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Browse Prior Art Database

Amplitude Modulator

IP.com Disclosure Number: IPCOM000081909D
Original Publication Date: 1974-Aug-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 40K

Publishing Venue

IBM

Related People

Taub, DM: AUTHOR

Abstract

One disadvantage of simple amplitude modulator circuits is that their output contains not only the desired carrier and sideband frequencies, but the modulation frequencies as well. The latter are generally removed by filtering, but if the modulating signal contains frequencies greater than half the carrier frequency it becomes impossible to filter them out without, at the same time, losing part of the lower sideband. The circuit shown overcomes this difficulty by balancing out the modulation frequencies.

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Amplitude Modulator

One disadvantage of simple amplitude modulator circuits is that their output contains not only the desired carrier and sideband frequencies, but the modulation frequencies as well. The latter are generally removed by filtering, but if the modulating signal contains frequencies greater than half the carrier frequency it becomes impossible to filter them out without, at the same time, losing part of the lower sideband. The circuit shown overcomes this difficulty by balancing out the modulation frequencies.

In the circuit shown in Fig. 1, resistors R2 and R3 are each twice the value of resistor R1, and resistor R4 has the same value as resistor R5. The current I from the generator divides between transistors T1, T2 and T3, a proportion depending on the modulation voltage v(m) passing through T1, and the remainder dividing equally between T2 and T3.

Ignoring base current, the currents in these transistors are:

(Image Omitted)

If v(m) varies as shown in Fig. 2a, i1, i2 and i3 will be as shown in Figs. 2b and 2c. The carrier voltage v(c) (Fig. 2d) is large enough to switch i1 completely between transistors T4 and T5, giving collector current waveforms for these transistors as shown in Figs. 2e and 2f.

The currents i6 and i7 flowing through R4 and R5 are given by:

(Image Omitted)

Summing the appropriate waveforms in Fig. 2, the waveforms of 16 and 17 will be as shown in Figs. 2g and 2h. They are seen to contain: (i) A DC component. (ii) A component a...