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NOISE AT NARROW BAND amplifier
Noise at narrow band amplifier.
In post #53 is showed the output of a broadband preamp when the AGC maintains its gain
near to the threshold of saturation with noise.
http://www.geotech1.com/forums/showt...898#post165898
The given below image shows the output of a narrowband amplifier whose gain is controlled by
the same AGC. You can see one sinusoid whose frequency is in the center of received bandwidth. One
inexperienced researcher would think that this is oscillation due to some parasitic positive feedback. To see whether this is a generation, you should gradually reduce the gain. If the amplitude also
decreases gradually, this is not generation. Decrease of gain interrupts generation suddenly.
The noise we see on the output of narrow band amp only looks like a sine wave.
If we look with caution, should be apparent the chaotic amplitude modulation and the angle modulation. The maximum modulating frequency is equal to the bandwidth. Angle modulation of sine wave by narrowband noise is difficult to discern in preamp for metal detector if the bandwidth is 16Hz. The reason for this is that modulation
index appears too small compared to the center frequency 4kHz or more. Amplitude modulation is
more noticeable especially if the bandwidth is below 16Hz. Then the vision has not merged images
so the chaotic variation of the sinusoide is visible.
Noise at narrow band amplifier.
In post #53 is showed the output of a broadband preamp when the AGC maintains its gain
near to the threshold of saturation with noise.
http://www.geotech1.com/forums/showt...898#post165898
The given below image shows the output of a narrowband amplifier whose gain is controlled by
the same AGC. You can see one sinusoid whose frequency is in the center of received bandwidth. One
inexperienced researcher would think that this is oscillation due to some parasitic positive feedback. To see whether this is a generation, you should gradually reduce the gain. If the amplitude also
decreases gradually, this is not generation. Decrease of gain interrupts generation suddenly.
The noise we see on the output of narrow band amp only looks like a sine wave.
If we look with caution, should be apparent the chaotic amplitude modulation and the angle modulation. The maximum modulating frequency is equal to the bandwidth. Angle modulation of sine wave by narrowband noise is difficult to discern in preamp for metal detector if the bandwidth is 16Hz. The reason for this is that modulation
index appears too small compared to the center frequency 4kHz or more. Amplitude modulation is
more noticeable especially if the bandwidth is below 16Hz. Then the vision has not merged images
so the chaotic variation of the sinusoide is visible.
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