John Byrns wrote:
In article , Patrick Turner
wrote:
John,
You seem to be limiting your considerations to the 'tangential clipping'
and not to other distortions that will occur.
On the contrary I alluded to the other problems in my post above where I
said "while the traditional RC circuit has its problems". The problem is
that Patrick has a very thick head, so I am trying to keep things simple
so he might get the point.
Unfortunately, you have not yet done a proper comparison measurement
of a traditional detector driven off an IFT secondary, and compared
the results to what I have proposed and posted using two CF tubes.
I'm not sure what "proper comparison measurements", or "CF tubes", have to
do with the theoretical aspects of tangential clipping?
It means to build samples of two comparable circuits, and
thoroughly measure and observe the workings of each,
and make your conclusions.
That isn't too hard now surely?
Unfortunately I
don't possess an AM generator that is adequate for making these
measurements, that is a generator that will do 100% negative modulation,
or anywhere near it, with low distortion. It isn't clear that you possess
such a generator either, and you seem to have engaged in a certain amount
of shucking and jiving with respect to the actual performance of your
detector.
I have two such generators, a old Topward, which uses chips to easily get
100% modulation of any signal between 2 Hz and 2 Mhz with a 400 Hz tone.
The other is a tube one I built, which is also capable of 100% modulation,
but the thd in the AF envelope is around 3% at the onset of 100% modulation.
Why don't you describe the AM generator you are using, how close it gets
to 100%, and what the distortion is at that point? I know you are using a
CRO to check for distortion, rather than a distortion analyzer, but still
if the generator has serious distortion at the extremes of modulation it
could mask some of the faults in a detector.
If you have a dual trace CRO, you should be able to view the
input gene RF signal on one trace, and the recovered audio from the detector
on the other trace.
Each trace can be overlaid, and at low % of modulation, the recovered
audio trace should perfectly outline the input RF signal modulation,
so if there is any thd in the input modulation shape, it doesn't matter.
The two traces should remain locked close when the amplitude of
the input RF and or modulation % is increased at least to an equivalent
level of signal to a strong local station when its received using 10 metres of
wire
as an antenna, and perhaps -8 volts of AGC is being generated.
I don't have the time to spend on discussions that get nowhere with
someone who hasn't the time to connect a handful of parts on a bench, and
do some real work, instead of endlessly talking around the subject,
I am working on such a project, but first I must solve the AM generator
problem. I am beginning to get an idea or two as to how I can overcome
the AM generator problem. You have nearly pushed me to the point of
action in my workshop, as Danger Dave did a few years back with respect to
the workability of an amplifier design I had been contemplating for
several years. Once I built it, Danger Dave was quickly proved to be full
of it, and I expect a similar result again, once you have motivated me
sufficiently. But first lets hear more about the AM generator you are
using for your tests?
Its a simple triode oscillator with a grid LC, with tap on the LC for the cathode
current.
This feeds a 6BX6 RF amp, which has an LC in its anode circuit.
The AF is fed into the 6BX6 grid circuit to alter the anode current at AF, and
modulate the output at the anode.
The anode LC has a secondary winding to reduce the output impedance.
One two gang tuning cap from an old radio is used.
It took about a fortnight to build, and a fortnight to de-bug, and to get the
sawtooth oscillator working, so when switched to 455 kHz,
the Fo could be wobulated 40 kHz each side to display the IF bandpass contour.
I used about 10 x 68v zener diodes operating at a DV lower than the zener voltage
to make a varicap diode to give a high enough C shift to cause the wanted F
deviation.
Some wobulators use a spinning tuning cap driven by a motor, but I wanted
no mechanical parts which could wear out.
and making incorrect statements about skull bone thickness.
The process taught me all about my own cerebral bone thickness.
I took aim as to what I wanted from thre test gear, and I didn't stop
until I achieved it.
Probably just as well I never had any involvement with discussion groups
back in 1993 to 2000, I was learning by doing, and I wasn't able to load all my
dumb
questions, about 20 per day, onto any news group.
I answered many questions myself.
As they say, "if the shoe fits wear it"! I remember the "thick as a
brick" thread from earlier this year, where you clearly demonstrated the
thickness of your skull. For those don't remember, that adventure might
have been called the "octave" matter. It was related to the slope of the
attenuation curve of an RF tank circuit, IFT, or other similar circuit.
Phil Allison and I were quite correct in our assessment about
attenuation rates in RF tank circuits, and I was the one to measure a typical
LC taken from an old radio and post the results at the binaries groups,
to prove and define what I was saying, leaving no room for any doubt, or BS.
Workshops, simulations, and what not didn't enter into the matter because
you had conveniently measured, plotted, and posted the response curves for
an AM aerial circuit which made a perfect example for discussion. The
trouble started when you and your fellow countryman Phil Allison claimed
that the slope of the attenuation curve of a tank circuit was stepper
close to resonance and that the slope of the attenuation progressively
became less steep as you moved away resonance.
Well indeed the rate of attenuation is steeper near Fo, and then becomes less.
There is only 6 dB /octave attenuation when you are 20 dB or more away from Fo.
Say you have a tuned LC with Fo = 1,200 kHz, then at 600 kHz, the rate of
attenuation
is 6 dB /octave, so that between 600 kZ and 300 kHz, there is only 6 dB of
attenuation.
But close to Fo, within a few kHz, and if the Q of the LC is say 50,
the rate of attenuation is far far greater than 6 dB / octave with regard to RF
frequencies.
The rate of audio F carried by a modulated carrier follows the RF attenuation
shape.
I had been under the
impression that the slope of the attenuation curve actually increased as
you moved away from resonance, and asymptotically approached a slope
determined by the order of the filter. After a few back and forths it
became obvious to me that the problem was one of the different frequency
reference points we were using, you and Phil were using Zero frequency as
your reference, while I was using the center frequency of the filter as my
reference. At that point I said I completely agreed with your
conclusions, given your frame of reference, but you refused to accept my
concept of using the filter center frequency as an alternative view of the
situation, and told me it just wasn't valid. That is a perfect example of
a thick skull, since generally there are alternate definitions for things,
and as long as they are consistent with the facts, in that case your
measured and posted results, they are just as valid as what you may
consider to be a more conventional viewpoint, although in the case of the
"octave" matter I am not entirely sure yours was the conventional
viewpoint, but the bottom line was they both worked, and you denied that
my approach had validity.
I have seen no reference of your interpretive methodology in any text books,
and the text book methods to which I adhere to explain it all nicely,
and I don't have any intention of going right through all that long and tortuous
discussion again.
I been there, done that, and I have moved on.
Meanwhile, I am probably wasting far too much time as it is re-building an
old Stromberg Carlson, giving it a real good Turnerization, with singing lessons
included.
But slowly the work looks promising....
It will never be as good as my other radio circuit which I posted last week
because
the IF tube is a vari mu 6G8, and I have AGC applied......
Patrick Turner.
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/