In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been
mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

I supect it might, and one article in Wireless World refered to using
10.7 MHz.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar. What
exactly did Wireless World say was so great about using a 10.7 MHz IF for
a MW AM receiver?

Wide AF response was easily achieved.

Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed, although in the old days they
often did have articles by people who knew what they were talking about
with respect to radios.

I differ. WW and what it became, Electronics World wasn't just an
amateur's magazine. It had cutting edge articles about electronics
from 1917 onwards, and I suggest you park yourself beside a
pile of all the old copies and have a good read.
Most of the info was only comprehensible by very well university educated
professionals, or intellectuals, and most ideas were backed up with
mathematical
proofs which nearly all the general public couldn't understand.

I am reasonably familiar with Wireless World, I have 3 & 1/2 of those copy
paper boxes full of old issues from the 1930's through the 1950's. I
would estimate that I have at least half the issues from that period whcih
was probably the golden age of AM receiver technology. I have to take
serious exception to your characterization of the "mathematical proofs"
included in their articles. There may have been the odd article with some
mathematical depth, but those were few and far between. The math
presented seems to have been just enough to go over the head of the
average reader, but was hardly complex enough to be "only comprehensible
by very well university educated professionals, or intellectuals". I
suspect this light weight approach just slightly above the level of the
man in the street was carefully calculated to impress the average reader
without putting the material at a level where he couldn't understand it at
all. That is not to say that they didn't have many excellent authors who
knew all the math, but it is a serious stretch to imply that they included
any real mathematical depth, they included only enough to look impressive
to the untutored reader.

I suspect that the reason Wireless World might
have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an
easy way for a hobbyist, who both doesn't have a clue what he is doing,
and doesn't have the necessary test equipment, to get a super wide
bandwidth.

I leave you to your suppositions.


OK, but for all practical purposes my "supposition" seems to be identical
with your statement above that "Wide AF response was easily achieved",
which I take to be a quote from the actual Wireless World article?

Damping reduces Q, and increases BW.
But it also reduces Z at Fo, thus reducing gain in an amp
which must be a current source, like a pentode or j-fet,
to realise the best selectivity for the LC circuit.

This is a half truth, what matters is that the filter is correctly
terminated, not that pentode, triode or whatever drives it. As far as
stage gain goes, increasing the frequency from 455 kHz to 2.0 MHz is
likely to decrease the gain by a similar amount to widening the 455 kHz
filter to the same bandwidth as the 2.0 MHz filter.

What I said was what I said.
You are confused.

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

I know enough about IFT design, after having built my own radio.

That isn't clear at all, you seem to be obsessed with "Q", and hardly if
ever mention "k", and how it relates to "Q" in determining the
characteristics of an IFT. You occasionally mention "critical" coupling
but haven't tied that concept in with the "Q" and "k" of an IFT, nor have
you mentioned the related concept of "transitional" coupling. I would
expect to hear more mention of these concepts from someone who knows
"enough about IFT design".


Regards,

John Byrns


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