[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. This takes advantage of the fact that licensed broadcasters
today must broadcast on specific frequencies, every 10 khz in North
America and 9 khz in Europe and elsewhere. So, instead of trying to
be able to continuously tune across the BCB spectrum, we can think
outside the box for the moment and consider the alternative of
building reasonably optimized tuning circuits for each listened-to
frequency. There'd be a switch to select from a number of channels,
each associated with a specific frequency the user wants to listen to
(suggesting a plugin mini-board for each channel, but there are other
possible configurations.)

I infer from what Patrick said that it is unnecessary for a single
frequency AM tuner to be a super-het design, and that (I assume) a
much simpler two RF amp TRF design is sufficient for good to excellent
audio quality and good to excellent sensitivity and selectivity. (John
Byrns implies the same in his various comments on TRF AM tuners.)

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.
The board will contain the few components whose values *independently*
change as a function of frequency. They probably will have trimmers
for fine calibration of the center frequency and other bandpass filter
characteristics. We may need multiple mini-boards for each channel
(one for each tuning stage) if necessary for shielding purposes (to
prevent oscillation by stage-to-stage interference if that is a
problem.) And if higher frequency channel boards require some minor
changes in the circuitry configuration, and not just component value
changes, that can easily be done, too. In principle, this tuner might
even be able to extend a little beyond (on both sides) the 500-1800
khz MW band -- just plugin the right mini-board circuitry for the
frequency desired.

Of course, others here will probably have much better ideas as to
how to implement the channel approach.

Thoughts? Comments? Criticisms?

Jon Noring


(It's interesting to think of doing the same "channel" approach for
an FM tube tuner. Will that also confer several advantages in
simplifying the circuit design for the same overall performance
level?)