Noise floor automatic gain control

A noise floor automatic gain control system for radio receivers that receives a modulated signal and converts the modulated signal to a demodulated signal whose amplitude is controlled by a first gain control signal. The first gain control signal is the larger of a second automatic gain control signal that is provided by automatic gain control circuit that is activated when the demodulated signal is present, or a third automatic gain control signal that is provided by a noise floor automatic gain control circuit that is activated when the demodulated signal is not present. There is a detector circuit for detecting the presence of the demodulated signal as well as a selector circuit to select either the second automatic gain control signal or the third automatic gain control signal for the first automatic gain control signal.

BACKGROUND OF THE INVENTION 
This invention relates to automatic gain control circuits for single 
sideband receivers and, in particular, to automatic gain control circuits 
for single sideband receivers that have included therein a noise floor 
automatic gain control circuit which maintains the output background noise 
level at a substantial percentage below the output level of a strong 
signal. 
There is no unique feature associated with single sideband voice 
communication signals to provide reliable squelch operation. The skilled 
operators of High Frequency (HF) radios do not normally use the prior art 
type squelch because of the danger of missing usable communications. If 
the Automatic Gain Control (AGC) is allowed to govern the audio output 
signal level, the noise level will be brought up to the normal speech 
level when the channel is idle. This high idle channel noise level is not 
only fatiguing to the operator, but the high acoustic noise level limits 
the number of receivers a single operator can monitor to two or three. The 
skilled HF radio operator therefore adjusts the radio frequency (RF) gain 
control of the radio receiver so that the AGC can no longer bring the 
background noise level up to the normal speech level. The output of the 
receiver now increases with signal level for some 20-30 dB above the noise 
level until the input signal level is at a high enough level to activate 
the AGC circuit. This procedure reduces operator fatigue and allows the 
operator to monitor three or four receivers simultaneously. Manual RF gain 
control adjustments are generally made hourly in order to maintain the 
background noise level at an acceptable level. However, there are no 
reliable, automatic, comparable AGC techniques known in the prior art that 
provide the volume control needed by unskilled HF operators, especially 
where the users have other functions to perform in addition to HF 
communications. 
SUMMARY OF THE INVENTION 
A noise floor automatic gain control system for radio receivers is 
disclosed that receives a modulated signal and converts the modulated 
signal to a demodulated signal whose amplitude is controlled by a first 
gain control signal. The first gain control signal is the larger of either 
a second automatic gain control signal that is provided by an automatic 
gain control circuit that is activated when the demodulated signal is 
present, or a third automatic gain control signal that is provided by a 
noise floor automatic gain control circuit that is activated when the 
demodulated signal is not present. There is a detector circuit for 
detecting the presence of the demodulated signal as well as a selector 
circuit for selecting either the second automatic gain control signal or 
the third automatic gain control signal as the first automatic gain 
control signal. 
It is the object of this invention to provide an automatic gain control 
system for a radio receiver that has a first automatic gain control signal 
which rapidly reduces the gain of the radio receiver when a strong signal 
is received, and slowly increases the noise level until full audio output 
level is achieved in the absence of a transmitted radio signal. 
It is another object of the invention to provide an automatic gain control 
system for a radio receiver that has a noise floor automatic gain control 
circuit which very slowly increases or decreases the radio receiver's gain 
in the absence of a receive signal, so that the output background noise 
level will be approximately 20 dB below the output of the strong signals. 
It is yet another object of the invention to provide an automatic gain 
control system whose gain is controlled from the larger of two automatic 
gain control signals, one being developed by a traditional automatic gain 
control circuit and the other being developed by a noise floor automatic 
gain control circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, to which reference should now be made, there is shown a block 
diagram of a radio receiver and, in particular, a single sideband HF radio 
receiver 10. Radio frequency signals are received by antenna 1 and applied 
to a mixer 3 where the received radio signal is mixed with a signal 
provided by a frequency synthesizer 5. The results of the mixing provide 
an intermediate frequency signal which is amplified by an IF amplifier 7 
which is a variable gain amplifier. The gain of the IF variable gain 
amplifier 7 is controlled by the signal level that is present on conductor 
9 and provided to it by the output signal from either an AGC circuit 19 or 
a noise floor AGC circuit 21. The output of the IF variable gain amplifier 
7 is applied to a single sideband demodulator 11 where the IF intermediate 
frequency signal is demodulated to a single sideband signal, and in the 
preferred embodiment, an audio signal which is applied to an output device 
13 such as a radio speaker. 
The output of the single sideband demodulator 11 is monitored by a signal 
presence detector 15 which is a circuit such as a squelch circuit and 
detects the presence, in the preferred embodiment, of audio signals and 
provides the results of this detection in the form of a digital signal on 
conductor 17. Both the first automatic gain control circuit 19 and the 
noise floor automatic gain control circuit 21 try to control the gain of 
the IF amplifier 7. However, a larger input selector 23 which is a pair of 
diodes arranged as an OR circuit allows the larger of the two gain control 
signals to control the gain of the IF variable gain amplifier 7. The AGC 
circuit 19 may be any one of the several single sideband gain control 
circuits known in the art such as those disclosed in U.S. Pat. No. 
2,921,188 or the technique discussed in Section 18-3 of the book entitled 
"Single Sideband Principles and Circuits" by Pappenfus, Bruene and 
Schoenike published by McGraw-Hill Book Company, New York, in 1964. 
The noise floor AGC circuit is selected to maintain the gain of the IF 
variable gain amplifier 7 so that the signal that is applied to the output 
device 13, in the absence of a transmitted signal being picked up by the 
antenna 1, is approximately 20 dB below the output level of a strong 
transmitted signal when received by the radio receiver 10. 
FIG. 2, to which reference should now be made, is a block diagram of the 
noise floor AGC circuit 21. The noise floor AGC circuit 21 operates on a 
weaker signal than does the AGC circuit 19. Consequently, an IF amplifier 
25 amplifies the signal by approximately 20 dB and applies it to an 
envelope detector 31. The output of the envelope detector 31, the detected 
envelope, is applied to a comparator 33 which compares the envelope 
provided by the envelope detector 31 against a threshold that is provided 
by a potentiometer 35 and a voltage source 37. The output of the 
comparator 33 is either a positive or a negative signal which is applied 
to an integrator 39. The integrator 39 integrates at a constant positive 
rate whenever the output of the comparator 33 is greater than the 
threshold that is established by the voltage source 37 and the 
potentiometer 35 and at a constant negative rate whenever the envelope 
detector 31 is less than that established threshold. However, the 
detection of a signal presence as indicated by a logic 1 on conductor 17 
disables the integrator 39 and holds its output voltage at its previous 
voltage level. The integrator 39 includes an up/down (UP/DN) counter 41 
which will either count up (increase in values) or down (decrease in 
values) depending on the state of the output of the comparator 33 and the 
logic level on conductor 17. A pulse generator 43 provides a string of 
pulses to an AND gate 44 which in the absence of a positive signal 
indication provides a string of pulses to the clock input of the UP/DN 
counter 41 via an AND gate 45 which ANDs the carry output from the UP/DN 
counter 41 with the output of the AND gate 44. The output of the UP/DN 
counter 41 is converted to an analog signal by an A/D converter that 
includes a plurality of weighted summing resistors 47, 49, 51 and 53. The 
number of summing resistors is of course dependent upon the number of 
output bits from the UP/DN counter 41 and the resistances are determined 
by the amount of weight they are providing to the circuit. The output of 
the UP/DN counter 41 is summed by the adder circuit that includes the 
operational amplifier 55, and applied to a level shift circuit 59. The 
output of the level shift circuit 59 is applied to the larger input 
selector via conductor 24. 
Since the noise floor AGC voltage that is provided on conductor 24 is held 
fixed as long as the audio signal is detected, the integrator 39 is 
implemented digitally so that there will be no drift in the noise floor 
AGC voltage even after it has been disabled for many minutes. A flying 
capacitor or sample and hold circuit would have drift associated with it. 
In the preferred embodiment the output from the pulse generator is set at 
two pulses a second and the smallest counter step corresponds to the least 
significant bit of the UP/DN counter 41 and is approximately equal to 1/4 
dB. The gain adjustment rate is therefore+ or -1/2 dB per second, and it 
is anticipated that by adjustment of the output of the pulse generator 43 
that even slower rates may be achieved and even desirable. 
FIG. 3, to which reference should now be made, is a block diagram of a 
signal presence detector which is similar to the prior art audio squelch 
circuits. The audio input is applied via conductor 2 to the signal 
presence detector 15 which has a low band channel and a high band channel. 
The low band channel includes a low band bandpass filter 4 and a low band 
envelope detector 6 with the bandpass filter 4 in the preferred embodiment 
being selected to pass frequencies within the band of 300 through 900 Hz. 
The high band channel includes a high band bandpass filter 8 and a high 
band envelope detector 12. In the preferred embodiment, the high band 
bandpass filter 8 is selected to pass frequencies in the band of 2100 
through 2700 Hz. In both the high band and low band channels the output of 
the envelope detectors are applied to low-pass filters 34 which are 
designed to pass frequencies in the order of 2 Hz. The outputs of the 
low-pass filters are multiplied by multipliers 18 and 20 with a reference 
signal that is provided by the voltage source 14 and the potentiometer 16 
and applied to summing devices 22 and 26. The output of the summing 
devices is applied to comparators 28 and 30. When either channel has a 
presence of energy as indicated by the output of the summing devices 22 
and 26, the comparators 28 and 30 will have a logic 1 on either of their 
outputs and this logic 1 is detected by the OR gate 32. The output of 
or-gate 32 is applied to the conductor 17 for application to the noise 
floor AGC circuit 21 indicating the presence of an audio signal. The 
signal presence detector 15 of FIG. 3 takes the ratio of the band energies 
and if this ratio is appreciably higher or lower than with pure noise, a 
logic 1 is provided to the conductor 17. In the absence of signal energy 
when the noise on both the high channel and the low channels should be 
approximately equal and consequently there would be a logic zero on the 
conductor 17. 
Although the present invention has been described with respect to a 
particular embodiment thereof, it is not to be so limited as changes might 
be made therein which fall within the invention as defined in the appended 
claims.