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N Stage Amplifier as an AM Demodulator

IP.com Disclosure Number: IPCOM000079576D
Original Publication Date: 1973-Jul-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 71K

Publishing Venue

IBM

Related People

Rettinger, LJ: AUTHOR [+2]

Abstract

Described is an AM demodulator for high-data rate digital applications comprising an n-stage log amplifier. The demodulator has the advantage of accommodating a 50 decibel dynamic range for the input signal, and yet it has a fast setting time to enable the tracking of data pulses separated by 100 nanoseconds.

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N Stage Amplifier as an AM Demodulator

Described is an AM demodulator for high-data rate digital applications comprising an n-stage log amplifier. The demodulator has the advantage of accommodating a 50 decibel dynamic range for the input signal, and yet it has a fast setting time to enable the tracking of data pulses separated by 100 nanoseconds.

The demodulator is shown in Fig. 1 as used in the digital modem 3. The modem is connected by the tee connection 2 to the coax line 1, which is a data bus shared in common with other similar modems for communication therewith. The amplitude modulated digital signal is shown in Fig. 2a and a time-expanded view in Fig. 2b. The carrier is a 210MHz RF signal which is modulated at 10 megabits per second. As is shown in Fig. 2b, each data pulse on the modulated carrier varies in amplitude from zero to a maximum amplitude 15.

The modulated data signal is received over the data bus 1 through the 3db hybrid 4, and is amplified by the linear preamplifier 6 before being selectively passed by the bandpass filter 7. The modulated carrier is then directed into the n-stage log amplifier 8 where the demodulation process takes place.

Each stage of the n-stage log amplifier 8 is itself a log amplifier with a 7 decibel dynamic range. Each has the capacity to convert the amplitude of an input signal within its operating range, into an output signal having an amplitude which is the logarithm of the input amplitude, amplified by a 10db gain. The output signal from the first stage comprises the input signal for the second stage, where a second logarithmic function and 10db amplification is executed.

The output of each stage of the n-stage log amplifier 8, is connected to the summing amplifier 13. The output of the summing amplifier is the sum of the outputs from each log amplifier stage, which passes through decode logic 9 to line 12.

With minimum detectable signal input, the amplification of the modulated input signal by 10db at each stage will cause the last stage n to saturate, giving a constant output which is summed by the summing amplifier 13. As the maximum amplitude 15 of the input signal increases, the n-1, n-2, etc. stages go into saturatio...