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Constant bandwidth TRF circuit



 
 
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  #1  
Old June 13th 04, 01:47 AM
John Byrns
external usenet poster
 
Posts: n/a
Default Constant bandwidth TRF circuit

In article > , "Henry
Kolesnik" > wrote:

> Got any idea how it maintains constant BW as BW is a function of Q, a
> relative constant and frequency which varies? Also I don't understand your
> notation "12uh centertapped" (3uh persection).
> tnx


I have moved this response from "alt.binaries.pictures.radio" to
"rec.antiques.radio+phono" so that it will not be quickly deleted by the
server.

The following is my take on how these circuits work, if you don't like the
explanation consider that you got exactly what you paid for, as I thought
this explanation up all by myself, I did not find it in the RDH4, nor is
it handed down to me from the ancients.

I believe there are two ideas incorporated in this circuit. The first is
the idea of a tunable tank circuit whose Q, and hence bandwidth is
proportional to frequency, and the second idea is coupling two such
circuits, such that the coupling coefficient is inversely proportional to
frequency, to take advantage of the better shape factor that double tuned
circuits provide. If this could be done in practice we would have a
bandpass tuning circuit that would maintain constant bandwidth and
selectivity across the entire broadcast band.

Theoretically if we had perfect Ls and Cs with infinite Q, and if we
eliminated all shunt losses like diode detectors, antenna source
resistance, and coils with frequency dependent losses, we could build the
required tank circuits. A variable capacitor tuned tank circuit using a
coil of infinite Q, with the loaded Q controlled by a small series
resistance in the tank circuit will have the desired Q that is
proportional to frequency. At this point we could build a traditional TRF
type receiver using these constant bandwidth tank circuits alternated in
the traditional way with RF amplifier stages, making sure that we don't
load the tank circuits with any significant shunt resistance like a diode
detector, or an RF amplifier tube with a high input conductance. For the
detector we would use something like an anode bend detector, or reflex
detector to minimize the grid conductance. Of course in a practical radio
such a circuit is impossible, and can only be approximated, but we try to
do the best we can, accepting some broadening of the bandwidth at the
upper end of the band due to the inevitable shunt losses.

Since the response curve of each tank circuit is rounded, and when we
cascade several single tuned tank circuits the rounding and response roll
off increases, we realize that it would be a nice idea if we could couple
the tank circuits in pairs as is commonly done with the IF transformers in
superhetrodyne receivers to provide a better shape factor. For this to
work we need the coupling coefficient of the two coils to vary inversely
with frequency so that the product of "k" and "Q" remains constant vs.
frequency. Normal mutual inductance coupling as is typically used in IF
transformers won't work here because with mutual inductance coupling the
coupling coefficient remains constant with frequency. In a variable
capacitor tuned circuit what we need is a coupling reactance that is
independent of frequency, which will then cause the coupling coefficient
to vary inversely with frequency. There is not a real component that has
a fixed reactance vs. frequency, but we can simulate one to quite a good
degree of accuracy across the MW broadcast band by using an ordinary
capacitor in series with a negative inductor. The negative inductor acts
like a capacitor whose reactance increases with frequency, and when the
decreasing reactance of an ordinary capacitor is added to this decreasing
reactance, the result is a relatively constant coupling reactance across
the MW broadcast band, thus providing the desired decrease in "k" or
coupling coefficient vs. frequency. It should be noted that the reactance
of both a capacitor and a negative inductor have the same sign, which is
negative. Now the only problem is where to find the mythical "negative
inductor"? In the context of coupled circuits the effect of a negative
inductor is easily simulated by using a center tapped inductor where the
two halves of the inductor are closely coupled with k = 1, and connecting
the two tuned circuits to opposite ends of the tapped inductor, the
capacitor then goes in series with the tap, and we have the desired
result.

Now in the real world we find that we can't really build our perfect
series loaded tank circuits, and some shunt losses intrude, causing the
tank Q to not increase as much as we would like at the high frequencies,
which results in a somewhat wider bandwidth at the top of the dial. I
suspect that the designers of these sets made an effort to compensate
somewhat for this effect, by choosing Qs that made the bandwidth slightly
narrower than optimal at the low end of the band, and then tweaking the
values of the coupling reactances, the capacitance and negative
inductance, so that the circuit becomes slightly under coupled at the high
end of the band, tending to narrow the bandwidth, although making the
response more rounded, and causing the circuit to be slightly over coupled
at the low end of the band widening the compromise bandwidth a little at
the expense of a slightly humpbacked response curve.

That's just my take on how these sets were designed, and obviously there
are a lot of moving parts which probably were adjusted in different ways
by different designers with different tastes in design.

I await Patrick's take on how these so called "band pass" double tuned TRF
circuits actually work.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/
Ads
  #2  
Old June 13th 04, 03:44 AM
Patrick Turner
external usenet poster
 
Posts: n/a
Default



John Byrns wrote:

> In article > , "Henry
> Kolesnik" > wrote:
>
> > Got any idea how it maintains constant BW as BW is a function of Q, a
> > relative constant and frequency which varies? Also I don't understand your
> > notation "12uh centertapped" (3uh persection).
> > tnx

>
> I have moved this response from "alt.binaries.pictures.radio" to
> "rec.antiques.radio+phono" so that it will not be quickly deleted by the
> server.
>
> The following is my take on how these circuits work, if you don't like the
> explanation consider that you got exactly what you paid for, as I thought
> this explanation up all by myself, I did not find it in the RDH4, nor is
> it handed down to me from the ancients.
>
> I believe there are two ideas incorporated in this circuit. The first is
> the idea of a tunable tank circuit whose Q, and hence bandwidth is
> proportional to frequency, and the second idea is coupling two such
> circuits, such that the coupling coefficient is inversely proportional to
> frequency, to take advantage of the better shape factor that double tuned
> circuits provide. If this could be done in practice we would have a
> bandpass tuning circuit that would maintain constant bandwidth and
> selectivity across the entire broadcast band.
>
> Theoretically if we had perfect Ls and Cs with infinite Q, and if we
> eliminated all shunt losses like diode detectors, antenna source
> resistance, and coils with frequency dependent losses, we could build the
> required tank circuits. A variable capacitor tuned tank circuit using a
> coil of infinite Q, with the loaded Q controlled by a small series
> resistance in the tank circuit will have the desired Q that is
> proportional to frequency. At this point we could build a traditional TRF
> type receiver using these constant bandwidth tank circuits alternated in
> the traditional way with RF amplifier stages, making sure that we don't
> load the tank circuits with any significant shunt resistance like a diode
> detector, or an RF amplifier tube with a high input conductance. For the
> detector we would use something like an anode bend detector, or reflex
> detector to minimize the grid conductance. Of course in a practical radio
> such a circuit is impossible, and can only be approximated, but we try to
> do the best we can, accepting some broadening of the bandwidth at the
> upper end of the band due to the inevitable shunt losses.
>
> Since the response curve of each tank circuit is rounded, and when we
> cascade several single tuned tank circuits the rounding and response roll
> off increases, we realize that it would be a nice idea if we could couple
> the tank circuits in pairs as is commonly done with the IF transformers in
> superhetrodyne receivers to provide a better shape factor. For this to
> work we need the coupling coefficient of the two coils to vary inversely
> with frequency so that the product of "k" and "Q" remains constant vs.
> frequency. Normal mutual inductance coupling as is typically used in IF
> transformers won't work here because with mutual inductance coupling the
> coupling coefficient remains constant with frequency. In a variable
> capacitor tuned circuit what we need is a coupling reactance that is
> independent of frequency, which will then cause the coupling coefficient
> to vary inversely with frequency. There is not a real component that has
> a fixed reactance vs. frequency, but we can simulate one to quite a good
> degree of accuracy across the MW broadcast band by using an ordinary
> capacitor in series with a negative inductor. The negative inductor acts
> like a capacitor whose reactance increases with frequency, and when the
> decreasing reactance of an ordinary capacitor is added to this decreasing
> reactance, the result is a relatively constant coupling reactance across
> the MW broadcast band, thus providing the desired decrease in "k" or
> coupling coefficient vs. frequency. It should be noted that the reactance
> of both a capacitor and a negative inductor have the same sign, which is
> negative. Now the only problem is where to find the mythical "negative
> inductor"? In the context of coupled circuits the effect of a negative
> inductor is easily simulated by using a center tapped inductor where the
> two halves of the inductor are closely coupled with k = 1, and connecting
> the two tuned circuits to opposite ends of the tapped inductor, the
> capacitor then goes in series with the tap, and we have the desired
> result.
>
> Now in the real world we find that we can't really build our perfect
> series loaded tank circuits, and some shunt losses intrude, causing the
> tank Q to not increase as much as we would like at the high frequencies,
> which results in a somewhat wider bandwidth at the top of the dial. I
> suspect that the designers of these sets made an effort to compensate
> somewhat for this effect, by choosing Qs that made the bandwidth slightly
> narrower than optimal at the low end of the band, and then tweaking the
> values of the coupling reactances, the capacitance and negative
> inductance, so that the circuit becomes slightly under coupled at the high
> end of the band, tending to narrow the bandwidth, although making the
> response more rounded, and causing the circuit to be slightly over coupled
> at the low end of the band widening the compromise bandwidth a little at
> the expense of a slightly humpbacked response curve.
>
> That's just my take on how these sets were designed, and obviously there
> are a lot of moving parts which probably were adjusted in different ways
> by different designers with different tastes in design.
>
> I await Patrick's take on how these so called "band pass" double tuned TRF
> circuits actually work.


I find the above dissertation too difficult to fully digest on a sunday.
I doubt I could fully explain how mutual coupling works in dual
LC circuits where the mutual element is a reactive shared element,
without an enormous dissertation with lots of formulas and equations, that not
even I know about.
But its enough for most folks to know that the basic config will give a wider than
single tuned
response, and then play with values and a plot the response to find out
experimentally what is possible


But double tuned circuits with mutual reactive coupling in the earthy
ends of the Ls are not all that easy to get right, and anyone attempting them will
run into difficulties, which is why
we never see any in old tube radios made for the majority of consumers in bygone
days.

In the above paragraphs, there is too much talk of perfect LC tank circuits and
"what ifs".

The facts are that no matter what we do with LC, R still exists to affect what we
do
in the real world, so R has to be included in all perceptions of LC workings at
all times.

There is enough ideas presently on the web about mutually coupled pairs of tuned
LCs anyone keen amoung you
to go to your workshops and build something which may turn out a complete waste of
10 sundays,
but then again if you emerge with something giving good selectivity, wide AF bw,
and low N&D, then you will have achieved something.
Its no good rabbiting on about rabbit eared response curves forever, the time for
action
is nigh, so away from the PC and to the workshop!

For TRF, I'd suggest trying a "loosely coupled" antenna input coil,
then 3 other identical LC coils so a 4 gang tuning cap is required, or a pair of
old
but identical twin gangs, with those large dia wheels attatched of the same dia,
allowing
the dial cord to be run around the two wheels.
Locating the coils and Cs is critical, and determining amp gain betwen stages.
Its easiest to use j-fets with low gain and 12volt supplies until a thourough
undertsanding of the practicalities ahve been attained.
breadboard construction would be fine at first.
All coils can be wound at home using 1.5" cardboard tubes and 0.4mm dia wire.
The coils need screening, so don't be tempted to use old steel tins from
the kitchen, that will damage the Q. Cans must be 1" away from windings, and
done with Al or Cu sheet, but need only be quite thin material.

This is a shirt load of work to trim for equal performance along the band.

I recommend a superhet damped specially treated 455 kHz IFTs,
or possibly 2MHz IFTs, or perhaps cascaded pairs of single tuned 2 MHz IF
LC circuits.

The simplest good performance tubed superhet AM tuner which offers quite fair
performance
is one using a moderately selective input coil with loose coupling to the antenna,

a 6BE6 converter, two IFTs, and a 6BA6 IFamp with AVC to both.
The detector using a 12AU7 plus germanium diode and tone control
should be as I described in detail last night in another post.

But 9 kHz AF BW is available from such a tuner, and in one I altered in a guys
old Kenwood AM/FM receiver, I used a series LC for a broad 9 kHz notch
filter which removed most monkey chatter as well as the carrier whistle
for distance listening.

The use of wide pass band ceramic filters is an explorable persuit.

The other simple type of AM receiver is a chip based synchrodyne, using
a simple double tuned input circuit for initial broad selectivity,
although one single LC input could be tried until the rest of the circuit is got
working,
so the effect of greater input selectivity can be later measured if a dual LC
input is tried.

The oscillator runs at the wanted station's F, and is held synchronised with a
PLL.
Final selectivity is attained with the audio filter following the detector.

One only needs one double gang tuning cap, and $10 worth of chips, and a
+15v supply, plus minor parts.

The hard part is understanding what you are doing, and applying what you have
learnt,
and ironing out the bugs, ie, getting things to work like they do in the text
books,
without any spurious behaviors.

In conclusion my answer is to build, observe, and learn.

Patrick Turner.


>
>
> Regards,
>
> John Byrns
>
> Surf my web pages at, http://users.rcn.com/jbyrns/


  #3  
Old June 13th 04, 04:42 AM
Bill
external usenet poster
 
Posts: n/a
Default

Patrick Turner wrote:

(SNIP, SNIP, SNIP)

>>Now in the real world we find that we can't really build our perfect
>>series loaded tank circuits, and some shunt losses intrude, causing the
>>tank Q to not increase as much as we would like at the high frequencies,
>>which results in a somewhat wider bandwidth at the top of the dial. I
>>suspect that the designers of these sets made an effort to compensate
>>somewhat for this effect, by choosing Qs that made the bandwidth slightly
>>narrower than optimal at the low end of the band


True.

(more SNIP)
>
> For TRF, I'd suggest trying a "loosely coupled" antenna input coil,
> then 3 other identical LC coils so a 4 gang tuning cap is required, or a pair of
> old
> but identical twin gangs, with those large dia wheels attatched of the same dia,
> allowing
> the dial cord to be run around the two wheels.


Be advised that the old 1920's multi-gang tuning caps are generally
found nowadays in very leaky condition. A thorough cleaning and baking
is quite often called for in order to have them behave respectfully.


> All coils can be wound at home using 1.5" cardboard tubes and 0.4mm dia wire.
> The coils need screening, so don't be tempted to use old steel tins from
> the kitchen, that will damage the Q. Cans must be 1" away from windings, and
> done with Al or Cu sheet, but need only be quite thin material.


Cardboard tubes are desirable if one is looking for MINIMUM Q. If your
humidity hovers above zero percent you can count on even lower Q...but
it won't be predictable :-) Nowadays a nice solid/consistent BCB
inductor can be made with an FT-82-61 toroid core with +/- 50 turns of
#26 enamelled wire approaching midband unloaded Q numbers of 300 or
better. I've yet to find any coil from an old BCB TRF set that comes
even close to this. Under 100 is not atypical.

An added advantage using toroids is that screening is not normally
required. Shoving a 1.5" solenoid coil into a box with only 1" of
spacing is a good way to kill the Q of the ckt.

But, I've described a method of getting 3kc selectivity at the low end
of the band that will likely be 20-25kc (measured, not a guess) at the
high end in a 2-stage set. The point is only to illustrate why this
isn't as good an idea as superhetting.

In the real world my experience says a 2 or 3 or 4 stage set works great
on a 650kc station with another strong local present at 700kc. But it
(the same scheme) will NOT work for your 1450kc station with a strong
local at 1500kc.


> This is a shirt load of work to trim for equal performance along the band.


Hasn't yet been accomplished in 80+ years of radio...


-BM
  #4  
Old June 13th 04, 10:03 AM
Patrick Turner
external usenet poster
 
Posts: n/a
Default



Bill wrote:

> Patrick Turner wrote:
>
> (SNIP, SNIP, SNIP)
>
> >>Now in the real world we find that we can't really build our perfect
> >>series loaded tank circuits, and some shunt losses intrude, causing the
> >>tank Q to not increase as much as we would like at the high frequencies,
> >>which results in a somewhat wider bandwidth at the top of the dial. I
> >>suspect that the designers of these sets made an effort to compensate
> >>somewhat for this effect, by choosing Qs that made the bandwidth slightly
> >>narrower than optimal at the low end of the band

>
> True.
>
> (more SNIP)
> >
> > For TRF, I'd suggest trying a "loosely coupled" antenna input coil,
> > then 3 other identical LC coils so a 4 gang tuning cap is required, or a pair of
> > old
> > but identical twin gangs, with those large dia wheels attatched of the same dia,
> > allowing
> > the dial cord to be run around the two wheels.

>
> Be advised that the old 1920's multi-gang tuning caps are generally
> found nowadays in very leaky condition. A thorough cleaning and baking
> is quite often called for in order to have them behave respectfully.


The last 1932 radio I serviced seemed to work as intended
without coil baking, ( or even frying or grilling ) , :-)


>
>
> > All coils can be wound at home using 1.5" cardboard tubes and 0.4mm dia wire.
> > The coils need screening, so don't be tempted to use old steel tins from
> > the kitchen, that will damage the Q. Cans must be 1" away from windings, and
> > done with Al or Cu sheet, but need only be quite thin material.

>
> Cardboard tubes are desirable if one is looking for MINIMUM Q.


I was going to suggest old toilet paper roll inners, soaked in varnish
before winding, and waxed after.
One don't want an extremely high Q.

> If your
> humidity hovers above zero percent you can count on even lower Q...but
> it won't be predictable :-) Nowadays a nice solid/consistent BCB
> inductor can be made with an FT-82-61 toroid core with +/- 50 turns of
> #26 enamelled wire approaching midband unloaded Q numbers of 300 or
> better. I've yet to find any coil from an old BCB TRF set that comes
> even close to this. Under 100 is not atypical.


Even with Q = 50, the BW at 550 kHz is 11 kHz, which is OK.
It only allows 5.5 kHz of audio, so hence you'd need two
LCs stagger tuned at the low end of the band.

>
>
> An added advantage using toroids is that screening is not normally
> required. Shoving a 1.5" solenoid coil into a box with only 1" of
> spacing is a good way to kill the Q of the ckt.


Again I have to pay homage to my ancestors and say that many an ancient old
radio had sufficient Q with RF coils which were a 1" dia solenoid inside a 3" can.
No worries.
Using ferrite reduces the size of the coil, and hence the can.

But DIYers wanting a spectacular radio should keep all the coils and vari caps
nice and big alongside old 6U7 pentodes with top caps.



>
>
> But, I've described a method of getting 3kc selectivity at the low end
> of the band that will likely be 20-25kc (measured, not a guess) at the
> high end in a 2-stage set. The point is only to illustrate why this
> isn't as good an idea as superhetting.


I have been saying all along that TRFing is a pita because of the unfixed bw
at different F.
Its only good for locals well spaced apart.

>
> In the real world my experience says a 2 or 3 or 4 stage set works great
> on a 650kc station with another strong local present at 700kc. But it
> (the same scheme) will NOT work for your 1450kc station with a strong
> local at 1500kc.


Fortunately, no such situation occurs here in Oz for local strong metropolitan
stations.

But in regional areas, its all a bit different, and its where the superhet kills all
TRFs.

>
>
> > This is a shirt load of work to trim for equal performance along the band.

>
> Hasn't yet been accomplished in 80+ years of radio...


The radio of mine has stagger tuning of two LC circuits simply cascaded, both before a
resistance loaded
cascoded triode amp, which feeds the mixer of a supehet.
The first LC has a loose coupled untuned primary winding for the antenna,
so the antenna capacitance/inductance does not much affect the secondary, which
is tuned by a gang cap.

Then the output from the top of this LC has a 39k 1 W carbon resistor to couple
to the next LC, tuned by a second gang cap.
The R cas the right amount of stray C and R to make a good couple.

Both LC coils are lowish Q, and the attenuation just outside the 22 kHz pass band
at anypoint of the BCB is steeper than having a single tuned LC,
and provides some initial selectivity to stop a strong station
cross modulating a weaker station in the mixer.

I could have soldered on to make similar set up with 2 or 3 twin gang caps
each with identical tuning caps.
I think it wopld have worked OK.
It'd have been harder to align.

Patrick Turner.

>
>
> -BM


  #5  
Old June 13th 04, 10:18 AM
William Sommerwerck
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That was really interesting. Thanks for going to the trouble of writing it out.
  #6  
Old June 13th 04, 06:09 PM
Henry Kolesnik
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John

I'm from a school that taught me that Q was a physical characteristic of a
coil and I still believe that.
Q = {2 * pi * f)/R Selecting the optimum sizes of wire, number of turns,
and diameter to maximize Q is well documented. However in this Miller TRF
circuit I see no mechanism for changing any physical characterisic of the
coils. There are many ways of reducing Q and I see none in the circuit.

I assume your neat little one stage mock up works and it's interesting that
L1/T2 and L2 are in seperate shielded cans but T1 is open. I'm curious as
to how RF is coupled from L1/T2 to L2 by T1. I've struggled to try to
simplify this simple circuit further but so far no cigar. Is it possible
for you to tune this to a known stations and then measure the variable
capacitor so that the Ls can be worked out from f = 1/(2 * pi * (LC)^0.5)

tnx

--
73
Hank WD5JFR

"John Byrns" > wrote in message
...
> In article > , "Henry
> Kolesnik" > wrote:
>
> > Got any idea how it maintains constant BW as BW is a function of Q, a
> > relative constant and frequency which varies? Also I don't understand

your
> > notation "12uh centertapped" (3uh persection).
> > tnx

>
> I have moved this response from "alt.binaries.pictures.radio" to
> "rec.antiques.radio+phono" so that it will not be quickly deleted by the
> server.
>
> The following is my take on how these circuits work, if you don't like the
> explanation consider that you got exactly what you paid for, as I thought
> this explanation up all by myself, I did not find it in the RDH4, nor is
> it handed down to me from the ancients.
>
> I believe there are two ideas incorporated in this circuit. The first is
> the idea of a tunable tank circuit whose Q, and hence bandwidth is
> proportional to frequency, and the second idea is coupling two such
> circuits, such that the coupling coefficient is inversely proportional to
> frequency, to take advantage of the better shape factor that double tuned
> circuits provide. If this could be done in practice we would have a
> bandpass tuning circuit that would maintain constant bandwidth and
> selectivity across the entire broadcast band.
>
> Theoretically if we had perfect Ls and Cs with infinite Q, and if we
> eliminated all shunt losses like diode detectors, antenna source
> resistance, and coils with frequency dependent losses, we could build the
> required tank circuits. A variable capacitor tuned tank circuit using a
> coil of infinite Q, with the loaded Q controlled by a small series
> resistance in the tank circuit will have the desired Q that is
> proportional to frequency. At this point we could build a traditional TRF
> type receiver using these constant bandwidth tank circuits alternated in
> the traditional way with RF amplifier stages, making sure that we don't
> load the tank circuits with any significant shunt resistance like a diode
> detector, or an RF amplifier tube with a high input conductance. For the
> detector we would use something like an anode bend detector, or reflex
> detector to minimize the grid conductance. Of course in a practical radio
> such a circuit is impossible, and can only be approximated, but we try to
> do the best we can, accepting some broadening of the bandwidth at the
> upper end of the band due to the inevitable shunt losses.
>
> Since the response curve of each tank circuit is rounded, and when we
> cascade several single tuned tank circuits the rounding and response roll
> off increases, we realize that it would be a nice idea if we could couple
> the tank circuits in pairs as is commonly done with the IF transformers in
> superhetrodyne receivers to provide a better shape factor. For this to
> work we need the coupling coefficient of the two coils to vary inversely
> with frequency so that the product of "k" and "Q" remains constant vs.
> frequency. Normal mutual inductance coupling as is typically used in IF
> transformers won't work here because with mutual inductance coupling the
> coupling coefficient remains constant with frequency. In a variable
> capacitor tuned circuit what we need is a coupling reactance that is
> independent of frequency, which will then cause the coupling coefficient
> to vary inversely with frequency. There is not a real component that has
> a fixed reactance vs. frequency, but we can simulate one to quite a good
> degree of accuracy across the MW broadcast band by using an ordinary
> capacitor in series with a negative inductor. The negative inductor acts
> like a capacitor whose reactance increases with frequency, and when the
> decreasing reactance of an ordinary capacitor is added to this decreasing
> reactance, the result is a relatively constant coupling reactance across
> the MW broadcast band, thus providing the desired decrease in "k" or
> coupling coefficient vs. frequency. It should be noted that the reactance
> of both a capacitor and a negative inductor have the same sign, which is
> negative. Now the only problem is where to find the mythical "negative
> inductor"? In the context of coupled circuits the effect of a negative
> inductor is easily simulated by using a center tapped inductor where the
> two halves of the inductor are closely coupled with k = 1, and connecting
> the two tuned circuits to opposite ends of the tapped inductor, the
> capacitor then goes in series with the tap, and we have the desired
> result.
>
> Now in the real world we find that we can't really build our perfect
> series loaded tank circuits, and some shunt losses intrude, causing the
> tank Q to not increase as much as we would like at the high frequencies,
> which results in a somewhat wider bandwidth at the top of the dial. I
> suspect that the designers of these sets made an effort to compensate
> somewhat for this effect, by choosing Qs that made the bandwidth slightly
> narrower than optimal at the low end of the band, and then tweaking the
> values of the coupling reactances, the capacitance and negative
> inductance, so that the circuit becomes slightly under coupled at the high
> end of the band, tending to narrow the bandwidth, although making the
> response more rounded, and causing the circuit to be slightly over coupled
> at the low end of the band widening the compromise bandwidth a little at
> the expense of a slightly humpbacked response curve.
>
> That's just my take on how these sets were designed, and obviously there
> are a lot of moving parts which probably were adjusted in different ways
> by different designers with different tastes in design.
>
> I await Patrick's take on how these so called "band pass" double tuned TRF
> circuits actually work.
>
>
> Regards,
>
> John Byrns
>
>
> Surf my web pages at, http://users.rcn.com/jbyrns/



  #7  
Old June 14th 04, 01:55 AM
John Byrns
external usenet poster
 
Posts: n/a
Default

In article >, "Henry
Kolesnik" > wrote:

> John
>
> I'm from a school that taught me that Q was a physical characteristic of a
> coil and I still believe that.


There are two values of "Q" associated with a coil, ignoring that the "Q"
may change with frequency. The first is the "Q" of the coil by itself,
which is what you describe above, but there is also the "loaded" or
"circuit" "Q", which includes the effects of external circuit elements
like the resistors that Patrick has mentioned.

> Q = {2 * pi * f)/R Selecting the optimum sizes of wire, number of turns,
> and diameter to maximize Q is well documented. However in this Miller TRF
> circuit I see no mechanism for changing any physical characterisic of the
> coils. There are many ways of reducing Q and I see none in the circuit.


I would have to look but I believe you are correct about the Miller
circuit. I don't know what the "Q" of the Miller coils was, perhaps they
were not all that great, so they didn't have to add any series resistors
to reduce the "Q" at the low end of the band. I don't know, these designs
are not perfect, they are just one way to go.

> I assume your neat little one stage mock up works and it's interesting that
> L1/T2 and L2 are in seperate shielded cans but T1 is open. I'm curious as
> to how RF is coupled from L1/T2 to L2 by T1. I've struggled to try to
> simplify this simple circuit further but so far no cigar. Is it possible
> for you to tune this to a known stations and then measure the variable
> capacitor so that the Ls can be worked out from f = 1/(2 * pi * (LC)^0.5)


It's not my "neat little one stage mock up", it's Robert Casey's, but L1
and L2 are coupled in the same manner as in the W.E., Miller, and the
other similar tuners, by means of a common impedance in the ground end of
L1 and L2, this common impedance is T1 & C2 in Robert's design.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/
  #8  
Old June 14th 04, 02:00 AM
Telamon
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In article >,
(John Byrns) wrote:

Please delete rec.radio.shortwave from the news group header.

Thanks.

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Telamon
Ventura, California
 




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