[Flexradio] 144 and 432 - Low vs High Power Design

[Jim Lux]

Quoting Brian Lloyd <brian-wb6rqn at lloyd.com>, on Wed 02 Jul 2008  
12:40:10 PM PDT:

>
> On Jul 1, 2008, at 11:23 PM, David Hilton-Jones wrote:
>
>> I suggest that there are two important issues to consider:
>>
>> 1) Power output. I have a strong preference for 50-60 watts - that
>> allows driving of a 13db triode to 1kw. Intermediate amplifiers are
>> undesirable.
>
> You also want lower outputs needed by mixer stages for microwave
> converters.


Well.. that's sort of a system design/configuration thing.  You're  
going to need gain to get from the 0 dBmish levels out of the QSE up  
to your +60dBm and that gain's going to be in steps.  So the question  
is do you put it inside in the F5K or inside the amp or as a separate  
box.

I'd advocate, generally, for a design that puts out +10dBm or so,  
because that lets you pad it a bit and drive a mixer's IF port  
(assuming the LO comes from somewhere else).  Maybe a bit more,  
depending on if you want to do a passive divider scheme to drive  
multiple converters..  Maybe +20dBm?

After that it's just figuring out what the optimum (for each user)  
arrangement of gain blocks is.  The +13dB gain in a box for the high  
power amp is kind of a holdover from earlier days when a) HF amps are  
limited in gain by the FCC and b) they'd be just a single stage.

I would think that a modern UHF amp building block wouldn't use a  
+47dBm (50W) drive... even a watt seems like a lot.  In the microwave  
area, a HPA gain of 40 or 50 dB (e.g. 0dBm in for 100W out) isn't  
unusual for a TWTA.

Yes, there's a substantial installed base of low gain HPAs out there,  
but I would think that one would want to design the building blocks  
for a more modern system architecture.  Most commercial amps (not ham)  
seem to have a fair amount of gain (e.g. the Harris HF amps take 0dBm  
to +10dBm in for 1kW out, ditto the Amplifier Research amps we use at  
work).

Jim, W6RMK