John Byrns wrote:
What exactly are the advantages of a 2.0 MHz IF from a
selectivity/bandwidth point of view?

Both pros and cons to this John.

Since the bandwidth is a percentage of the center frequency,
the shape of the bandwidth will change based on the distance
from the center frequency (as a percentage.)

Assuming just for the moment +/- 5 KHz.
At 455 KHz that's about 1% above and below.
At 2 MHz, that's now only .25% above and below.

As you get further from the center frequency, percentage
wise, the shape of the curve as it transitions from inside
to outside of the band pass is going to look different at
the upper frequency than it does at the lower frequency.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar.

With the notable exception of the difference in shape of the
roll off above and below the center frequency.

In the world of designing filters (and overall system performance)
this is called group delay. A shorter, perhaps more recognizable
term would be linear phase shift over the entire band pass of the
filter.

Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed

They were under the same constraints as the Weekly World News.
"If it wasn't true, they couldn't print it." Note smiley face
here. ;-)

Yes, although I have some reservations about the use of the term "Q", that
is obvious, but so what, what difference does it make?

Back to the original comments about Q. In a perfect world, it
would only be a matter of the LC ratio setting the bandwidth
of a tuned circuit. Of courses, there are other things that
get in the way to reduce the overall Q of a circuit. Nasty
little things like the series resistance of the coils, dielectric
losses in both the coil forms and capacitor insulation material.

Back to the original "ideal" values of Q. 15 at 455 KHz and
67 at 2 MHz. It is physically "more challenging" to get higher
Q at a higher frequency. All of the various losses of the
components tend to get in the way. Wire losses, dielectric
losses and any losses of the ferrite used in the core materials.

Also, what does "damped" mean in this context? I would
have to do some research, but I suspect that "damping" is more related to
filter bandwidth than to the center frequency, and both filters are aiming
for the same bandwidth.

"Damped" means adding some form of resistance across the reactive
components of a circuit. As an example, if you were to assemble a
nice 455 KHz IF transformer and found that the bandwidth was too
narrow, a fast method of widening it would be to place parallel
resistors across the windings.

Another point about "damped" is that if a tuned circuit has too
high a Q, a sudden transient will tend to make it oscillate.
In communications receivers, this is obvious that a signal sounds
more like you're ringing a bell, than simply turning a tone on
and off. (Kind of like using the sustain pedal on a piano.)

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.

Lets not go in that direction.

An IF transformer is simply a two pole butter worth filter.
That it can have different input and output impedance just
makes it really convenient for taking the source from a plate
and connecting it to a grid for a load.

By definition, a butter worth filter has a smooth curve with
only one peak (in the middle.) And the shape (steepness) of
the band pass is related to the overall Q of the circuit.

The next type of filter, would be Chebychev, This is no more
than a "predistorted" butter worth filter network. By allowing
a certain amount of ripple in the pass band, the shape of the
rejection can be made sharper. The obvious trade off is the
amount of distortion to the signal within the pass band.

A simple example of this would be stagger tuned IF coils.
Two or more peaks, and a dip (or dips), ripple, in the middle.

While I can't claim to have designed the filter I used, I have actually
built a transistor superhetrodyne AM tuner using a 455 kHz block filter
with a 30 kHz IF bandwidth. Will the 2.0 MHz IF work better than this?
Have you tried a properly designed wideband 455 kHz IF filter to see how
it worked? The filter I used came out of a 2-way land mobile radio and I
think it was about an 8 pole filter.

The point you've probably overlooked in land mobile operations is that
it was NEVER designed as a "hi-fi" system. There's a reason for the
term "voice grade." Having as much a 3 dB of ripple in a band pass
filter is meaningless especially when the filter is in the midst of
a limiting IF strip for FM recovery, and on AM demodulation. What
really matters here is limiting the bandwidth of the received signal to
ONLY include that of the wanted (in channel) information and none of
the unwanted (adjacent channel) information to get to the discriminator.

So what it boils down to is that you haven't tried a wideband 455 kHz
filter while I have, and I haven't tried a 2.0 MHz IF filter, which you
may or may not have done. I at least have cited some concrete facts about
IF filters, while you have only muttered about Q, without indicating how
it actually relates to the problem. I am not a "filter jock" (tm) but I
think it is generally desirable that the Q of the components used in a
filter be high, especially when we get beyond simple double tuned
transformers. What you are calling Q is more related to how the filter is
terminated, which is a different matter than the Q of the components that
make up the filter.

You should take the time to read up on "filter jockeying" John.
You're making a lot of incorrect assumptions on how they work.

The primary requirement on the Q of individual components in
filter design is only such that their value of Q be high enough
to not materially effect the overall Q of the circuit. As an
example, (and without getting into cryogenic treatments and
styrofoam cups) A speaker system sounds better through 25 feet
of #12 AWG wire than it does through 25 feet of #18 AWG wire.
And that's strictly due to the resistive loss of the wire in
comparison to the losses in the actually speaker design and
implementation.

I had a electronics instructor in college that would show you
"The secret of electronics" that he kept hidden, and locked,
inside a small jewelry box if you "caught on" during his course.

With some fanfare, he would slowly open the box and you would
see an inductor, a resistor and a capacitor.

And it's really just that simple. What gets complicated is when
you forget that all three items have hidden values of the others
contained within them. (I.e. the difference between practical
and theoretical parts.)

Jeff



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