Posted by
Richard Ferch on
Dec 15, 2008; 4:34pm
URL: http://elecraft.85.s1.nabble.com/f-w-2-73-1-98-power-calibration-tp1651339p1658884.html
Julian, G4ILO said:
> I don't think it is anything to do with this revision. I first noticed
this
> a few months ago, after one of the updates that made some changes to the
> ALC. Adjusting TXG VCE may be the answer, but the other change I made was
to
> avoid tuning at 5.0 or 50W. The K3 recalibrates the power when you press
> TUNE at those exact power levels, and if it is looking into a mismatched
(or
> varying, as you adjust the ATU) load then it mucks up the calibration.
Except when performing a power calibration, I never set my requested power
to 5.0W or 50W for the reason Julian describes.
Actually, this is not a power calibration issue at all, but it is a
difference in the radio's performance as a result of the firmware revision.
Even after performing the power calibration at 50W into a dummy load to
eliminate calibration as a concern, I find that version 2.73/1.98 delivers
significantly higher power than 2.67/1.96 at the same PWR control setting.
As AD7OG did, I observe that with a PWR control setting of 48 watts and TXG
VCE at 0.0 dB I get a reading of 90 watts (9 bars) delivered power at an
audio frequency of 1000 Hz in DATA A mode. The IMD in PSK31 is
correspondingly bad (at 90W delivered power, the measured IMD is around -18
dB instead of the desired -24 dB or better). The delivered power is not
quite this high at higher audio frequencies, but still well above the
requested 48 watts.
AFSK A mode behaves similarly to DATA A, but unlike PSK31 the consequences
in RTTY are relatively minor (no IMD issues to worry about - if I were using
low tones, I would hold the delivered power to 100 watts or less simply to
avoid the possibility of transmitting audio harmonics, but with high tones
this is not likely to be a problem).
In order to hold the delivered power down to no more than 50 watts so as to
give acceptable IMD values for PSK31, I must either adjust the PWR control
down to a setting much lower than the power I actually want, or set TXG VCE
to a value of -2.0 dB or so as suggested by NW8L. The precise power
delivered varies somewhat with audio frequency, but settings of 48W for the
PWR control and -2.0 dB for TXG VCE seem to deliver reasonable PSK31
performance here. I expect that these settings may continue to change with
future firmware revisions.
73,
Rich VE3KI
K3 #1595
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