Topband: dominating the spectrum

[Tom W8JI]

> Is anyone using these ruggedized Freescale LDMOS transistors in any new 
> amplifier designs?
>
> <http://www.youtube.com/user/digikey?v=KZF-FR8b71s&feature=pyv&ad=14013206369&kw=ldmos>
>
> Mike N2MS

Mike,

We should understand the You-tube videos and things about 40:1 SWR at full 
power we see or read are all marketing showmanship and hype.

The Freescale LDMOS FET's are virtually the same performance as the old 
Motorola TMOS devices, and any device they sell has to be duty cycle, 
voltage, and/or SWR limited or it will fail.

The Freescale demonstration video, and their specs, are for pulsed 
non-linear applications. Something with high duty cycle, or with SSB or CW, 
is different. In the 1990's, I designed a medical device with two MRF150's 
that ran over 1000 watts pulse output at HF. It could tolerate a shorted 
load, open load, or anything in between for hours. It had no SWR protection, 
except for the anemic power supply needed for pulsed duty cycle and AGC from 
a forward output power sensor that always held forward power at the FDA 
allowed peak power rating. I suppose Motorola could have advertised 1000 
watts from a MRF151 Gemini pack into any SWR.  :-)

There is a big difference between the real world, where heat and peak 
voltage are issues, and a marketing department's pulsed power video or 
creative specmanship.

If you really ran full rated power on the device in a normal duty cycle 
communications system, it would need protection for SWR and temperature like 
any other device or it would fail. Even a 2:1 SWR can kill an unprotected 
device. Also, the power rating for linear clean SSB is about 70% of what 
they claim, because they are giving class C ratings (that's why efficiency 
is so high). Things are not so rose colored when you read and understand the 
data sheet.

73 Tom