100% sound modulation depth- is this wanted?

[oldeurope]

Good evening,
I finished the new (adjustable to London/Paris/Berlin) modulator.
It has a sound ALC and needs no sound level adjustment.
Making the sound modulation deepth to 100% gives some destroied signal.
The fault is not the modulator, it is the receiver.
Attached some calculationes what maximum undestroied modulation deepth is possible at some common TV sets. I hope the calculation is ok. The calculated result is very close to the measured result.
To get a clean output signal, modulation deepth should not be much more than 80%.

Kind regards,
Darius

continued...

[oldeurope]

https://www.vintage-radio.net/forum/...ead.php?t=3723

In this thread you can see how modulation deepth is detected.
To drive the electronic sound level pot the modulation deepth detector transistor Q2 must react at a little bit more than 80%. This is set by an additional resistor (100K) from base to GND. The LED shows when the electronic potentiometer is adjusted. This circuit works very fine and gives a good and strong sound from the receiver. The time constant is made long to make sure that the ALC is not noticed and dynamic is not limited.

Kind regards,
Darius

[jim_beacon]

Hi Darius,

100% mod depth on transmission is never a good idea, as unless you can accuratly control the AF input to your transmitter, you run the risk of over-modulation, resulting in "splatter" (spurious modulation sidebands, outside of the sound channel bandwidth). If you have EXTREMELY good AF level control, then you can probably run at 90% (we do this on our latest generation of UHF transmitters, but they have a VOGAD (ALC) circuit, which is capable of holding the mod depth at 90% for an input of -22dBm to +10dBm, and clipping if the signal level exceeds +10dBm. On older transmitters, we run at 75% mod depth.

Jim.

[oldeurope]

Originally Posted by jim_beacon

Hi Darius,

100% mod depth on transmission is never a good idea, as unless you can accuratly control the AF input to your transmitter, you run the risk of over-modulation, resulting in "splatter" (spurious modulation sidebands, outside of the sound channel bandwidth). If you have EXTREMELY good AF level control, then you can probably run at 90% (we do this on our latest generation of UHF transmitters, but they have a VOGAD (ALC) circuit, which is capable of holding the mod depth at 90% for an input of -22dBm to +10dBm, and clipping if the signal level exceeds +10dBm. On older transmitters, we run at 75% mod depth.

Jim.

Good morning Jim,
I decided to keep mod depth between 80%- 85% from an input of -12dBm to more than +6dBm (my audio test equipment stops there). I don't want to much AF gain after a longer quiet sound period. But this depends much in what the thing is used for. What is the use for your system? AM at UHF?

There is another very interesting aspect I found out.
Look at the detector circuit. At the top of the calculations I draw two sine wafes. The left with low mod depth is ok, the other is clipped at the positive swing. How much it is clipped depends in the ratio between the load resistor of the detector diode and the AC load of the cap and the following resistance of the sound stages. Some very vintage receivers do have a 1Meg detector load a 10nF cap and a 1Meg volume pot too keep damping of the detector tuned circuit low. These sets start clipping at 50% mod depth.
(BTW: The signal from "Deutschlandfunk DLF" is always destroied in these sets because DLF controlls the carrier level to get always to get full modulation depth for saving transmitting energie.)
Which side is clipped depends in the direction of the detector diode.
So we can not say clipping takes place in the positive (or negative) swing,
it is better to say the low carrier level and high carrier level swing.
If you do understand that the low level swing is generated by the load resistor of the detector and the high level swing is generated by the diode,
you will come to an interesting conclusion!

The high carrier level swing can be bigger than the low level swing because there is plenty of energie that can be provieded by the last IF stage to feed the detector diode.
My modulator uniquely supports this. It has the capability to produce 130% modulation depth at the high level swing. This is set by the biasing of the modulator transistor Q64. I am only controlling the modulation depth of the low level swing by Q2 to make sure that it does not go over 80%-85%.
I don't care about the high level swing. It can go up to 130%
The advantage of this circuit can not be measured out with a sine wave.
But with sound signal it is, because the peak levels of the positive and negative swings are not the same. The average is the same of course.
The result is a good modulated sound signal and this is importand for sets that drive the output valve directly from the detector.

(hope you do understand my german-english)

Kind regards,
Darius