[Obligatory Telamon rec.radio.shortwave on-topic statement: The
following reply touches upon aspects of TRF design as they apply to MW
DXing, particularly sensitivity and selectivity of tube-based TRF
designs. MW DXing is on-topic to r.r.s. discussion. I appreciate
Telamon's efforts to keep r.r.s discussion strictly on-topic per the
written r.r.s. Charter.]


Patrick Turner wrote:
Jon Noring wrote:

Patrick Turner noted the following in a thread from early this year,
when someone asked about a single frequency BCB tuner:

"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."

The design I evolved for my variable frequency superhet leaves
behind all of the many AM radios I have repaired as new and tested
in my kitchen for comparison. Even an old Quad AM tuner I have is no
better.

Patrick, have you published the schematic for your radio? How amenable
is your design for turning it into a kit? And what are its overall
specs?

It sounds like a good MW DXer combined with high audio quality.


A TRF fixed F tuner with say two j-fets could be cobbled up
with preset RF transformers, in the same format as the double tuned
IFTs and with necessary couplings and input LC to broaden the pass
band. Miniature sized coils and cans could be used and each module
with the RF amps would fit on a board about 70mm x 40 mm, which is
2,800 sq.mm.

Thanks. The single frequency tuning board you sized is somewhat close
to the size I essentially guessed at. So we'll go with your estimate
of 70mm x 40mm.


120 such modules could then fit on a board about 350,000 sq.mm,
so that 10 boards would each be 35,000 sq mm, and about 190 mm x 190
mm, and thus all fit in a box the same size as a variable tuner
radio from the 1940s. Chuck in an extra board with tubed detector
and audio preamp, and 120 position switch, and a PS, and you're
done.

Another way to look at this is by a volumetric analysis, since one can
take advantage of plugging modules in a motherboard-like fashion.
Let's assume the modules will be 70 mm by 40 mm, and let's assume we
have to space the modules 19 mm apart because of the height of the
components soldered on. (Is this reasonable? -- 19 mm is the spacing
between PCI boards on a PC MB.) This means each module must minimally
take up a volume of 53,200 mm^3. 120 modules therefore will occupy a
minimum of 6,384,000 mm^3 (6.4 liters). This works out to a cubic box
186 mm on a side, or in English units 7.3" on a side (alternatively,
it works out to about 390 cubic inches for those not used to working
in metric.)

Of course, a cube is aesthetically and practically not the way the
modules would be distributed. So let's assume we plug the modules into
the "motherboard", with the long side (70 mm) sticking up. This would
yield a single motherboard footprint of 142 square inches, or about
one foot on a side, and about 3" tall. If we split it into two
motherboards, each holding 60 modules, then the footprint would be
8.4" on a side, with a height of about 6".

Yes, this is not an insignificant volume, but it is not a huge volume.
I notice that the three gang tuning air capacitor on my Philco 37-670
occupies a space of 3"x4"x7", or about 84 cubic inches, about 1/5 the
volume of the 120 module "box" (in a channel TRF, the tuning capacitor
would not be used.) Also remove the small volume taken up by the IF
section (no idea how much volume that typically takes up in a tube
set, but it is not tiny.)

Now, let's look at the 20 module motherboard. Here, the necessary
one-level motherboard would have an area of 24 square inches. Thus, a
6"x4" motherboard (make it 7"x5" for some clearance) will hold 20
modules, plugged in. The height will be 3". Now this seems reasonable
when the end-user only intends to tune in 20 local stations. It is
smaller in volume than the 3-gang tuning air capacitor on my Philco.
Of course, it will only tune 20 stations, and nothing in-between.

From the perspective of tuners in general (not specific to tube type
tuners, but also solid state and digital), the channel TRF does not
make sense. But with respect to a tube-based tuner, it does seem to
make sense for *some* applications. Since the only ones who will even
buy or build an AM tube tuner are tube-o-philes or tube-o-holics
(those who are attracted to tube-based equipment for whatever
aesthetic reason), the aspect of "commercial application" as we
understand it for ordinary radios does not enter the picture.

Those who simply want to get some job done with a radio (listening to
local stations, DXing, etc.), and are not overly enamored with any
particular under-the-hood architecture, will certainly NOT gravitate
to any tube-based tuner because of the much better and cheaper options
out there in the marketplace (digital and SS designs -- I don't know
of any tube-based high-end general coverage receiver being built
today -- and I'd be surprised if someone is attempting it.)

As Patrick noted, and which I agree wholeheartedly, pure digital is
the future of radio for utilitarian purposes (if BCB and FM radio
itself even has a future!)

But that's the point. In this discussion we are not talking about
building a radio for those who want to get a job done, but those who
are enamored with tubes and want the best possible sound out of the AM
tuner. For this purpose, the channel TRF is certainly a viable
candidate, along with a tuned TRF (as John Byrns is apparently working
on), as is the more traditional IF design (which Patrick says he is
working on.)


If the components for each tuner board cost $20, then about $3,000
for the 120 + PS, box, etc, all would be a steal, and a quite cheap
sort of "high-end" price. An asian maker of boards might reduce the
cost by 20 dB to $2 each.

Obviously, the component cost is significantly higher than for a
traditionally tuned circuit because one is using a larger number of
components, most of which will not even be powered while the tuner is
selected to a particular channel frequency. So in a sense, this is a
significant inefficiency.

But for a tuner intended to tune in local stations, the channel TRF
tube tuner appears to have some things in its favor. As a
tube-o-phile myself, one can make several strong arguments in favor of
the channel TRF tube tuner: 1) the circuitry is "clean", no IM mixing,
2) the bandpass filters are *perfectly* optimized for each channel --
no compromises (this is a *huge* attraction), 3) provides the ability
to plugin different bandpass filters for a particular station (if
needed), and 4) *may* be more amenable to a kit than would a full-
blown superhet design.

Now, if a tube-o-phile wants a tube tuner for serious MW DXing (for
whatever reason -- I would not use a tube tuner for *serious* DXing),
then the channel TRF is not down and out, but certainly has its work
cut out for it to try to compete with the continuously-tuned TRF, and
of course with traditional superhet designs. The need to include all
120+ BCB channels does work against the "channel TRF".

(On the other hand, I can see a serious MW DXer build a single-
channel TRF design of three or four RF amp stages where the bandpass
sections are "swappable" to tune the channel wanted to monitor.
Here the design will simply have a single slots for each bandpass
filter stage -- no channel switches. Just swap the mini-boards to
retune to a different frequency.)


I eagerly await your completion of a prototype of just one single
iddy biddy TRF tuner board which has all the discussed and wanted
capabilities with respect to audio BW, distortions, sensitivity,
selectivity to allow local station listening where weak and powerful
stations exist which are only 40 kHz apart, all without spurious
noise, interference, cross modulation, etc.

First, I assume that sensitivity is largely a matter of the RF amp
itself (and number of RF amp stages), not the bandpass filter itself
(although the filter should not overly get in the way of RF amp gain.)
But if the bandpass filter plays a greater role in sensitivity than I
realize, shouldn't an optimally tuned bandpass filter in the channel
TRF concept significantly outperform the limited and sub-optimal
single or double stage bandpass filters one is *forced* to use for
continuous tuning?

Second, each tuning module (for a single frequency) is, by and large,
independent of all the other modules. Thus, this simplifies the design
process since one doesn't have to share the same bandpass component
values from channel to channel, except maybe the RF transformers. This
should make it much easier, not harder, to achieve the performance
goals.

In the channel TRF, we are no longer constrained to single or double
tuning -- we can, for example, have the equivalent of quintuple
"tuning" for a 5th order bandpass filter if we want. I'm assuming
that, for a given frequency, the designer will have full control over
the values of all the LC components (and not just one or two,
excluding the RF transformer, though) in the bandpass filter, thus
making it much easier to achieve selectivity, distortion and other
performance goals, all the while simplifying the main part of the
circuit -- to make it cleaner -- fewer kludges needed.

(I keep looking at advanced radio circuits and see such a spiderweb of
wiring between the various stages, wondering why the hell it is all
there -- I wonder how much of that complexity is due to not being able
to properly optimize the RF bandpass filters for a given frequency,
thus requiring all sorts of work-arounds to get good overall
peformance.)

It is a remarkable achievement for a radio designer to meet the
several specification goals Patrick listed for an AM BCB tuner (his
list appears to be an "all things for all users" dream list) and which
is continuously tunable from 500 khz to 1800 khz (thus necessitating
most of the tuning components be shared.) I have no doubts that
Patrick has come up with a great design.

Superhet definitely helps with accomplishing this feat, but from what
I see, there are a lot of ****ty superhets out there, so if superhet
alone were sufficient, the perfect radio would have been designed
years ago. (Isn't the AA5 that perfect radio? -- it is, depending upon
the definition of "perfect" -- it is "commercially" perfect for the
masses.) IF is not the magic bullet (albeit it is a powerful one), but
simply a nifty tool to get from here to there. But like all nifty
tools, they have their limits and their place. One does not use a
hammer to drive in a screw, for example.


Almost the entire amount of AM radio reception theory that has ever
filled the minds of conscious humans has been repeatedly explained
so far in this thread, so you have all the knowhow you ever wanted,
so what's the hold up? Stop dithering, and go to it man!

Actually the know-how has not been explained in full! :^)


Thanks for your feedback. It is definitely adding useful information
to this thread.

Jon Noring