiting.
It ~ill be appreciated that the Schmitt trigger means need not necessarily be a type 7400 integrated circuit or even an integrated circuit device. However, it should be selected so as to switch at a rate which is higher than the intermediate frequency used in the transmitter. A fast acting gate could be used.
In transmitters, the intermediate frequency is usually in the range from 200KHZ to 15~Z. A 3.395 MHZ
filter was used in the prototype but it can be of any suitable type, e.g. mechanical, quartz crystal, etcO If the rise-time and the fall-time of the Schmitt trigger means is controlled then a low pass filter may not be required. However, a low-pass filter may have to be inserted to reduce shoak excitation 2~ by the steep wavefront ~rom the Schmitt trigger means - From a theoretical inspection of the waveform generated it should be possible that a single sideband signal could be generated by passing a suitable PWM signal through an SSB filter. The resulting signal would exhibit the top limiting characteristics only and the carrier would have to be suppressed by the filter.
As mentioned above it has previously been attempted to improve the operation of single-sideband trans-mitters, particularly for amateur radio, by utilizing IF clip 3a- ping teGhniques and bandwidth restoration without exceeding the peak handling capacity or generating "splatter" or ~ i - ~7~3~L

excessively wide signal in the frequency domain. Typical increases in aommunication effectiveness of six decibels were obtained by twenty decibels of clipping of a voice signal.
However, in the prior systems noise became.more pronounced with heavier clipplng and the maximum effectiveness could not be reali~ed because the system noise "captured" the limiter stage, especially between transmitted bursts of voice signals.
If we assume that the original transmitter signal-ko-noise ratio was forty db, then with thirty db of clipping the signal-i lO to-noise ratio was reduced to lO db and this is obviously too low.
In the illustrated embodiment of the invention, ' it was found that operation was not so adversely effected due .ll to background sounds on the voice circuits, tempest related signals, or power supply ripple, etc. It was found that the I average power output was increased for a single-sideband ¦ suppressed-carrier ~SSB) transmitter ~where the SSB signal is I generated at an intermediate-frequency) whilst not exceeding ¦ the peak power handling capacity of the final stage of the 2~....... transmitter. Thus, the system achieved "infinite clipping"
¦ where the talk power was increased with the degree of clipping (peak power held constant) and system noise was subskantially excluded In a variation of the circuit illustrated in : Figure 1, a low pass filter was inserted between the ~ Schmitt trigger ~or clipper) stage 20 and the sideband : : selective filter stage 18. This appeared to prevent shock-excitation and ringing of the selective elements whereas~
~` in some cases, t~e xinging had shown up as apparent audio distortion or harshness, at higher levels of clipping.

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It will be appreciated that~ in sorne situations, operation of the described apparatus may be improved by providing temperature compensation of the DC coupled stages to improve operation from a col~ s;tart and redu~e the few minutes delay ..
which is required for settling down of the operation of the apparatus Furthermore~ it will be appreciated that the provision of remote switching may be advantageous~ One way o providing this facility is shown in Figure 3. With the moveable arm 50 making contact with the fixed terminal5~ , the circuit functions so as to provide a signal along line 56 which has been both top and bottom limited. When the arm 50 is moved into contact with fixed terminal52 the signal appears on line 58 with top (or no limiting) limiting only.
It will be appreciated that the described embodi-ment has broad application. It could be used on any transmitter employing processing, e.g. clipping or limiting, on a single sideband signal whether this is done internally or externally.
The primary application is, of course, to singla sideband, voice or data.
The described apparatus could be u~ilized externally, e.g. in a F.~. system before the audio in~ut by taking the audio signals and synthesizing them by ~op and bottom limiting and then feeding the signals into the respective input in the normal way and this may be particularly ~seful for security purposes - it will be understood that diodes alone will give top limiting only~and for top and bottom limiting one could utilize a Schmitt trigger device.
In use on eighty meters impxoved operatio~ was obtained using only one watt. The res~ s oF a "two-tone"

test demonstrated~a ten percent differe~ce between a single tone and two tones in transmitter power output, whereas ~ 9 ~7~3~

an unmodified transmitter required that average power must be reduced ~y sixt~ percent when going from single to two-tone conditions to maintain the same peak po~er. Thus, the illus-trated circuit, whether used as a modification to an existing single-:sideband transmi~ter or included as part of a new desiyn, appears to enable communication to be established using less peak power than with an unmodi~ied transmitter.
This is particularly important where ssveral transmitters and receivers operate in alose proximity since the use oE lower power will reduce the generation of intermodulation and desensitiæation in a nearby receiver. Because the peak output of a transmitter will not exceed a predetermirled value no matter what the modulation waveform is, there will not be over-drivihg of ~he transmitter on peaks and therefore much less intermodulation products generated within the ~ransmitter.
Furthermore, the fact that the noise exclusion ability of the device permlts "infinite clipping" whilst minimizing emanation of distractiny noises or comp.romising sounds would appear to make the circuit of particular interest so far as military communication systems are concerned. Additionally, the peak to average power characteristic makes IF clipping of ~: possible in-terest for multi~tone data transmission as well as : for voice 'cransmission.
In the aase of speech, the single sideband signal will comprise: (1) the desired intelliyence but a-t a high peak to average power ratio, and ~2) undesired sounds . such as hiss and hum from the microphone amplifier, IF carrier :~ ~ generator and balanced modulator as well as noises picked up :by the amplifier - other voices, sounds/ music, cooling fan, 3~.. street noise, overElving a.c., etc.

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To lmprove khe signal to noise ratio a linear amplifier could, of course, be used but in,for example, amateur radio this involves quite a high additional cost and also i5 liable to generate TVI, BCI and HI-FI types of interference in a domestic city environment The described apparatus appears to be advantageous in reduc:ing the peak to average ratio so that a higher "talk power" or communications effectiveness may be achieved without resorting to higher peak power by using a linear amplifier.
In the above desaription reference has been made to a voice modulated waveform. It will be appreciated that the described embodiment, possibly with modifica~ion, is applicable also to multi-tone voice frequency carrier tele-graph systems (VFCT) which may benefit from top and bottom limiting. Asswning all tones are o~ equal amplitude, the peak power output with N tones present simultaneously is ! believed to be 20 loglON decibels greater than the single tone I output, unless limiting is used. The system would appear to be ¦ effective to "ragenerate" the signal by removing hum and hiss, even though there are no background sounds, as well as making peak and average power almost equal.

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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Signal producing apparatus adapted to produce an output signal consisting of pulses of constant amplitude but of varying duration, having a balanced modulator and a succeeding first stage of amplification including circuit means interposed between said balanced modulator and said first stage of amplification, said circuit means comprises a first emitter-follower stage having an output connected through crystal filter means to a linear integrated circuit means, the output of the linear integrated circuit means being connected through transformer means to a second emitter-follower stage whose output is connected to the input of a digital integrated circuit Schmitt trigger circuit having an output connected to the input of said first stage of amplification.

2. Apparatus according to claim 1 including a first potentiometer in the input circuit of said second emitter follower stage to determine the operation point of said second emitter-follower stage.

3. Apparatus according to claim 2 including a second potentiometer in the output circuit of said Schmitt trigger circuit to determine the drive level to said first stage of amplification.