[1000mp] ALC

Sun, 4 Apr 2004 11:20:28 -0400

Tom,

Certainly understood on afsk and psk. My concern was the
difference in alc reading doing the same modes (cw, fsk)
between the Field and an 870. I haven't dug into it yet
but guessing the Field uses alc to limit output power as
opposed to the 870 having a carrier level control for
these modes in addition to a power control for ssb. I
just found it curious that if adjusted for instance to 10
watts output the alc reads 3/4 scale. I guess this is normal
but it's not addressed in the manual and was a departure
from what I'm used to.

73
Mike WA2E

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Tom McDermott
Sent: Thursday, April 01, 2004 9:11 AM
To: [email protected]
Subject: Re: [1000mp] ALC

Mike,

	You can run a fair amount of ALC on digital modes that only transmit
a single carrier at any instant in time - such as RTTY, MFSK, and CW. You
should not have any significant ALC on modes that transmit multiple carriers
simulataneously, or that shift the carrier phase suddenly - such as PSK,
Digtrx, MT63, etc.

	The reason is that ALC causes compression of the waveform. When
multiple tones are present, the compression will cause generation of
intermod products between the tones. If these IMD products fall in the audio
range they cause your signal to broaden a lot. What you get is more power
going into your interference products, and less power into your desired
signal.

	On phase-shifted signals, compression causes spectral regrowth,
which is seen on PSK as a really broad signal. The worst possible IMD on PSK
caused by ALC is -13 db - you see that a lot on the air.