On 05/25/2015 12:46 AM, Jim Thomson wrote:
/* snip */
## it takes aprx 11 db of NFB to reduce IMD by 10 db.....its almost a 1:1
correlation. A lot of the older existing xcvrs can have their imd improved by
increasing their idle current by 30-100% . This doesn’t always work..since
most older rigs,
like my FT-1000D only have one bias adjustment pot..for the final pa
section...and no provisions to tweak the bias for the driver transistors.
The entire chain has to work as a systems approach. A method to get around
the ALC induced imd, is
to use external 0 to –7 vdc alc volatge, then feed it to thre xcvrs rear alc
input. That way you can dial up any PO you desire, and not have to develop alc
voltage..after the fact. A simple setup with a 9 vdc battery feeding the outer ends
of a 50-100k pot works great. The wiper and one end is the V divider supplies
the 0 to –9vdc to the alc jack. if you want say 65 watts pep.... just set the
PO control on the xcvr to max.....then adj the alc voltage to 65 watts pep. Done deal.
Scream all you want, with mic gain cranked up..and wattmeter never goes higher
than 65 w pep.....or what ever you set it for.
I think this orange button business is related to and because of
the use of FET power amplifiers, probably LDMOS devices normally
operated in class "AB." (except when the orange button is depressed)
Transceivers such as the FT-990 and it's predecessor the FT-1000
use a pair of bipolar transistors operated in push pull class "B"
and if reasonably matched need only be biased to 1/100 of their
maximum collector current for good operation.
The setup is for .1 Ampere idle for 10 Amp of full drive per device.
(some people/person on this list have said 1/3 maximum current I
have no idea where they got that figure)
The push pull bipolar pair operated in class "B" has little to gain
from being operated in class "A." (it would limit the output power)
Two parallel bipolar transistors operated in class "A" would have a
much greater second harmonic content than the push pull bipolar pair
operated in class "B."
Every HAM transceiver of almost every brand that I have ever worked on
for friends used a pair of bipolar transistors operated in push pull
class "B" as the output stage and the driver has always been a single
bipolar transistor operated in class"A." (which is why the "driver" is
so frequently the cause of transmit failures, they didn't cool it
adequately)
More recent transceivers such as my FT-857D and FT-450D use use a
pair of bipolar transistors operated in push pull class "B" as the final
amplifier, the only exception is the FT-450D which has a push
pull bipolar pair operated in class "B" driver stage.
--
Ron KA4INM - Youvan's corollary:
Every action results in unwanted side effects.
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