In article ,
wrote:

John Byrns wrote:
What exactly are the advantages of a 2.0 MHz IF from a
selectivity/bandwidth point of view?

Both pros and cons to this John.

Since the bandwidth is a percentage of the center frequency,
the shape of the bandwidth will change based on the distance
from the center frequency (as a percentage.)

Assuming just for the moment +/- 5 KHz.
At 455 KHz that's about 1% above and below.
At 2 MHz, that's now only .25% above and below.

As you get further from the center frequency, percentage
wise, the shape of the curve as it transitions from inside
to outside of the band pass is going to look different at
the upper frequency than it does at the lower frequency.

Didn't I explicitly point out this difference in asymmetry in several
earlier posts? I also specifically stated that we were dealing with audio
bandwidths, so that someone wouldn't try to build a video IF at 455 kHz
running head long into extreme asymmetry. As long as we keep to audio
bandwidths the asymmetry is what I would consider minor, but I realize
that different people have different degrees of obsession with respect to
issues like this, and that is the main reason I posted some calculated
numbers, so that everyone could judge for themselves, using their own
standards.

In the world of designing filters (and overall system performance)
this is called group delay. A shorter, perhaps more recognizable
term would be linear phase shift over the entire band pass of the
filter.

The curves for phase shift vs. frequency for the 455 kHz and 2.0 MHz IFs
look virtually identical to me, I will have to pull the numbers off and
see exactly how similar they are, I expect that the only difference in the
phase response of the two filters will be a slightly greater asymmetry for
the 455 kHz filter, as with the amplitude response, but not really a
significant difference between the two.

An IF transformer is simply a two pole butter worth filter.
That it can have different input and output impedance just
makes it really convenient for taking the source from a plate
and connecting it to a grid for a load.

Are you sure that it is always simply a Butterworth, that it can't ever be
something else?

By definition, a butter worth filter has a smooth curve with
only one peak (in the middle.) And the shape (steepness) of
the band pass is related to the overall Q of the circuit.

Hmm, Patrick often speaks of IF response curves with "rabbits ears" on
them, are these "Butterworth"?

The point you've probably overlooked in land mobile operations is that
it was NEVER designed as a "hi-fi" system. There's a reason for the
term "voice grade." Having as much a 3 dB of ripple in a band pass
filter is meaningless especially when the filter is in the midst of
a limiting IF strip for FM recovery, and on AM demodulation. What
really matters here is limiting the bandwidth of the received signal to
ONLY include that of the wanted (in channel) information and none of
the unwanted (adjacent channel) information to get to the discriminator.

I think you are over simplifying FM systems, but that's a different
thread, this one is about AM systems.

I am not a "filter jock" (tm) but I
think it is generally desirable that the Q of the components used in a
filter be high, especially when we get beyond simple double tuned
transformers. What you are calling Q is more related to how the filter is
terminated, which is a different matter than the Q of the components that
make up the filter.

You should take the time to read up on "filter jockeying" John.
You're making a lot of incorrect assumptions on how they work.

The primary requirement on the Q of individual components in
filter design is only such that their value of Q be high enough
to not materially effect the overall Q of the circuit.

I was trying to avoid getting into this issue, but I agree that your
simple statement is a better way of putting it than mine was. It is my
understanding that infinite Q components would be ideal, but lower Qs are
acceptable as long as they don't materially impact the response of the
filter. With a simple double tuned IFT I don't think component Q is as
important because there are no interior components to affect the response
of the filter, and any losses in the few components used can be considered
to be part of the filter termination. Of course you are in trouble even
with a simple IFT if the component Q is so low that the losses of the
components alone are greater than the desired terminations.


Regards,

John Byrns


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

John Byrns wrote:

In article , Robert Casey
wrote:

Just posted a schematic of the Miller TRF receiver, with the "secret"
inductance values filled
in. a.b.p.radio

Hi Robert, that's certainly a cute little radio you have there. An
interesting point is that the separate "negative mutual coupling"
inductance, the one with the "secret" value, isn't even necessary and the
part can often be eliminated from the circuit. All you need to do is wind
L1 and L2 like a typical double tuned IF transformer, and if the coupling
coefficient is correctly chosen to yield the required value of mutual
inductance, and if the two windings are phased correctly to make the
mutual inductance "negative", then the separate coil like you used isn't
necessary, although you must retain the capacitor in the common lead of
"L1" And "L2", since that is part of the "secret". This scheme will work
in a circuit like the Miller "High Fidelity" Crystal Tuner where L1 and L2
are just single winding coils, there would obviously be problems applying
the idea to your circuit because of the extra winding you put on L1,
making it into a transformer by itself.

I think there is a reason why the CT choke is used for coupling the earthy
ends of two Ls in a pair of LC circuits.
I tried to use the mutual coupling solely via a 0.1 uF cap,
or about that value, and the reponse doesn't always come out nice and flat and
symetical
each side of the centre F.

Its not as easy as ypou think to get this sort of circuit to work like the
text books say.



I listened to the WABC "jpg" you posted, and the tuner certainly has a
good bandwidth, although I wonder how much pre emphasis WABC might have
been using and how well your receiver matches it, I will have to listen to
it again to see how correct the de emphasis seems to me, there was also
some background noise at several points, I will have to listen again to
see if it was part of the audio at points, or if it was interference of
some sort. The thing I didn't like about it was that it had pretty
horrible levels of distortion, and while this could be WABC's fault, I
have found that it is typical of these so called "High Fidelity" crystal
receivers. I have a couple of J.W. Miller "High Fidelity" crystal tuners,
and they have the same distorted sound. I think this is because the
crystal detectors produce really horrendous distortion, unless you have a
big enough antenna to get the audio output level up to at least the 2
volts RMS that Patrick recommends.

The type of detector used does affect the audio quality.

A germaniun diode detector should never be driven straight off
a tuned secondary IF circuit.
It should be driven by a low impedance source, ie a buffer stage such as a
cathode follower,
which is a 12AU7, 12AT7, or a trioded 6AU6 or some such.
The CF grid is direct connected to the active end of the last IF or RF tuned
circuit.
The earthy end of this LC is connected to a +30volt source from a resistance
divider from the B+,
and bypassed with 100 uF to 0V.

In my radio, the CF is 1/2 a 12AU7 and has a 15 k to 0V.

The diode anode is connected to the tube's cathode, and the diode's cathode
feeds
a 270 pF cap with 1M in parallel to 0V.

Then a 0.022 uF couples the audio output of the 270 pF to a series 100k and
screened lead
a 500k log pot to 0V.
This is the volume control.

The wiper of the volume pot feeds the grid of the other 1/2 of the 12AU7
via a screened lead.
This tube is set up as a normal plate loaded gain stage with 3k3 for Rk,
bypassed with 0.01 uF.
The DC carrying R = 47k, and B+ = 235v, but could be more.
The screened leads and miller C of the 12AU7 gain stage act as a second C to
filter the 455 kHz ripple voltage from the recovered audio signal from the
detector.

The 12AU7 gain stage plate signal is cap coupled via 0.033 uF to a
120k in series with one 1/2 of a dual gang 100k + 100k linear pot,
which is in series with the other 1/2 of the pot which forms
a resistance total of 320k to 0V.

The centre join of each 1/2 of the two pot tracks is the output to the power
amp.

There is a 180 pF compensation cap between the earthy side of the
0.033 uF and the join of the two tracks of the pot.

Each wiper of each pot section has a 0.001 uF cap taken to the join of
the two tracks of the pot.

The action is that the wipers each move up and down the pots in the same
directio
at the same time, so when at the top, the top track is bypassed with 0.001 uF,
and the botton track
has its 0.001 uF shunted, so the treble is boosted.

when both wipers are swung downm the top 0.001 uF is shunted,
and there is a 0.001 uF from output to 0V, thus the treble is cut.

Try this out you guys, you'll really like the low thd detection and tone
control!

Because a nearly constant current flows through the detector diode, its always
turned on,
so the forward conduction curve of the diode does not affect the signal
detected, even when the signal
is really low level.

For AVC, I have a 25 pF taken directly off the anode of the IF amp
to an IN914 with its cathode at 0V.
A negative going voltage is developed at the anode of the diode,
and its passed via 1M to the AVC control of the RF input tube.



I would try for another 10 dB or so of

audio output above that level before being completely happy myself. But
no way do these simple crystal sets sound "High Fidelity" to me because of
all the nonlinear distortion. The distortion probably does help give them
their bright sound though, sort of a psycho acoustical trick if you will,
but it does wear on one.

Crystal sets need quite some refinement to be hifi.

Using a cathode follower detector as I have outlined would work well to keep
the diode distortion out of the audio signal.

The system I use removes the loading effect of the diode and RC
filter from the tuned circuit. Even when a tiny RF or IF signal is present
at the CF grid, the ripple voltage on the 270 pF won't vary much
compared to when there is a huge RF or IF signal present.
If say the RF/IF signal is 40v peak to peak, then the
direct voltage level across the 270 pF rises from its 35v at no signal to
55 volts, no problem for the CF tube.
You can easily get a very nice 10vrms of audio signal from a
large % modulated signal.


Patrick Turner.





Regards,

John Byrns

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

Randy and/or Sherry wrote:

Patrick Turner wrote:

I thought white noise had a rising amplitude as F rose.

as "burnt toast" as I am tonight - someone could claim white noise comes
from Procol Harum and pink noise from Pink Floyd - and I'd agree.

Here is the quote from the NRSC-1 spec for bandwidth testing - as it
relates to "source"...

Section: 6.3.2 Use of Standard Test Signal. Audio bandwidth shall be
measured using a test signal consisting of USASI (United States of
America Standards Institute) noise that is pulsed by frequency of 2.5 Hz
at a duty cycle of 12.5%. See Figure 4. USASI noise is intended to
simulate the long-term average spectra of typical audio program
material. Pulsing of the noise is intended to simulate audio transients
found in audio program Material. USASI noise is a white noise source
[note 4](i.e. noise with equal energy at all frequencies) that is
filtered by (1) a 100 Hz, 6 dB per octave high-pass network and (2) a
320 Hz, 6 dB per octave low-pass network. too Figure 4. A pulsed USASI
noise generator is shown in Figures 5 and 6. Using the attenuator pad,
the ratio of peak-to average amplitude shall be 20 db at the audio
output of the pulser. [snip]

Note 4. Acceptable white noise sources include GenRad Models 1382 and
1390B; Bruel & Kjaer Model 1405; and National Semiconductor IC No. MM5837N.

[end NRSC-1 quotes]

Lordy Lordy, please protect us from engineers who speak with a foreign
tongue....

Can't they say it simpler?????





If you can find specs on any of those generators or that IC - then
you'll find what they think white noise is.

I won't bother; I am three tired.



Right now it's approaching midnight - just went through the emotionally
draining experience of watching a old family friend's funeral on TV...
and Sherry and I are toast - so someone else can look them up.

I hope your friend found a nice spot on a nice cloud in heaven to park his
weary soul,
for earth is a trying place to be for too long.

Regards,

Patrick Turner.



best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

John Byrns wrote:

In article ,
wrote:

John Byrns wrote:
What exactly are the advantages of a 2.0 MHz IF from a
selectivity/bandwidth point of view?

Both pros and cons to this John.

Since the bandwidth is a percentage of the center frequency,
the shape of the bandwidth will change based on the distance
from the center frequency (as a percentage.)

Assuming just for the moment +/- 5 KHz.
At 455 KHz that's about 1% above and below.
At 2 MHz, that's now only .25% above and below.

As you get further from the center frequency, percentage
wise, the shape of the curve as it transitions from inside
to outside of the band pass is going to look different at
the upper frequency than it does at the lower frequency.

Didn't I explicitly point out this difference in asymmetry in several
earlier posts? I also specifically stated that we were dealing with audio
bandwidths, so that someone wouldn't try to build a video IF at 455 kHz
running head long into extreme asymmetry. As long as we keep to audio
bandwidths the asymmetry is what I would consider minor, but I realize
that different people have different degrees of obsession with respect to
issues like this, and that is the main reason I posted some calculated
numbers, so that everyone could judge for themselves, using their own
standards.

In the world of designing filters (and overall system performance)
this is called group delay. A shorter, perhaps more recognizable
term would be linear phase shift over the entire band pass of the
filter.

The curves for phase shift vs. frequency for the 455 kHz and 2.0 MHz IFs
look virtually identical to me, I will have to pull the numbers off and
see exactly how similar they are, I expect that the only difference in the
phase response of the two filters will be a slightly greater asymmetry for
the 455 kHz filter, as with the amplitude response, but not really a
significant difference between the two.

An IF transformer is simply a two pole butter worth filter.
That it can have different input and output impedance just
makes it really convenient for taking the source from a plate
and connecting it to a grid for a load.

Are you sure that it is always simply a Butterworth, that it can't ever be
something else?

By definition, a butter worth filter has a smooth curve with
only one peak (in the middle.) And the shape (steepness) of
the band pass is related to the overall Q of the circuit.

Hmm, Patrick often speaks of IF response curves with "rabbits ears" on
them, are these "Butterworth"?

I don't know if dog eared, rabbit eared cat eared, or mule eared IFT
response shapes have butter on them or not.

But such curves are detrimental to the audio when the set isn't tuned exactly
correctly, so the class A pentode amp isn't loaded quite right...

the twin peaked response of the first IFT is supposed to compensate
for the nearly flat but rounded shape of the second IFT, so a flat final response
comes out.
Easier said than done, I can assure you.

Patrick Turner.


The point you've probably overlooked in land mobile operations is that
it was NEVER designed as a "hi-fi" system. There's a reason for the
term "voice grade." Having as much a 3 dB of ripple in a band pass
filter is meaningless especially when the filter is in the midst of
a limiting IF strip for FM recovery, and on AM demodulation. What
really matters here is limiting the bandwidth of the received signal to
ONLY include that of the wanted (in channel) information and none of
the unwanted (adjacent channel) information to get to the discriminator.

I think you are over simplifying FM systems, but that's a different
thread, this one is about AM systems.

I am not a "filter jock" (tm) but I
think it is generally desirable that the Q of the components used in a
filter be high, especially when we get beyond simple double tuned
transformers. What you are calling Q is more related to how the filter is
terminated, which is a different matter than the Q of the components that
make up the filter.

You should take the time to read up on "filter jockeying" John.
You're making a lot of incorrect assumptions on how they work.

The primary requirement on the Q of individual components in
filter design is only such that their value of Q be high enough
to not materially effect the overall Q of the circuit.

I was trying to avoid getting into this issue, but I agree that your
simple statement is a better way of putting it than mine was. It is my
understanding that infinite Q components would be ideal, but lower Qs are
acceptable as long as they don't materially impact the response of the
filter. With a simple double tuned IFT I don't think component Q is as
important because there are no interior components to affect the response
of the filter, and any losses in the few components used can be considered
to be part of the filter termination. Of course you are in trouble even
with a simple IFT if the component Q is so low that the losses of the
components alone are greater than the desired terminations.

Regards,

John Byrns

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

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been
mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

I supect it might, and one article in Wireless World refered to using
10.7 MHz.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar. What
exactly did Wireless World say was so great about using a 10.7 MHz IF for
a MW AM receiver?

Wide AF response was easily achieved.

Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed, although in the old days they
often did have articles by people who knew what they were talking about
with respect to radios.

I differ. WW and what it became, Electronics World wasn't just an
amateur's magazine. It had cutting edge articles about electronics
from 1917 onwards, and I suggest you park yourself beside a
pile of all the old copies and have a good read.
Most of the info was only comprehensible by very well university educated
professionals, or intellectuals, and most ideas were backed up with
mathematical
proofs which nearly all the general public couldn't understand.

I am reasonably familiar with Wireless World, I have 3 & 1/2 of those copy
paper boxes full of old issues from the 1930's through the 1950's. I
would estimate that I have at least half the issues from that period whcih
was probably the golden age of AM receiver technology. I have to take
serious exception to your characterization of the "mathematical proofs"
included in their articles. There may have been the odd article with some
mathematical depth, but those were few and far between. The math
presented seems to have been just enough to go over the head of the
average reader, but was hardly complex enough to be "only comprehensible
by very well university educated professionals, or intellectuals". I
suspect this light weight approach just slightly above the level of the
man in the street was carefully calculated to impress the average reader
without putting the material at a level where he couldn't understand it at
all. That is not to say that they didn't have many excellent authors who
knew all the math, but it is a serious stretch to imply that they included
any real mathematical depth, they included only enough to look impressive
to the untutored reader.

I suspect that the reason Wireless World might
have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an
easy way for a hobbyist, who both doesn't have a clue what he is doing,
and doesn't have the necessary test equipment, to get a super wide
bandwidth.

I leave you to your suppositions.


OK, but for all practical purposes my "supposition" seems to be identical
with your statement above that "Wide AF response was easily achieved",
which I take to be a quote from the actual Wireless World article?

Damping reduces Q, and increases BW.
But it also reduces Z at Fo, thus reducing gain in an amp
which must be a current source, like a pentode or j-fet,
to realise the best selectivity for the LC circuit.

This is a half truth, what matters is that the filter is correctly
terminated, not that pentode, triode or whatever drives it. As far as
stage gain goes, increasing the frequency from 455 kHz to 2.0 MHz is
likely to decrease the gain by a similar amount to widening the 455 kHz
filter to the same bandwidth as the 2.0 MHz filter.

What I said was what I said.
You are confused.

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

I know enough about IFT design, after having built my own radio.

That isn't clear at all, you seem to be obsessed with "Q", and hardly if
ever mention "k", and how it relates to "Q" in determining the
characteristics of an IFT. You occasionally mention "critical" coupling
but haven't tied that concept in with the "Q" and "k" of an IFT, nor have
you mentioned the related concept of "transitional" coupling. I would
expect to hear more mention of these concepts from someone who knows
"enough about IFT design".


Regards,

John Byrns


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

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been
mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

I supect it might, and one article in Wireless World refered to using
10.7 MHz.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar. What
exactly did Wireless World say was so great about using a 10.7 MHz IF for
a MW AM receiver?

Wide AF response was easily achieved.

Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed, although in the old days they
often did have articles by people who knew what they were talking about
with respect to radios.

I differ. WW and what it became, Electronics World wasn't just an
amateur's magazine. It had cutting edge articles about electronics
from 1917 onwards, and I suggest you park yourself beside a
pile of all the old copies and have a good read.
Most of the info was only comprehensible by very well university educated
professionals, or intellectuals, and most ideas were backed up with
mathematical
proofs which nearly all the general public couldn't understand.

I am reasonably familiar with Wireless World, I have 3 & 1/2 of those copy
paper boxes full of old issues from the 1930's through the 1950's. I
would estimate that I have at least half the issues from that period whcih
was probably the golden age of AM receiver technology. I have to take
serious exception to your characterization of the "mathematical proofs"
included in their articles. There may have been the odd article with some
mathematical depth, but those were few and far between. The math
presented seems to have been just enough to go over the head of the
average reader, but was hardly complex enough to be "only comprehensible
by very well university educated professionals, or intellectuals".

I never learnt any electronics maths at high school.
I was told that if I went into a career as an electronics design engineer,
I'd have to be far better at maths.

I agree, many articles don't have much maths, but a lot do,
and all one can do is read between the lines of incomprehensible maths.
By the mid 1970s, there were many young bright mainly british stars
who showed off their mathematical abilities, most probably to appear
to be stars, and top of the bloomin heap, and to foster good future employment with
major electronics firms.

The readers' letters section daown the back of the mag had the arguments between
engineers who
couldn't agree. Plenty of that alright.

The internet changed all that, along with everyone trying to keep progress secret
as possible,
and only for eyes of the financial backers.


I
suspect this light weight approach just slightly above the level of the
man in the street was carefully calculated to impress the average reader
without putting the material at a level where he couldn't understand it at
all. That is not to say that they didn't have many excellent authors who
knew all the math, but it is a serious stretch to imply that they included
any real mathematical depth, they included only enough to look impressive
to the untutored reader.

Well, there was a pile of stuff I couldn't understand.

I just got the general idea and built stuff, and got very nice results as good
as anyone with all that math ability would.



I suspect that the reason Wireless World might
have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an
easy way for a hobbyist, who both doesn't have a clue what he is doing,
and doesn't have the necessary test equipment, to get a super wide
bandwidth.

I leave you to your suppositions.

OK, but for all practical purposes my "supposition" seems to be identical
with your statement above that "Wide AF response was easily achieved",
which I take to be a quote from the actual Wireless World article?

Not my quote of WW.
But basically, one of the aims was expressed to mean thy same as I said.



Damping reduces Q, and increases BW.
But it also reduces Z at Fo, thus reducing gain in an amp
which must be a current source, like a pentode or j-fet,
to realise the best selectivity for the LC circuit.

This is a half truth, what matters is that the filter is correctly
terminated, not that pentode, triode or whatever drives it.

Well most IFTs made for tube radios would perform abysmally is driven
with triode amps with Ra = say 10k.
This would over damp the LC circuit in most cases.

Try damping an ordinary radio's IFTs with 100K, then 47k and finally 22k
for each of the 4 IF coils in a set.
Tell me what you find.

As far as
stage gain goes, increasing the frequency from 455 kHz to 2.0 MHz is
likely to decrease the gain by a similar amount to widening the 455 kHz
filter to the same bandwidth as the 2.0 MHz filter.

Use 3 x IFTs, and an extra stage of IF amplification.
I still reckon the 2MHz will work, and when I have time,
I'll try the idea, and tell everyone about it.

But to know any earlier, try it out for yourself.



What I said was what I said.
You are confused.

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

I know enough about IFT design, after having built my own radio.

That isn't clear at all, you seem to be obsessed with "Q", and hardly if
ever mention "k", and how it relates to "Q" in determining the
characteristics of an IFT. You occasionally mention "critical" coupling
but haven't tied that concept in with the "Q" and "k" of an IFT, nor have
you mentioned the related concept of "transitional" coupling. I would
expect to hear more mention of these concepts from someone who knows
"enough about IFT design".

I don't need to use k to confuse everyone.

An IFT is a simple RF transformer operating at a fixed F.
The magnetic lines of force from one coil react to transfer
some power from a primary LC to a secondary LC.

The coupling and insertion loss is whatever you are gonna get.
The looser the coupling, ie, the further apart the coils, the
sharper is the nose shape of the two circuits.
Let's assume you have a current source, ie, high impedance
signal source, or generator for the primary.
Assume the output from the sec goes to a high impedance load, like the grid
of a pentode tube, with little miller capacitance.
The load of the sec LC is transfered to the pri, depending on the closeness
of coupling.

Far apart gives a large insertion loss, and lowest RL for the pri signal source,
but the response shows the attenuation
is twice that of a single LC as you move away from the centre F.
Then as you bring the coils together, the insertion loss and load value reduces,
and the
response suddenly becomes flat topped, but the attenuation out of the pass band is
still
twice that of a single circuit.
Then with coils even closer, the insertion loss is low, but there are two peaks in
the response,
but outside the two peaks the response remains twice that of a single LC circuit.

k isn't needed to be considered since we are dealing practically with what you get
when you
use LC circuits arranged as they are in IFTs.
We simply wanna know what happens.
Its also spelled out in numerous old radio books, and there are maths for those
inclined.

The beauty of audio and radio engineering is that a lot of it can be done using
only very basic maths, and following well known practices and precautions.
It isn't as complex as rocket science.
We don't have to worry if we shoot some dude off into space, and find that our
equations were
wrong, and he spends eternity orbitting Mars with no way back.

Patrick Turner.



Regards,

John Byrns

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

I built a kit from Antique Electronics for a hi-fi crystal set AM tuner.
It was from a company called 'Peebles Originals' model number POCR-AM3.
(I'm not sure if this kit is still in their mosy recent catalog.)

It was fun to build. I also got the pine cabinet which I lined with tin
foil to minimize noise. With a long wire antenna it sounds quite good
having a clarity missing from most modern tuners. However, living in
Los Angeles with a very crowded AM band the poor selectivity is an issue. Also, the output is very low so you need a fair amount of gain to get to a decent line level.

You might want to check it out.

I have also noticed that there is a schematic for a "Tubeless HiFi
Tuner" in volume one of _Audio Anthology_.

- Paul
In rec.audio.tubes Jon Noring wrote:
william_b_noble wrote:

I guess that I must be in the minority - it seems to me that for
best AM fidelity (not selectivity, nor sensitivity), you would use a
crystal set with tuned RF stages, no IF, no heterodyne of any kind.
Use the tubes for RF amps if needed, and for audio amplification,
and use a tube diode for the detector.

Actually, this setup intrigues me for local reception, since it
appears to be a quite simple circuit. Are there any schematics of such
a circuit -- any commercially made radio of yesteryear using this
design approach?

Jon

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

Damping reduces Q, and increases BW.
But it also reduces Z at Fo, thus reducing gain in an amp
which must be a current source, like a pentode or j-fet,
to realise the best selectivity for the LC circuit.

This is a half truth, what matters is that the filter is correctly
terminated, not that pentode, triode or whatever drives it.

Well most IFTs made for tube radios would perform abysmally is driven
with triode amps with Ra = say 10k.
This would over damp the LC circuit in most cases.

That is true of "IFTs made for tube radios" when they are being used as
originally intended, but what happens when we hobbyists modify them for
High-Fidelity use by increasing "k" and decreasing the circuit "Q" by
adding resistors? In this case since we need external termination
resistors anyway, all we need do is connect the resistor between the anode
of the triode and the input of the IFT and all will be well, there is no
need for a current source to drive the filter, ideal would be a source
with just the required termination resistance. There is no reason why the
required termination resistance can't be connected between a low impedance
source and the input of the filter, it does not have to have one end
earthed.

Try damping an ordinary radio's IFTs with 100K, then 47k and finally 22k
for each of the 4 IF coils in a set.
Tell me what you find.

Every filter, be it an IFT or something more complex, is designed to be
terminated in specified impedances, which may be a specified resistance,
an open circuit, or even a short circuit, what matters is that the
termination is correct, not that the filter is driven by a pentode.

As far as
stage gain goes, increasing the frequency from 455 kHz to 2.0 MHz is
likely to decrease the gain by a similar amount to widening the 455 kHz
filter to the same bandwidth as the 2.0 MHz filter.

Use 3 x IFTs, and an extra stage of IF amplification.
I still reckon the 2MHz will work, and when I have time,
I'll try the idea, and tell everyone about it.

But to know any earlier, try it out for yourself.

I never said 2.0 MHz wouldn't work, in fact I specifically stated at least
once that I thought 2.0 MHz would work. If 2.0 MHz is what floats your
boat then that's what you should use, although I notice that you choose to
use the traditional 455 kHz in your radio design. What I said was simply
that 455 kHz would also work in a wideband High-Fidelity AM radio.

What I said was what I said.
You are confused.

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

I know enough about IFT design, after having built my own radio.

That isn't clear at all, you seem to be obsessed with "Q", and hardly if
ever mention "k", and how it relates to "Q" in determining the
characteristics of an IFT. You occasionally mention "critical" coupling
but haven't tied that concept in with the "Q" and "k" of an IFT, nor have
you mentioned the related concept of "transitional" coupling. I would
expect to hear more mention of these concepts from someone who knows
"enough about IFT design".

I don't need to use k to confuse everyone.

An IFT is a simple RF transformer operating at a fixed F.
The magnetic lines of force from one coil react to transfer
some power from a primary LC to a secondary LC.

The coupling and insertion loss is whatever you are gonna get.
The looser the coupling, ie, the further apart the coils, the
sharper is the nose shape of the two circuits.
Let's assume you have a current source, ie, high impedance
signal source, or generator for the primary.
Assume the output from the sec goes to a high impedance load, like the grid
of a pentode tube, with little miller capacitance.
The load of the sec LC is transfered to the pri, depending on the closeness
of coupling.

Far apart gives a large insertion loss, and lowest RL for the pri signal
source,
but the response shows the attenuation
is twice that of a single LC as you move away from the centre F.
Then as you bring the coils together, the insertion loss and load value
reduces,
and the
response suddenly becomes flat topped, but the attenuation out of the
pass band is
still
twice that of a single circuit.
Then with coils even closer, the insertion loss is low, but there are
two peaks in
the response,
but outside the two peaks the response remains twice that of a single LC
circuit.

k isn't needed to be considered since we are dealing practically with
what you get
when you
use LC circuits arranged as they are in IFTs.
We simply wanna know what happens.

If you simply "wanna know what happens" why do you even need to consider
"Q"? Your narrative description above sure makes it sound like "k" is
important, you just haven't tied "Q" and "k" to the response shapes you
describe.


Regards,

John Byrns


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

In article ,
(John Byrns) wrote:

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

Thanks.

--
Telamon
Ventura, California