On Sun, 9 Oct 2005 20:11:09 +0000 (UTC), "Reg Edwards"
wrote:


"Owen Duffy" wrote various ways and means of automatically
controlling the power output of transmitters. Thanks very much.

Presumably control is needed to limit the power dissipated in the
output transistors which involves a time delay. And, which is a
different thing, to place limits on the instantaneous
collector/emitter voltage.

Another method of PA protection is thermal protection of the PA,
sometimes integrated with cooling fan control. In my experience, where
drive control is deployed, it reduces drive to prevent the heatsink
exceeding about 60 deg C. I have found such protection commonly on
commercial FM land mobile equipment (two way radios), some amateur FM
mobiles, and less frequently in HF equipment.


In my Icom 735 transceiver the first control requirement appears to be
met by by placing a limit on the DC collector current or power input.

You are right, some radios do incorporate over-current protection of
the PA. It is in some Icom radios (IC706IIG, IC7400 / IC746PRO), and I
suspect many other Icoms will share this feature.

Whilst probably intended for PA protection, it has the great benefit
that the radio is less likely to draw excessive current while
adjusting an ATU, and tripping over-current protection in a power
supply. A radio that needs 16A in operation, but 25 during tune is a
pain in the butt on a nominal 20A power supply with over current
protection that kicks in above 22A!

The second requirement appears to be met by feeding back a
gain-control voltage proportional to the magnitude of the reflection
coefficient of the load impedance. I may not be entirely correct.

As I described, and for example, from the IC706IIG service manual:
"The reflected wave signal appears and increases on the
antenna connector when the antenna is mismatched. The
HF/50 MHz reflected signal level is detected at D10 (FILTER
board), and is amplified at the APC amplifier (IC1091c) and
applied to the ALC circuit as the reference voltage."

You are probably (almost certainly) correct about the 735 having a
"reflected power" limiting circuit, but Icom's view seems to be that
if you want to read their description of how the radio works (if you
had an interest in that sort of thing), you need to purchase the
service manual.

Where is all this leading Reg?

Owen
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