John Byrns wrote:

In article , Robert Casey
wrote:

Looks like an envelope detector can perform well. See my post
over in a.b.p.radio with diagrams. Below is the text without
the diagrams:

After reading Brian's web page on modulation acceptance
http://n2.net/k6sti/ma.htm
I decided to try some simulations of the AA5 envelope detector and
got results similar to Brian's page before any mods.
Then I decided to try using that second diode 12AV6's and 12SQ7s
come with. That second diode will do the AVC function, thus
leaving the first diode doing only the audio envelope detection. As
Brian described, the AVC filter cap tends to back bias the envelope
detector in convential AA5 circuits, causing clipping at low valleys of
the modulation. Moving the AVC to its own diode gets rid of that
problem. Also reducing the envelope detector resistance value to 150K
from 500K also helps (you will lose about 3dB of audio, which isn't that
much). And with the AVC now separate this will only mildly affect the
AVC (by about 1dB). And when you move the AVC pick-off cap to
the plate of the IF tube, this drop goes away. This was observed in a real
radio I tried this mod in.

There are a couple of other interesting effects that taking the AGC
voltage from the primary of the last IFT has. At first as we tune toward
the center of the pass band the voltage across the primary of the IFT
increases as expected, but then as we get close to the center the voltage
starts dropping, reaching a minimum when the carrier is centered in the
pass band. The AGC voltage follows and the effect is to sharpen the
apparent aural tuning characteristic because the decrease in AGC voltage
when center tuned increases the gain at that point.

If a tuning eye is fitted to this type of radio it is almost universally
connected to the audio detector rather than the AGC detector, which is
unfortunate from the AC loading point of view as the filter for the eye
tube often presents an even worse AC load than the AGC circuit. For the
longest time I couldn't figure out why the designers of most of these
radios made this apparently dumb mistake in connecting the eye tube, but I
eventually realized that if the eye tube was connected to the AGC line it
would have an unnatural and non intuitive visual tuning characteristic,
the dip at center tune might confuse the average consumer, although a
radio nut would probably delight in the effect as an indicator of exact
tuning.

But there is rarely a dip in the tuning character of most AM sets.

But I often derive the AGC voltage separately from
a small cap&diode off the anode of the last IF amp, where more AGC voltage is
available because the envelope amplitude is greater than at the secondary of the
IFT.

The IFTs can be aligned with a VTM attatched to the AGC voltage,
and the tuning done to generate the highest AGC.
There is usually only one peak, and if two tuning peaks are seen,
its often because of mis-alignment of the IF, or
the circuit has been configured for wide BW and has slightly over critically
coupled IFTs,
which rarely occurs in any ordinary domestic AM set.
Some sets with variable IF selectivity had a switched tertiary on IFT1,
so first you tuned with the eye for max peak, then switched the tertiary in for
wide band audio,
or for locals.

Patrick Turner.





Regards,

John Byrns

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