Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to electronic communication
systems. In particular the invention relates to a carrier
isolation system in a receiver for amplitude modulated
carrier waves.
. S BACKGROUND OF THE INVENTION
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Many communication systems require the isolation of the
carrier portion of the modulated wave. For example, in
reduced carrier (A3A) transmission the carrier is transmitted
along with the sideband frequencies at a reduced amplitude,
generally 10 to 20 db below the peak sideband level. In
many situations, this carrier wave is used to demodulate the
sideband signal in a product demodulator. This insures that
the recovered frequency components will be accurate and will
protect the system against frequency errors due to the
frequency doppler shifts. Such systems have been in use for
communication for many years and have generally given good
, service.
one of the problems connected with this type of
transmission system is that it is necessary that the band
~ 20 width of the isolating filter be wide enough to follow drift
; and frequency errors but narrow enough so that desired side--~ band components do not disturb the performance of the system.
This re~uires, in single sideband systems, that the carrier
filter pass only the desired frequency plus or minus a shift
of 100 Hz. In most transmitters which carry voice frequencies,
the frequency response is restricted so that the lowest audio
frequency is greater than 300 Hz. If the transmitter filter
circuits are sufficiently selective and are stable, this
system works fairly well.
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However, there are some cases where the low pass -
filter has changed, due to temperature changes or aging, and
there is some response at frequencies as low as 50 Hz. Also,
there are cases where the carrier fre~uency drifts to the -
edge of the sideband filter, thereby permitting the low
frequency components of the sideband to fall within the pass-
. band of the carrier filter. In this case, the sideband
~ components will produce objectional phase modulation components
; and introduce distortion. The invention described herein
greatly reduces this problem, making such systems relatively
free of such difficulties.
The invention is based upon the fact that frequency
division or re~uency multiplication does not change the
;i spacing of the sideband frequencies relative to the carrier.
For example, if a single-sideband transmitter is modulated
; with a 100 cycle tone, the lowest separation between the
sideband and the carrier is 100 Hz. This separation is main-
tained no matter how many times the signal is multiplied or
divided in frequency.
In the Marine radio telephone field, a specification
of - lO0 cycles per second for the accuracy of the carrier
frequency has been standardized for certain types of modula-
tion. For this reason, the band width of the carrier channel
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must be at least + 100 Hz in order to handle the transmissions
properly. Since it is possible that the transmitter carrier
- wave be at the edge of its tolerance, say 100 Hz low, upper
: sideband components at 200 Hz from the carrier just pass the
`;` selectivity re~uirement~ These sideband components may be
,. . .~- fairly strong and, since in the A3A transmission the carrier
is at least 10 db below the sideband, it is possible that
distortion may result.
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The basic idea of the invention is to divide the
frequency of the carrier fre~uency wave. Frequency division
reduces the amount of frequency error which is permissible in
the system. The carrier wave is first filtered by a crystal
filter having a band width of + 100 Hz. Then, the resulting
wave is passed through a limiter to remove all traces of
amplitude modulation and then the carrier frequency is divided
by 8 by any of the known division circuits. If the original
carrier had a frequency of 100 KHz, the divided wave has a
; 10 frequency of 12.5 KHz. A fre~uency error at the inputfrequency of 100 Hz will now be an error of 12.5 Hz. The
band pass filter which lies between the dividing and multi-
plying circuits should have a band width of only - 12.5 Hz.
; An ordinary band pass circuit filter can be used for this
purpose, but it is believed that a phase locked loop is
preferable. Other active circuits, such as a locked oscillator,
have been considered, but it is believed that the phase locked
loop is the best. The phase locked loop as used in this
situation acts like a tracking filter which follows frequency
errors witnin the range of + 12.5 Hz; however, due to the
circuitry that restricts its speed, effectively cuts off
undesired sideband components.
; Additional details of the invention will be disclosed
in the following description, taken in connection with the
' 25 accompanying drawings.
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` SUMMARY OF THE INVENTION
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In accordance with the invention, in a receiver for
amplitude-modulated carrier waves, there is provided a carrier
` isolation system for separating a carrier wave from undesired
accompanying components. The system comprises a carrier wave
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filter connected to a source of amplitude modulated wave
signal for passing substantially only the carrier frequency :
wave, said filter having a frequency width sufficient to
accomodate the expected carrier frequency shifts. A frequency
divider circuit connected to the filter for dividing the
carrier frequency wave by a predetermined amount. A bandpass
filter connected to the output of the divider circuit for
restricting the divided frequency wave to a predetermined
, :
variation and thereby attenuate undesired frequency components.
; 10 A new system also includes a multiplying circuit coupled to
the bandpass filter for multiplying the frequency of the "
carrier wave by the same factor as used by the divider circuit
to restore the carrier wave to its original frequency. `-
Finally, the system includes a demodulation circuit connected
,
to the multiplying circuit and the source of modulated wave
; 3ignal for demodulating said wave signal to produce an audio
~ frequency wave signal.
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For a better understanding of the present invention,
together with other and further objects thereof, reference
is made to the following description, taken in conjunction
with the accompanying drawings, and its scope be pointed out
-~ in the appended claims.
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~ BRIEF DESCRIPTION OF THE DRAWINGS
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` Figure l is a schematic wiring diagram in block showing -
one embodiment of the invention.
Figure 2 is a schematic wiring diagram also in block
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showing the addition of a phase locked loop which may be used
instead of the band pass filter in Figure l.
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Figure 3 is a graph showing the frequency and amplitude
relations between the carrier wave and the region in which
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sideband waves can exist. This figure is for the normal ;-
reduced carrier with single sideband (A3A). ~
Figure 4 is a graph similar to Figure 3, but showing - -
the carrier wave after having been divided by 8 and a side-
band region also having l/8 the normal frequency values.
, DETAILED ~ESCRIPTION
Referring now to Figure l, input terminal 10 indicates
the connections which are to be made to a circuit which
supplies an incoming SSB modulated wave. This signal is
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applied to a carrier filter 12 and to a sideband filter 13.
The carrier filter 12 may be very narrow but the sideband
filter 13 must have considerable band width since it must pass
frequencies which include from 100.2 KHz to 102.5 KHz, this
, region being indicated by the rectangle 14 in Figure 3. The
~, 15 carrier filter 12 is rated to pass a nominal 100 KHz wave,
but due to the frequency instability of some of the sending
~ components, the filter must pass fre~uencies within the range
; of 100 KHz + 100 Hz, this range and amplitude being indicated
by the rectangle 15 in Figure 3.
~ 20 The output of the carrier filter 12 is applied to a
i limiter 16 which removes all traces of amplitude modulation
-~ i there be any. The output of the limiter 16 is then applied
to a divider circuit 17 which divides the frequency by 8.
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~,; This divider may be a three-stage bistable multivibrator or
any other well known divider means. The frequency in the
~` output circuit of this divider is a nominal 12, 500 Hz, but it
, is subject to the same percentage variation as the input or
12.5 cycles per second. The output of this divider circuit 17
is indicated by the narrow rectangle 18 in Figure 4, this wave
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being applied to a band pass filter circuit 20 which may be ~-
a crystal filter.
The output from the band pass filter is now applied
to a multiplier circuit 21 which multiplies the frequency by
; 5 8 and produces the same carrier frequency as the original
input signal. This wave is next applied over conductor 22
to a product demodulator 23. The demodulator 23 also receives
sideband frequencies from filter 13. The output from the
demodulator 23 may be amplified by an audio amplifier 25 and
then sent to terminal 26 and a load 27 which may be a loud
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speaker or a voice recorder.
The phase locked loop 24 is an alternate circuit for
use in filtering the signal after division and may be used
, instead of circuit 20. This circuit 24 is shown in Figure 2
;, 15 and includes a phase detector 30, a low pass filter 31, and a
voltage controlled oscillator 32. The phase locked loop as
used in this circuit acts like a tracking filter which will
' follow frequency errors due to the 12.5 cycles per second
variations, yet due to the circuitry that restricts its speed,
' 20 it effectively attenuates all the sideband components lying
outside the desired the desired range.
The phase detector 30 receives one of its inputs from
the divider circuit 17. The other input comes from the -
voltage controlled oscillator 32 and is a 12.5 KHz wave. The
'' 25 oscillator 32 is part of the loop 24 and receives its input
energy from the low pass filter 31. The filter 31 receives
its energy from the phase detector 30. The range of frequencies
passed by this loop circuit is determined by the cut-off
re~uency of the low pass filter 31. A typical cut-off
re~uency is 25 Hz. Thé output of the filter 31 controls the
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frequency of the oscillator 32 which is sent back to the phase
detector 30, thereby closing the loop.
It should be noted that the region 14 in Figure 3 and
a similar region 33 in Figure 4 are areas where the sideband
waves may occur. When there is no modulation of the carrier,
~ there is no wave energy at all in this range. When a pure
; tone o say 500 Hz modulates the carrier at a 50 percent
modulation, a single sideband 34 having a frequency of 100,500
Hz and a relative amplitude of .5 results. Both sideband
- 10 regions 14 and 33 are drawn to indicate a sideband range of
200 Hz to 2.5000 Hz.
The essence of the invention is contained in the ~ -
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process of segregating the carrier wave from its sideband
components, dividing its fre~uency by an integer factor greater
than one, iltering the resulting wave by the use of a narrow
efficient filter, multiplying the filtered wave by the same
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integer factor, and combining the resultant carrier wave with
; the sideband components.
Although various embodiments of the invention have
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been described herein in detail it will be understood by those
skilled in the art that variations may be made thereto without
departing from the spirit of the invention or the scope of
the appended claims.
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