Diff: Historic Papers

Differences between version 29 and previous revision of Historic Papers.

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version 29

Historic Papers

AM

• About Diode Demodulators, Jim Tonne, 2009. Though not historic, this paper reviews the operation of diode based envelope detectors for AM. It reviews several envelope detector designs and looks at the causes of distortion of the demodulated waveform on the negative peaks. The paper references "the slew rate problem" where the capacitor of the low pass filter used to remove residual RF after the diode has a low resistance charge path (through the diode) and a high resistance discharge path (typically a resistor to ground).

Another perspective, not mentioned in the paper, is that an envelope detector should work without distortion if the output resistance of the demodulator (before the low pass filter) is zero, or at least very low when compared to the input resistance of the low pass filter. If, for example, the RF driving the diode has a source resistance of 0 ohms, which then drives an ideal diode followed by a resistor to ground of 1 ohm, driving a low pass filter with an input resistance of 100 ohms or greater, the capacitors in the low pass filter would "see" equal resistances for charge and discharge. It is interesting that none of the sample circuits take this approach. It is interesting to compare the envelope detector to a synchronous detector.

A synchronous detector, of course, multiplies the incoming modulated waveform by a sine wave that is in phase with the original carrier. If, instead, the incoming modulation were multiplied by a square wave instead of a sine wave, and that square wave is biased up so that the low side is at zero volts, the resulting output is a half wave rectification of the input waveform. This, of course, is the first part of envelope detection. Moving to the frequency domain, multiplication of the incoming modulated waveform by a sine wave that is in phase with the carrier results in a DC component proportional the the product of the two waveforms plus a sine wave at twice the carrier frequency (difference frequency of zero, sum frequency of twice carrier). The low pass filter removes this twice carrier component, leaving the DC proportional to the amplitude of the incoming waveform. As this amplitude varies, the "DC" varies. This comes out of our low pass filter as the demodulated waveform.

Use of a square wave instead of a sine wave results in higher harmonics of the "difference" frequency. If the carrier is 1MHz, and we use a square wave, it has components at 1MHz, 3MHz, 5MHz, etc. Using the "sum and difference" result of analog multiplication, we find the "sum" results coming out of the analog multiplier as 2MHz, 4MHz, 6MHz, etc. The "difference" frequencies (other than zero) land on the same frequencies. Each of these are removed by the low pass filter. (comments by Harold Hallikainen)

• AM Transmitter Performance Without Filter Chokes, Wallace W. Wahlgren, Electro Engineering Works, San Leandro, CA. From 1964 National Elecronics Conference Proceedings. Scanned by John Lyles. 1.6MB
• The WLW 500kW Transmitter, Proceedings of the Institute of Radio Engineers, Volume 22, Number 10, October 1934. Describes the design and installation of the transmitter. Among the unique features are use of high level plate modulation with a class B modulator (with modulation reactor), a series fed vertical radiator, ability to operate at reduced power with portions of the transmitter removed from operation for repair, and use of coax between the transmitter and the antenna tuning unit. Another intersting feature is that the tower lighting AC wiring is enclosed in the output inductor of the antenna tuning network. A similar technique has been used relatively recently to get a variety of signals (AC and RF) across the base insulator of series fed towers. File provided by Stanley Adams. File size 5.4MB.
• A New High-Efficiency Power Amplifier for Modulated Waves - by W. H. Doherty, Bell Telephone Laboratories, Presented at the annual convention of the Institute of Radio Engineers, Cleveland, Ohio, May 1936.
• Ground Systems As A Factor In Antenna Efficiency - by G.H. Brown, R.F. Lewis, and J. Epstein of RCA, Proceedings of IRE, June 1937.
• How Radio Grew Up from Radio Broadcast, December 1925. From http://www.coutant.org/radiogrow . 27.5MB.
• The Pulse Duration Modulator: A New Method of High-Level Modulation in Broadcast Transmitters, H. Swanson, Gates Radio Company, IEEE Transactions on Broadcasting, December 1971. Scanned by John T. M. Lyles.