*** The Ampliphase Page ***
Ampliphase - A quick description....
Ampliphase transmitters quickly acquired the nickname of "Amplifuzz" due to the quirky nature of setting them up and maintaining consistant performance, and became the transmitters engineers loved to hate. Although the technique became obsolete for broadcast use in the 1970's, outphasing has undergone a bit of a renaissance in recent years in the microwave spectrum. It is used as an efficient and accurate means of producing phase/amplitude (QUAM) modulated wideband carriers, necessitated by todays digital communications systems.
The notes on these pages refer to the BTA-50H, although the same modulator chassis was used in the 50G/50H1 and 100B AM broadcast transmitters. If you can add any information (or spot any mistakes) to these pages, please feel free to contact me at the address at the bottom of the page. I am, in particular, keen to hear from you if you are going to be disposing of an Ampliphase transmitter, or have parts available.
From the block diagram (above), it can be seen that the frequency source is a crystal oscillator (at the operating frequency) based on an 807. This oscillator module was common to many other RCA BTA series transmitters from the 1940's through to the 1960's. The first part of the ampliphase process is applied by a 5693 pentode which drives a transformer load,
producing bi-phase outputs, 180 degrees apart. These outputs are then fed to two identical modulation chains, each comprising of four cascaded stages comprising of a 5693 "modulation shifter" amplifier and 5692 "phase modulator" triode. The first stage is "DC modulated" by a variable resistance (no triode in this stage) which is used to set the quiescent phase difference at 135 degrees,
hence the transmitters nominal output power. The three subsequent stages have audio fed to the grids of the triodes. The anode load of each pentode amplifier is effectively a tuned circuit comprising of a
parallel L/C network with a series component formed by the anode/cathode conductance of the triode. As audio is applied to the triode grid, its conductance thus varies. This has the effect of being a variable resistance in the L/C resonator, hence producing instantaneous phase change which corresponds to the applied audio.
Cascaded stages are used to ensure linearity of modulation, as a single stage could not provide enough phase change in a linear mode. Equal but opposite modulation is achieved by feeding the two chains with anti-phase audio derived from dual windings on the audio input isolating transformer. As the total phase change required is +/-45 degrees
one chain must reach +45 degrees whilst the other reaches -45 degrees. Each stage of active modulation therefore produces just 15 degrees of modulation.
Below : The modulator sitting on my dining room table!
In 1969 RCA brought out the BTA-50J which had a newly developed solid state exciter/modulator. This unit could also be retrofitted to the 50H, to make it a 50HS. (S for solid state) It was a very complex piece of kit, and there are mixed reports of its performance. Some idea of its complexity can be gauged from
the fact the handbook for this modulator is actually bigger than the handbook for the entire transmitter! Its principal of operation was also somewhat different from the cascaded phases modulators of the original system. Following buffering, the crystal frequency was converted into a symmetrical triangular wave, and fed through a "slicer" comparator circuit. Also fed into the comparator was the audio. The output from the slicer was a pulse width modualted waveform, on the carrier frequency.
A narrow pulse width corresponded with high levels of positive audio, and a wide pulse width with negative levels of audio. The rising and falling edges of the PWM wave were separated and differentiated to produce two "spikes". For positive audio (narrow pulse) the spikes would be close together, for low level audio, the spikes would be further apart. These two spikes were then used to trigger separate monostable circuits, each monostable timed to have a duty cycle of 120 degrees at the carrier frequency.
The outputs of the monostables were used to drive the two Class-C amplifier chains of the transmitter as described above. This exciter arrangement was essentially broadband, though the triangle waveform had to be set to be symmetrical on the carrier frequncy, and the two monostables needed to be adjusted to be 120 degree duty cycle. A separate solid state "baseband" amplifer module within this exciter provided for the drive regulation control to the grids of the driver tubes.
The pro's and con's of ampliphase are varied. It can achieve excellent modulation fidelity and high transmitter efficiency compared to a plate modulated transmitter (no need for 25KW audio amplifiers and modulation transformer!) but does require two identical RF PA stages and a combining network, and can be a problem to set up and maintain identical performance
for the two modulation chains (any mismatch in the phase shift or level between the two chains results in excesive phase shift of the final carrier). It was also necessary to maintain all the tuned stages within the transmitters as broad as possible to prevent any inadvertant phase distortion which would result in sideband splatter on the final output.
Today, of course, the whole modulation stages could be built into one chip of silicon, complete with RF frequency synthesizer, audio processing, logic-control and probably most of the studio playout system(!), although digital modulation systems, resulting in 90% or better TX efficiency have rendered the technique obsolete for broadcast use.
I hope the above description is accurate - I have written it from studying the BTA50H handbook and various materials produced by RCA some 40 odd years ago. Please feel free to contact me with any comments, corrections, or other info. As stated earlier, I would love to make contact with engineers who worked on this rigs in their hey-day, or anyone who may have parts or other information to share. - A big thank you to John, Barry, Don, Duffy, Grady and others who have already assisted me.
This page will be updated as and when I have extra information to add. Other pages on this website are updated more frequently, please use the menu bar on the left hand side.
My E-mail address is : "ampliphase [at] radio [dot] fm" or use the links to the feedback form. Thanks!
Some links to other fine and notable transmitter sites:-