[AMPS] L4B

[km1h@juno.com]

On Fri, 16 Oct 1998 09:50:32 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>
>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.



But Peter, you would not expect much in the way of temperature drift in
the short term. Any heating from either current flow or mounting location
would cause a slow and gradual change. Since the operating bias for
3-500Z's is usually defined over a fairly wide range...70-120ma per tube
depending on where you read the info....I still fail to see any reason
for concern with 3A diodes.....I would and did not recommend 1A diodes of
the 1N400x class.

73   Carl   KM1H



>
>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
>
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>

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