Last night I posted a couple messages regarding the proposed "channel
TRF" AM tube tuner, focusing on the plug-in mini-board idea as one way
among several possibilities to implement it.

The idea underlying the channel TRF concept is to build bandpass
tuning circuitry specific to, and optimized for, each frequency in the
BCB, instead of fixing that circuitry to some "average" value and
trying to vary it using a traditional variable air capacitor (or
variable inductor) for continuous tuning. A switch would be used to
select the bandpass circuitry for the particular frequency channel the
listener wants to hear.

This would allow, in principle if not in practice, the ability to
very precisely optimize the bandpass circuitry (to maintain a quite
constant bandwidth and shape) for every broadcast frequency in the BCB
(from 500 khz to 1800 khz.)

The "mini-board" variation of the concept would place the bandpass
circuitry for each channel (frequency) onto a small plug-in PCB board.
Depending upon the type and order of bandpass filter used, the number
of components on the mini-board may be quite small, maybe a couple
capacitors, a resistor or two, an inductor, etc., having the optimal
values, and with one or more trimmers for fine adjustment of the
center frequency.

Clearly there are several implementations of the general concept, one
of which is a well-known hybrid that allows continuous tuning in the
more traditional and familiar way. The ones I think of at the moment
a

1) Traditional continuous tuning: Divide the wide BCB into several
sub-bands, such as 5 or even more, each sub-band having optimized
bandpass circuitry for the sub-band, and then use the traditional
variable capacitor or inductor to tune within the narrow sub-band.
Although each channel will no longer have the most optimal bandpass
configuration, it will be closer to optimal.

2) Single Board, True Channel: It may be possible, instead of having
120+ totally independent channel circuits each placed on a separate
mini-board, to put them all onto one larger board, but still keep
all circuits otherwise separate on the board. A lot of components,
and probably a lot of trimmers.

3) Single Board, Shared Components: As a combination of items (1) and
(2), channels which are adjacent to each other (in their own
"sub-band") could probably share a lot of common bandpass
components, thereby reducing the number needed on the board. Only
the large number of trimmers for individual channel calibration
will remain.

The original idea of mini-boards is most advantageous when the user
of the TRF tube tuner only plans to listen to 10-20 stations (such
as local, higher-power stations). They only install the channel
mini-boards they want to listen to.


*****

I do have a couple questions of both John and Patrick (and anyone else
caring to chime in) related to this.

1) In the single frequency TRF tube receiver (a TRF designed strictly
to listen to a single frequency), is there a need for double tuned
circuits? Or will singly tuned circuits be sufficient for
excellent performance (audio quality, sensitivity and
selectivity)? If not, how do double tuned circuits benefit the
overall performance of the single frequency TRF receiver?

2) Let's assume that we decide to design a Mark I TRF AM tube tuner
kit designed solely for more local, higher power stations (thus the
sensitivity is less critical than a tuner to also be used for
casual DXing.) How will this further simplify the optimal single
frequency TRF receiver design? Will only one RF amp stage be
necessary, or will we still need two? The focus now will be on very
high-quality audio reproduction of local stations, which I believe
tubeophiles will be most interested in.


Thanks.

Jon Noring

How many stages you need depends on the selectivity you need because of your
geographic location and antenna Two stations close in frequency will
interfere with each other unless you have enough selectivity. A weak local
station that is strong enough to be heard may get splatter from a distance
station close in frequency that has 50KW or more and a pattern that
concentrates on your area! High Q, double tuning, addtional stages all add
to selectivity. The spuerhet solved this problem.

But if you want to satisfy your demons with a mdoular approach you might
want to consider salvaging a couple of turret type tuners from 1950s TVs.
These have clip in moddules with silver contacts and appropriate LC for each
channel. With todays ferrites you should be able to squeeze-in a LC for AM
BCB. Each tuner will give you 12 channels.

--
73
Hank WD5JFR

"Jon Noring" wrote in message
...
Last night I posted a couple messages regarding the proposed "channel
TRF" AM tube tuner, focusing on the plug-in mini-board idea as one way
among several possibilities to implement it.

The idea underlying the channel TRF concept is to build bandpass
tuning circuitry specific to, and optimized for, each frequency in the
BCB, instead of fixing that circuitry to some "average" value and
trying to vary it using a traditional variable air capacitor (or
variable inductor) for continuous tuning. A switch would be used to
select the bandpass circuitry for the particular frequency channel the
listener wants to hear.

This would allow, in principle if not in practice, the ability to
very precisely optimize the bandpass circuitry (to maintain a quite
constant bandwidth and shape) for every broadcast frequency in the BCB
(from 500 khz to 1800 khz.)

The "mini-board" variation of the concept would place the bandpass
circuitry for each channel (frequency) onto a small plug-in PCB board.
Depending upon the type and order of bandpass filter used, the number
of components on the mini-board may be quite small, maybe a couple
capacitors, a resistor or two, an inductor, etc., having the optimal
values, and with one or more trimmers for fine adjustment of the
center frequency.

Clearly there are several implementations of the general concept, one
of which is a well-known hybrid that allows continuous tuning in the
more traditional and familiar way. The ones I think of at the moment
a

1) Traditional continuous tuning: Divide the wide BCB into several
sub-bands, such as 5 or even more, each sub-band having optimized
bandpass circuitry for the sub-band, and then use the traditional
variable capacitor or inductor to tune within the narrow sub-band.
Although each channel will no longer have the most optimal bandpass
configuration, it will be closer to optimal.

2) Single Board, True Channel: It may be possible, instead of having
120+ totally independent channel circuits each placed on a separate
mini-board, to put them all onto one larger board, but still keep
all circuits otherwise separate on the board. A lot of components,
and probably a lot of trimmers.

3) Single Board, Shared Components: As a combination of items (1) and
(2), channels which are adjacent to each other (in their own
"sub-band") could probably share a lot of common bandpass
components, thereby reducing the number needed on the board. Only
the large number of trimmers for individual channel calibration
will remain.

The original idea of mini-boards is most advantageous when the user
of the TRF tube tuner only plans to listen to 10-20 stations (such
as local, higher-power stations). They only install the channel
mini-boards they want to listen to.


*****

I do have a couple questions of both John and Patrick (and anyone else
caring to chime in) related to this.

1) In the single frequency TRF tube receiver (a TRF designed strictly
to listen to a single frequency), is there a need for double tuned
circuits? Or will singly tuned circuits be sufficient for
excellent performance (audio quality, sensitivity and
selectivity)? If not, how do double tuned circuits benefit the
overall performance of the single frequency TRF receiver?

2) Let's assume that we decide to design a Mark I TRF AM tube tuner
kit designed solely for more local, higher power stations (thus the
sensitivity is less critical than a tuner to also be used for
casual DXing.) How will this further simplify the optimal single
frequency TRF receiver design? Will only one RF amp stage be
necessary, or will we still need two? The focus now will be on very
high-quality audio reproduction of local stations, which I believe
tubeophiles will be most interested in.


Thanks.

Jon Noring

Henry Kolesnik wrote:

How many stages you need depends on the selectivity you need because
of your geographic location and antenna. Two stations close in
frequency will interfere with each other unless you have enough
selectivity. A weak local station that is strong enough to be heard
may get splatter from a distance station close in frequency that has
50KW or more and a pattern that concentrates on your area! High Q,
double tuning, addtional stages all add to selectivity. The superhet
solved this problem.

Hmmm, the "channel TRF" approach may help with one stage selectivity
since it appears we can now use a perfectly optimized higher order
tuned filter on the channel mini-board, while in a traditionally tuned
circuit, implementing that same bandpass circuit to apply across the
whole BCB will be much more difficult, and I would guess be near
impossible (too many circuit components which need to be varied
simultaneously as one varies the reception center frequency.)

With the channel TRF approach, the tube-o-phile can mix and match
bandpass filter types from station to station depending upon the
circumstances. For example, they could use the default, wider-band,
gentler, bandpass filter plug-in board (one which has better linear
phase) for a local station which doesn't have adjacent interference,
and for a more difficult station (with adjacent interference) they can
use a bandpass filter plug-in board with a shape factor closer to
unity (which probably has more ripple and worse linear phase). (Even
for the default "wider-band" filter, because we can now use a
frequency optimized higher order filter, we should be able to achieve
reasonably good selectivity, at least sufficient for local station
reception, even with one RF amp stage.)

There appears to be a lot more freedom given to the circuit designer
when the necessity of tuning a fixed set of tuning components over a
frequency range is removed, such as using higher order bandpass
filters. (Of course, this is one reason for IF, but even superhets
have at least one tuned RF amp before the mixer, so the same issue
applies to superhets, but is not as critical.)

I now wonder that with a single TRF RF amp stage, and with a higher
order bandpass filter optimized for a particular frequency, if we can
now dispense with the RF transformer? Or does an RF transformer confer
other benefits that it should remain? I thought its main benefit was
for improved bandpass shaping, but then I may be wrong here (likely
with high probablity -- RF transformers do help with isolation of
stages for DC, so I've read, but don't know how that would benefit
real tuner circuit design.)

Jon Noring

In article , wrote:

*****

I do have a couple questions of both John and Patrick (and anyone else
caring to chime in) related to this.

1) In the single frequency TRF tube receiver (a TRF designed strictly
to listen to a single frequency), is there a need for double tuned
circuits? Or will singly tuned circuits be sufficient for
excellent performance (audio quality, sensitivity and
selectivity)? If not, how do double tuned circuits benefit the
overall performance of the single frequency TRF receiver?

The advantage of the double tuned circuit over two single tuned circuits
is that it has a better shape factor, the pass band response is flatter.
This advantage applies to single frequency as well as tunable receivers.
Even better response curves can be achieved with three, four, or even six
section filters.


Regards,

John Byrns


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