On Wed, 2009-08-19 at 18:21 +0200, F8EHJ wrote:
> Hello,
>
> After reading jupiter documentation and some posts on the mailing
> list, I don't understand how does ALC indicator work ?
> In CW mode : ALC is lighting with dot or dash transmit.
> In SSB mode : ALC is blinking with voice peak, the MIC gain shall be
> adjusted for this.
> PSK31 : ALC LED must not be lighted.
Basically ALC (Automatic Level Control) detects RF output power or
voltage and lowers the transmitter gain to keep the output power or
voltage in a linear region of the amplifiers. Its like AGC on receive
which keeps the radio from blasting your ears. On CW that linearity is
less critical so the drive can be harder, however too much ALC will
change the envelope of each dot and dash. On SSB you just want to tickle
ALC to keep the rig most linear (least splatter) but some will use ALC
as a compressor to increase the average output power which sometimes
increases the transmitter bandwidth to several times the filter
bandwidth from overdriving the PA (or earlier stages) and the chopping
of the envelope when the ALC feedback voltage takes effect.
On PSK, transmitter linearity is most critical to data accuracy, and PSK
works well with low power so the drive is kept out of the ALC control
region. PSK detectors have built in logic to check for intermod from
overdriven transmitters and it shows on the waterfall display when a rig
is driven too hard.
>
> I noticed sometimes that (in SSB mode) ALC LED didn't light even if
> Mic Gain was at maximum. For ALC blinking, I shall decrease output
> power from 100 to 92 %.
> I noticed this case when SWR was about 1.6 , 1.7
The load seen at the ALC detector can be different due to the output low
pass filters. They act sort of like a lumped transmission line and when
that line is effectively an odd quarter wave long it does a considerable
impedance transformation of mismatched loads. Most often the ALC
detector is looking at voltage, (because that's simpler) rather than at
power which does make the load impedance seen at the ALC detector
crucial for proper ALC operation.
>
> Why ?
>
> Thanks for your help.
>
> Best 73's
>
> Herve F8EHJ
>
73, Jerry, K0CQ
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