MODULATION OR NOT MODULATION?
Why a Hamletic title? Below I will try to explain it.
In re-reading the various articles, including patent, written by
Lacault, on the functioning of the Modulation System, or the
Ultradyne, made me doubt on its real working. If then I compare
it with the operation described by Jouaust in his patent and
related documents (Ref. 2), doubts have increased. Below I have summarized what I
understood and interpreted.
Lacault explained the operation of
the Modulation System as follows:
- The plate-filament space of the Modulator Tube acts as a
resistance, whose value is varied by the RF input signal
(antenna) imprinted on the grid. Since the plate is fed only
from the high frequency signal of the local oscillator the
intensity of this signal varies depending on the value of the
plate-filament resistance. It is the antenna signal that
modulates the local oscillator -
The description of Jouaust was the following:
- The "modulation" in reception consists in vary periodically,
with a local oscillator, the RF signal strength. It is the local
oscillator signal which modulates that of the RF -
As you can see they are conflicting, which one was right?
Below is how I think that the circuit functions:
We know that the switching mixer interrupts or shorts the RF
signal at the local oscillator frequency. In the Ultradyne,
mixing occurs by causing a periodic interruption of the antenna
RF signal on the plate of the tube. This is not done with an
electronic switching device, how we use nowadays, but simply by
feeding the plate, through the intermediate frequency
transformer, by the high frequency voltage of the local
oscillator rather than with the classic DC anode voltage. With the
positive half-wave of the oscillator, the RF signal is present
on the plate (so the tube is active), with the negative one it
is absent, thanks to this rhythmic interruption that occurs the
mixing and the intermediate frequency generated. Consequently,
it is the local oscillator that "modulates" the antenna signal.
The tube works both as an RF amplifier and as a pseudo "switch",
alternately, ingenious! We can consider it a ”switching mixer”.
I think the description of Jouaust was more correct, although
Lacault, talking about variation of the tube internal resistance
by the polarity of a periodical signal (however the signal
was the RF not that of the local oscillator) was closer to a
“switching mixing action”. Jouaust spoke of "the phenomenon of
modulation" in reception and Lacault of "modulation system" in
the superheterodyne receivers. Usually "modulation" means the
imparting information (audio or other) on a RF carrier, which is
actually a mixing, then ……."Modulation or not Modulation"? Call
it as you will, but it is thanks to the gentlemen Jouaust and
Lacault that we are aware of this ingenious RF mixer, invented,
it must be remembered, more than ninety years ago!
simplicity lies the genius.
1- www.radiopharos.it -
The ingenious circuit
of Monsieur Lacault - Part I
The Modulation System and the Ultradyne
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1st Issue - September 2014