Robert Casey wrote:
Jon Noring wrote:

I guess the question to ask is how much better can be done when
continuous tuning is eliminated, and one uses an optimum bandpass
circuit for each channel frequency? (It appears possible to get
almost uniform bandwidth and shape across the entire BCB spectrum.)
For high-fidelity audio purposes in the channel TRF concept, what
order and type of RF bandpass filter circuitry suggests itself?

The upside of several single channel TRF circuits is that each
channel's TRF circuits can be custom tuned for frequency and
bandwidth. The downside is that this will require a lot of parts
for more than a few selected channels, and that if someone moves to
another radio market a lot of retuning is required if all channels
aren't built to begin with.

Yes, this is a downside, but as I see it now, it may not be that much
of a burden -- it depends upon the use. It also opens up many
interesting opportunities for the hobbyist.

There are two general approaches to wiring up the independent bandpass
filters for some or all of the BCB channels:

1) Hardwire all the filter components for all the BCB channels onto
one large board (we'd have an "American" board and a "European"
board, both premade PCB.) This is not trivial, and we could have
upwards of 1000 small RLC components needing to be soldered on the
board, depending upon the order of the filter we want to use.
That's a whole lot of work. It is also inflexible -- the whole
board must be committed to one particular bandpass filter type and
order (e.g., it must be a 4th order Butterworth -- bandwidth is
adjusted by altering the values of the components soldered in as
the supplied "chart" will indicate.)

2) The mini-board idea, where the filter components for a single
channel frequency are put onto a small PCB mini-board. The user
plugs the mini-board into a slot to connect it to the RF amp
section (probably with antenna tuning as well.) We could imagine
having a large PCB "motherboard" which has up to 130+ plugin slots
(not unlike those used for PCs, but we need only have a small
number of contacts per slot -- the number I can't guess at the
moment.) A switch will also be needed (is an electronic switch a
possibility?) Of course, a smaller board with 20 slots, with a
twenty position switch, could be made for those who do not
anticipate tuning anymore than 20 channels. For local listening
(especially for the simple 1 RF amp stage tuner where it won't
be very sensitive), this is probably more than enough channels.

The advantage of this approach is that the user needs only to get
boards for the BCB channels they will listen to, and will have the
ability to alter the bandpass characteristics for a particular
channel (just wire up a different mini-board tuned to that
frequency.) For example, one could have a 7th order Chebychev for
1130 khz with a bandwidth of 15 khz, and a 4th order Butterworth
for 750 khz with a bandwidth of 10 khz. At a later time, the user
can change the bandpass filter used for any particular channel --
just swap mini-boards.

Now how big does the mini-board have to be? I don't have a good
feel for this, while the experienced radio builders out there will
have a much better idea. But let's look at what the mini-board
will contain. Essentially it will contain the RLC bandpass filter
components (plus a trimmer or two for fine calibration of the
center frequency). Depending upon the order of the filter used,
it may have anywhere from 5 to 10 RLC components (again just a
guess -- the very high order bandpass filters will have more.) So
the mini-board will need to be big enough to hold these components.
Again, I think most of them will be fairly small in size, so it is
not inconceivable for the mini-board to be as small as, for
example, 1" x 2" (again, only a guess -- anyone?)

I also foresee that there will be a standard bandpass filter for
the channel TRF tube tuner (a given type and order -- what would
you use for a single RF amp TRF tube tuner for local listening?)
One can have a large number of PCB boards made for that bandpass
filter. Then, for a given channel frequency (e.g., 830 khz), and
a chosen desired bandwidth, the kit-builder refers to the table of
values for each component (e.g., this resistor will be 50 ohms,
that capacitor 5 pf, etc.), solders them in, then fine calibrates
the center frequency. I don't imagine these boards, when made in
bulk, will be that expensive, neither the components be, nor will
it take much time to solder the components onto the mini-board --
maybe only a few minutes.

The biggest issue I foresee is the fine calibration of the bandpass
center frequency -- can that be done independent of the tuner (thus
allowing the kit-supplier to make them available on order) or must
the mini-board be plugged into the tuner? Since there will be some
distance between the RF amp tubes and the bandpass filter, with
intervening wire, a switch or two, and slot connectors, there will
be interwire resistance, capacitance, etc. For a real world tuner,
how important will this be?


As to the question of what the unused channels will do to the
receiver, I'd do the switching such that all unused channel circuits
are completely switched out, and maybe tied to ground.

Agreed, although there are probably other possibilities.

One interesting aspect of this design is that an enthusiast could add
a more traditional continuous tuner if they wanted to (e.g. with a
multiganged variable air capacitor or inductor -- just build it
separately and plug it into one of the slots.

I really do think there are other interesting things one might do
with the "channel TRF" tuner concept. I think we have just scratched
the surface.

Jon Noring