John Byrns wrote:
In article , Patrick Turner
wrote:
John Byrns wrote:
Now this seems like a reasonable concern about the effect of the diode
detector on the IF amplifier, unlike your concern about the current pulses
into the diode which are rendered irrelevant by the filtering/flywheel
action of the IFT.
The load varies during thre audio cycle
Yes, that is correct, the reason is that the diode isn't "perfect".
if the ripple voltage changes.
No, that is wrong, even with a "perfect" diode which would present the
driving circuit with a constant load over the complete audio cycle if such
a diode existed, the ripple voltage will vary, being very low at the
negative modulation peaks, and high at the positive modulation peaks, the
amplitude of the ripple tracks the envelope, it is just the natural way a
diode peak detector works, there is nothing to be done about it that I
know of.
There is something to be done about it.
Use my design of detector.
You have made conflicting statements about the 455 kHz ripple voltage out
of your detectors, first saying that your detector had a constant ripple
voltage over the audio cycle, then saying that the ripple voltage varied
over the audio cycle, and most recently you seem to be back to your
detector somehow having constant ripple voltage. This doesn't add up and
there is something fishy here.
Try my detector, and all will be revealed.
The variation of ripple voltage is much lower than with most other types of
detector.
So the IF amp gain changes, ie, there is intermodulation distortion caused by
the
detector.
Obviously the IF amplifier gain changes, I don't think anyone claimed
otherwise. Interestingly, one of the things I have learned as a result of
this discussion is that when a double tuned IFT is used to drive a real
diode the gain does not change in the direction you would expect due to
the impedance inverting properties of double tuned IFTs.
My concerns are not irrelevant.
I said your concerns about the diode RF current pulses are irrelevant in
my opinion, I didn't say that the varying load presented by the diode over
the modulation cycle is irrelevant. Interestingly the issue of the
varying load seems to be much more complex than I have seen mentioned in
any text book, I have never seen a text book which mentioned the impedance
inverting properties of double tuned IFTs in connection with the varying
diode load.
Using my detector, what becomes irelevant is the loading effect by the detector
circuit on the IFT and amp. My circuit removes such loading entirely.
When the signal level is low, or the negative modulation high, the non
linearity of the diode detector causes a lighter loading on the secondary
of the IFT. This of course is going to create envelope distortion at the
secondary of the IFT, and the plate of the IF amplifier tube, and many
authorities claim this distortion is a good thing because it implies that
given the finite source impedance of the IFT secondary, the drive voltage
to the diode detector will increase at these points, partially offsetting
the distortion of the envelope detector under these conditions.
I don't know what authorities say that distortion is a "good thing"
I didn't say that they said distortion was a good thing, I said that some
authorities point out that a finite non zero source impedance will work to
minimize the distortion caused by the diode peak detector at high
modulation levels, the fact that the signal wave form at the anode of the
IF amplifier tube becomes distorted as a result is irrelevant. Think of
it as a distortion cancellation mechanism if you like, I know you have
done work in that area yourself.
There is an optimal load range for an IF amp pentode, too high, or too low
results in greater distortion.
Its important to have the IF tube operate with a load within the range for a
fairly linear outcome, and at a not too high an output voltage, since there is no
negative FB that can be applied around the device to linearize it, except perhaps
by negative current FB
by usung an unbypassed Rk.
Pentodes have terrible distortion when the load is really high or low.
The text books speak of this mechanism
as if the diode were driven from a tube and a simple resistance, and it
seems rather obvious that the idea works in that case, but when you put a
double tuned IFT between the IF amplifier tube and the diode, then things
change and it is no longer obvious exactly what happens, and what the
final result is. I have crunched a few numbers and found that my
hypothesized impedance inverting effect does indeed occur, causing the
gain of the IF amplifier to decrease rather than increase as you might
expect, but so far it appears that the increase in voltage driving the
diode due to the higher load resistance presented by the diode more than
compensates for the lowered gain of the IF amplifier tube. One
unfortunate thing I have discovered is that the impedance inverting effect
is most pronounced at the carrier frequency, and less so at the sideband
frequencies, which means that this effect increases the modulation
percentage. Fortunately in a narrow band IF this increase in modulation
percentage is probably completely swamped by the bandpass effects of the
IFT on the sidebands, and this might partly account for John Doty's
comments that wideband AM IFs often sound more distorted. There are a lot
of numbers to be crunched with respect to the effects of the double tuned
IFT in this regard, it might make a good subject for someone's masters
degree thesis, that is if they can find a professor interested in
something like this, that has little relevance to current technology.
Maybe only about 3 ppl in the world care about this whole issue at this time.
I decided it'd be better to remove diode loading effects, period.
No more vague notions to spoil the music.
A cathode
follower would negate this effect.
You've been trying to tell me a CF isn't needed all along, but here you
say its
OK.
No, I didn't say it was OK, or for that matter that it wasn't,
???
what I was
saying is that if driving a diode envelope detector with a finite source
resistance does reduce distortion on weak or heavily modulated signals,
then using a cathode follower would negate the distortion reducing effect
of the finite source resistance, I didn't mean that a cathode follower
would negate the distortion!
Depends how the diode and resistances are set up.
But I would say the CF definately does negate, reduce, banish, expunge the
distortion
of the usual "conventional" detection circuit, so ppl should try it, if they can
live with an extra twin triode in their set, but if not, use a couple of emitter
follower
buffers, using darlington pair connected signal transistors.
These display very high input impedance, and much lower output impedance than any
tube,
and lower thd.
One thing that I haven't seen
mentioned is how the IFT plays into this since it has a sort of impedance
inverting characteristic, as we discussed earlier, if the secondary is
shorted, the impedance of the primary increases, just the opposite of what
we would expect.
I found that loading the secondary reduced the gain of the IF amp, ie,
the load is reflected to the primary.
When a double tuned IFT is used, the load reflected in the primary is
opposite to the actual load on the secondary, that is when the load
resistance presented to the secondary of the IFT by the diode goes up, the
tube sees a load resistance from the primary that is going down, just the
opposite of what you might expect.
Nope, when I placed 100k resistors on the IFT secs, the gain went down.
If by loading the secondary you meant
decreasing the load resistance, then you should have found that the power
gain actually went up, at least from the perspective of the primary. See
my discussion above for more detail.
If a 100k resistance is connected across the secondary of the last IFT LC,
then the the voltage ratio of the IFT increases. Less output IF voltage comes out,
so less AVC voltage, to the IF amp is biased more positively, so there is more
voltage produced
at the primary or anode connection of the IFT.
That's the only mechanism I know by which gain will increase on the IF amp tube,
if R loading is applied to the IFT sec.
If the AVC voltage is removed and replaced with a fixed bias then its a different
story, and loading the sec
of an IFT presents a lower load at the pri, and the gain falls.
That's what I have found, and there are limits about how much loading to apply,
or the gain reduces to too low a value.
100k is about right for most IFT LC circuits to widen bw.
47k is a bit too much, but might be OK if you had two IF amps.
If two IF amps are used, perhaps 22k could be used across the second IFT,
since the amp need only make a small amount of gain and a small output voltage
ahead
of the second IF amp.
The 3 IFTs would give better skirt selectivity.
Patrick Turner.
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/