Rich says:
>1N5400 series,3a avg. diodes are around $0.25 each. A string of 'em
>provides adjustable cathode bias, and can withstand 200a peak during a
>"glitch" -- a current that will destroy a 50w zener in mS.
They also provide a voltage which falls at 2mV/deg C per diode. So 10 diodes
will give a bias which falls by 200mV for a 10 deg C rise in temperature. If
you get a rise of 30 deg C, that's a drift of over 5% in the bias voltage.
Further, the dynamic resistance is given by 25/I, where I is in mA. For a
500mA plate current swing (i.e. the DC from no RF in to DC with a reasonable
amount of output), that's a non-negligible change in resistance.
Even so, the effects on the overall IMD are probably negligible because as
KM1H and K1TA have said, the IMD performance of most modern transceivers
both will dominate and is appalling!
As far as Type approval is concerned, the FCC is only concerned with
spurious emissions - that's harmonics and signals far out. By far out is
defined as the boundary between spurious emissions and Out of Band
Emissions. This boundary is 2.5 times the necessary bandwidth, the
'necessary bandwidth' being that bandwidth which the signal can be passed
through for a given degardation. So for SSB, this is taken as 3KHz, and
everything more than 7.5KHz away is spurious. BTW, the 7.5KHz is measured
from the assigned frequency, which 1400Hz away from the carrier. Out of Band
emissions are those produced by the modulation process, and are not to
confused with Out of Allocation emissions.
73
Peter G3RZP
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|