Comparing examples  A to B. In example A there is DC voltage on the modulation transformer secondary, with less voltage difference between primary and secondary. Also the voltage rating for capacitor " C1 " is much lower in example A than example B. If capacitor C2 is adequate in value there aren't any other meaningful differences. Therefore I use example A.

     Advantages of using an Audio Reactor De Coupling Choke ( Modified Heising Modulation) (L1) along with a modulation transformer

1. Audio power handling ability of modulation transformer is tripled. For example Take a UTC CVM-5 which is rated for 600 watts of audio and 1200 watts of unbalanced DC current on its secondary. When you remove the 1200 watts of DC current from the secondary the new Audio power rating is roughly 600 + 1200 or nearly 1800 Audio watts (more precisely 2.7 times as much). 

2. Speaker effect of modulation transformer greatly reduced.

3. B H distortion eliminated. IE when superimposing a alternating magnetizing force ( Audio ) on a very strong direct magnetizing force ( DC unbalanced current ) the permeability linearity of the steel transformer core is grossly upset. This non linearity is easily heard by A B listening comparisons.  B + H Curve distortion Radiotron Handbook

4. This allows cross laminating the core which will easily triple to quadruple the primary inductance passing many more lows.

   

Component Functions and values

C2 = Common Power Supply output filter capacitor must be large enough to be a very low impedance at the lowest modulating frequency  IE  usually 20 to 30 MFD minimum.

R1 = reduces ringing at the high audio frequencies  IE usually 10X the RF load impendence.

C1 = couples audio. Should be large enough to be a very low impedance at the lowest modulating frequency.

L1 = eliminates the unbalanced DC load ( the constant strong polarized magnet field ) from the modulation transformer secondary. Should be rated to carry the RF plate current and have enough inductance for a high impendence at 70 hertz. Many times its impossible to find real ( thinly laminated E, I core ) audio reactors so we usually use more commonly available Direct Current chokes designed for 120 cycle power supply filtering. This will produce more than reasonable results for voice frequencies .

     To determine the minimum audio reactor choke inductance  ( L1 ) I simply multiply the RF amplifier load impedance by .0045 for the total inductance in henrys required.  

      For example in my DX-100; plate voltage = 800, plate current = .3 amps equals a plate load impedance of 2,666 Ohms. Therefore  .0045 times 2,666 =  12 Henrys of Audio Reactor Choke required for L1 above.  Quite often we must series several power supply chokes to achieve this minimum inductance value. More isn't any advantage.

    To determine the minimum audio coupling capacitance  ( C1 ) I simply divide 10,000 by the RF amplifier load impedance for the minimum C1 capacitance in MFD. 

     For example in my DX-100; plate voltage = 800, plate current = .3 amps equals a plate load impedance of 2,666 Ohms. Therefore 10,000 divided by 2,666 = 3.75 MFD. More isn't any advantage.

      Here is a Quick Reference Chart for Cornering, Reactance and Resistance in Parallel   <<< here >>>  .  Print as landscape, 153 Kb.   We are often determining the Minimum size required for a capacitor or  the minimum primary inductance of our driver or modulation transformer or audio decoupling choke.  As we are using 20 Db. of mid band feedback, which will overcome many losses at the extreme lower and higher end, we simply locate twice the frequency ( 140 cycles )  on the chart, of the lowest audio frequency ( I use 70 Cycles ) we wish to pass for our frequency bandwidth. This 140 Cycles would be our 6 Db. low corner design frequency.  For my voice I use 6 Db. corners of 140 cycles Low and 4.5 KHz. High, which after closing the negative feedback loop affords more than a total flat bandwidth of 40 through 8 KHz.

A schematic using a audio reactor in a push pull audio coil, reactor modulator.   <<<here>>>

I strongly recommend adding  The ( so called ) 3 diode negative peak limiter   by Steve, WA1QIX    for Plate modulated Transmitters.

Measuring Audio Phase Shift in Audio Amps or Modulators with a Oscilloscope and audio tone generator   by W1ECO

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