(assorted snips because these threads get sooo long!) see reply below

Jon Noring wrote:

[Following up on a thread dating back to January, similar to one I
started recently. Responding to Patrick Turner's comments.]


Patrick Turner wrote in January 2004:

Jerry Wang wrote:


1. Even it is a single channel [AM] receiver, I would still suggest
the use of one or two intermediate frequency (IF) stages. Because
to achieve good sensitivity you need to have enough gain.


Since you only want one channel, there is no need for a frequency
converter or any IFTs or IF amps, and a TRF with four tuned circuits
in the form of two critically coupled RF trannies will do nicely.


Interesting.

As I noted in a recent message, it is very intriguing to build a
modernized, high-performance AM tube tuner using the "channel"
approach. T
So, with respect to the channel approach, the next question to ask is
if we can use the same two critically coupled RF transformers (as
Patrick notes), and *independently* vary several of the other smaller
components (e.g., capacitors, resistors, and even inductors) in the
two or three tuning stages (if we include the antenna tuner) so as to
maintain, from channel to channel in the BCB, reasonably optimal
bandwidth and other desirable tuning characteristics?

[With traditional continuous tuning, achieved with multiganged air
capacitors, we do indeed vary a few capacitors in the tuning
circuitry, but because all of them track each other, in reality we
only have one degree of freedom, leading to circuit design constraints
for continuous "single knob" tuning. Now imagine, for each channel
frequency, to *independently* vary the value of several components at
the same time -- we now have several degrees of freedom to play with
and thereby hope

to achieve reasonably constant (as a function of
frequency) bandpass characteristics.

Obviously, architecturally implementing this in a practical AM tuner
design is not trivial (we do benefit by throwing away the multigang
air capacitor.) However, several ideas suggest themselves. For
example, we can imagine having multiple plugin slots, where we plug
into each slot a PCB mini-board specific to a particular frequency.
Of course, others here will probably have much better ideas as to
how to implement the channel approach.

Thoughts? Comments? Criticisms?

Jon Noring

For a one-channel receiver it makes perfect sense. Beyond that any
advantage is lost.
Why would I say that? You can create a perfectly acceptable single IF
filter with not so much ado. Lets use 455kc as the example. Its
considerably easier to build a single 'custom' IF filter at 455kc to do
what you want to do than it is a bunch of modules at three or four times
that frequency.
Yes, you could do as you suggest but I see no advantage in doing so. It
would be more critical, more expensive and probably not yield as good a
result as a nice 455 filter.

One thing I haven't heard mentioned, and admittedly I have only been
grazing what has been a very windy thread, why not use a WIDE 455kc IF
with tunable traps on either side? You can get a very steep skirt on a
good hi-q trap...likely steeper than in a transformer configuration that
is inherently q-disadvantaged. This would come in handy at night when
dozens of adjacent channels stations will be struggling to find their
way into your wide bandpass - and this scenario alone is a huge negative
about any wideband scheme that needs to be addressed.

A savvy person might be able to 'gang' the two adjacent channel traps
for a single knob "bandwidth" control.

My apologies if I'm missing the point. I'm unsure if the motive of the
discussion is that of a wideband AM radio or a discussion of ways to
reinvent the wheel.


-Bill M