In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

Patrick, you are missing the point, the issue was the merits of a 2.0 MHz
IF frequency vs. a 455 kHz IF frequency with respect to
bandwidth/selectivity, my point was that for the sort of bandwidths we are
talking about for audio, a 455 kHz IF can provide virtually identical
"pass band and attenuation out of band" with exactly the same number of
IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design
specifications in both cases, and are what they are.

Sure. But the same Q would give wider BW at 2 MHz.
I have not ever done this, so I guess at what the final response could be.

But so what, I thought we were talking about IFs for audio here, not video
IFs? For an audio receiver I would think at the most we would want a 40
kHz bandwidth, more likely 30 kHz, or even 20 kHz in the US where the FCC
effectively limits the audio bandwidth to 10 kHz? What exactly do you see
as the advantage of a 2.0 MHz IF in an AM broadcast receiver?

The way I see it both 455 kHz IFs and 2.0 MHz IFs can be built with the
bandwidth necessary for High Fidelity AM audio reception. The stage gains
will be virtually identical for both the 455 kHz IFs and 2.0 MHz IFs of
similar bandwidth, with the exact stage gain depending somewhat on design
choices and practicalities. The wideband 455 kHz IF will have lower stage
gain than a normal narrow 455 kHz IF, but the 2.0 MHz IF also suffers from
lower stage gain. The wideband 455 kHz IF has the advantage that standard
RF front-end components like tuning capacitors and oscillator coils can be
used, while the 2.0 MHz IF will require special RF components.

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

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? 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 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.

To illustrate this consider the example of the following calculated
response curves for both a 455 kHz IFT and a 2.0 MHz IFT:

The only advantage the 2.0 MHz IFT shows is marginally better symetry of
responce about the ceter frequency, the response of the two IFTs is
virtually identical.

The equality in performance depends on a large Q difference, with
544 kHz Q much lower than 2MHz Q to get the same BW.

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

The Q of a typical 455 kHz IFT is higher than you have indicated, because
the impedance of the LC circuit at Fo is required to be high to suit
pentode loading, and to get high gain.

You also are going to sacrifice stage gain in the same way with a 2.0 MHz
IF, so this is no more of a problem for the wideband 455 kHz IF than for
the 2.0 MHz IF.

If the Q was real low, and hence the Fo impedance, you
would probably need 3 IFTs.

This is a consequence of the wide bandwidth, not the IF frequency, the
problem is identical at 2.0 MHz.

I have never tried 3 very damped IFTs.

The fact that you haven't tried something doesn't prove anything one way
or the other. 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.

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.

Build a radio with 2MHz and measure it, maybe it works better.

You are the 2.0 MHz IF advocate not me, you still haven't suggested any
reason why it might work better from a bandwidth/selectivity standpoint?

Just don't knock the idea before trying it, or condemn the idea
with postulations about what might be.

I'm not, I know it would work, what I don't understand is what the
advantages are over a 455 kHz IF of the same bandwidth? You are not
explaining yourself, cite some concrete facts.

These things must be tried and measured, to really know.

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. Back in the old days of land mobile
here in the US, wider channels with greater bandwidth were used than are
used today. Over time the channels were squeezed down to accommodate
additional channels in the same space, and block filters of several
different bandwidths were available to suit the changing allocations and
operating frequencies.

I have also built wideband single frequency TRF receivers using modified
double tuned IF transformers.

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.


Regards,

John Byrns


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