At 01:43 PM 2000-02-14 -0700, George K. Watson wrote:
>
>
>>>The only way to measure fundamental power without harmonic contribution is
>>>with a tuned receiver or spectrum analyzer.
>
>And the entire system has to be calibrated as just slapping a low pass on
>the line and measuring power will introduce losses at the fundamental.
Easy enough to do when the filter is included within the system when
"calibrating" the receiver or spectrum analyzer with the known-clean
signal. This is, in fact, the ONLY correct method of "calibrating"
equipment: include all adapters, modules, etc. that will be in-line during
the actual testing/measurement.
>>>If this is being observed with an untuned power meter, it is likely
>because
>>>MOSFETs and FETs readily generate copius amounts of odd-order harmonic
>>>energy, most notably 3rd and 5th harmonics.
>
>Isn't this a bit topology dependant? (Which harmonics are generated)
It's VERY topology-dependant, of course. But it's one of the reasons that,
until very recently, hybrid cable TV amplifiers, as one example, have been
primarily bipolars instead of FETs or MOSFETs. From my viewpoint, the
breakthrough occurred with the introduction, by NEC, of a line of four
GaAsFET broadband hybrid modules which exhibited slightly better odd-order
IMD generation compared to industry-standard bipolar hybrids. The
even-order distortion product levels were slightly higher but due to other
considerations of such applications, that turns out not to be a serious
drawback. Many vendors continue to push their own variations of FET
technology for these broadband, multi-carrier cable TV applications.
However, careful measurements of odd-order harmonic components generally
show their performance to be, at the least, inferior to the bipolar devices
they purport to be able to replace. Some have shown gross inferiority in
this respect due to inadequate vendor testing with multiple carriers such
as in a real-world application in cable TV amplifiers.
>>>Bipolar RF power transistors are built with and without emitter resistance
>>>which introduces negative feedback.
>
>Aren't there MOSFETS with 'built in' source resistors? I had heard this
>somewhere.
Yes, I don't recall off-hand the vendors doing this, though.
73, Steve K0XP
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|