In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

Better of course would be to have a CF tube to accept the IF envelope,
and the low impedance output from the CF can then power a crystal
diode, or
a tube diode
in a variety of ways I have previously explained in post on the matter.

You still haven't explained how this added cathode follower, to drive the
detector, helps matters? Many experts even make the claim that a finite
source resistance can be beneficial in reducing distortion, especially
high frequency distortion.

IN my case there *is* a finite source resistance which is the 100k
R across each IFT winding.

Yes, but the diode is driven from the low source impedance of the cathode
follower, not something on the order of 100k. Why not eliminate the
cathode follower and choose the diode detector load so that it looks like
100k to the IFT at 455 kHz?

I can see where a cathode follower could be
beneficial if we were trying to build a radio with an IF as narrow as
possible, in which case it would help keep the Q of the transformer
secondary as high as possible, but we are talking about a radio with wide
band audio, and are probably talking about adding loading resistors across
the transformers anyway, so why the cathode follower, why not just let the
load of the detector diode do the job? A cathode follower after the IFT
seems like a waste to me, better to use it after the detector, with a
negative cathode supply voltage, to buffer the detector from the AGC and
audio lines.

I do things to suit the desire for wide as possible AF bw, and the
R loading of the IFTs helps achieve that end.
I don't want severe selectivity and IFT gain; that only belongs
in Z grade AM radios and communications sets.

Yes of course, I was simply trying to point out one situation where a
cathode follower driving the diode might be useful.

Try using a CF buffer to power a detector with a germanium diode, you'll
hear the
difference!

Like others, yourself included, I am just prejudiced against some ideas,
and a cathode follower between the IFT and detector is just something that
I have little intention of trying.

Measurements will confirm the improvement.

Or they may only confirm that the cathode follower helps with your
detector design for some as yet unexplained reason, but doesn't help in
the general case, see my comments further along. Have you measured
identical detectors with and without the cathode follower? The same diode
driven at the same level, with the same DC bias applied, and the same
total load reflected to the IFT secondary at 455 kHz?

While many AM receivers have been designed in a cost conscious way, there
have also been a few where no expense was spared, and parts were freely
used, and yet I have never seen a cathode follower used as you propose in
a commercial design, I would think if it were beneficial someone would
have used it commercially, anyone know of any examples?

I have NEVER seen any ancient commercially produced radio or audio
product where
the sound quality was not compromised, often severely, with many lies told
by the market cowboys, after the maker had reduced the parts count to
reduce costs, to be able to compete.

There were certainly commercially produced AM tuners where the maker
didn't reduce the parts count at all in order to reduce the costs, the
sound quality may or may not have been compromised by your standards, but
if it was, it was due to a poor use of the parts, rather than to a lowered
parts count. There are certainly commercial AM tuner designs that use
significantly more parts than your tuner uses.

Some designs add
other relatively expensive parts to the detector circuit, one trick I have
seen whose effects might be worth looking into is replacing the second
capacitor in the peak detector & RF filter network with a series LC
network tuned to 455 kHz. I don't know how this circuit actually works,
but I assume that the idea is to improve the tradeoff of the total peak
detector capacitance vs. tangential clipping at high frequencies.

A series LC tuned to 455 kHz needs to be driven by a low impedance
to get a decent Q to reject the 455 kHz ripple,

This is patent nonsense, if anything just the opposite is true, if a
series LC to ground were driven by a very low impedance it would have
virtually no effect.

but it simply
is far easier to achive in well known ways with R&C.
Usually, the CRC arrangement of 100pF, 47k, and 100pF is entirely
adequate for removing RF detector ripple voltage.

Just goes to show that those manufacturers that included the series LC
weren't among the ones you are speaking of that reduced the parts count
wherever they could.

This is
something I will have to look into further. There are other detector
circuit subtleties like this that may, or may not, be worth while.

You need to use a soldering iron to find out about what I am promoting
about AM detection.
There is no other way.

Actually I am fairly certain that there are several other ways, like me,
you are simply prejudiced towards your own ways. I think I finally
understand what you are doing with the bias on your detector, and how it
works. I believe I have misunderstood what your biased detector was all
about and you haven't explained it. You have made statements like "This
method means that detection of weak signal lower than the forward voltage
of the Ge diode of 0.27v peak approx are not subject to the non linear
turn on of the diode, ie, there is no clipping by the diode." This lead
me to believe that you were using the bias to somehow "linearize" the
diode, which I didn't understand. The figure from the RDH4 which John
Stewart posted finally made me realize that you were doing exactly the
same thing with your bias as the RDH4 figure, except that you used a fixed
bias and left out the tracking feature. The distortion reduction you
claim makes sense in that context, because your receiver as described by
the schematic you posted has an extremely poor AC/DC load ratio and I am
sure the distortion is extreme without the bias. Your bias scheme
presumably partially compensates for the poor AC/DC load ratio, rather
than somehow improving the "non linear turn on of the diode" as I had
erroneously assumed from what you have said.

Now the only remaining question is, does the apparently pointless cathode
follower driving the diode also compensate in some way for another
unnoticed design flaw?


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/