[TenTec] (no subject)

[Dr. Gerald N. Johnson]

On Sat, 2007-09-22 at 10:32 -0500, Bob McGraw - K4TAX wrote:
> One of the nice things and one that is quite different than many other 
> radios is the fact that Tentec uses current monitoring of the PA stage. 
> Thus low current values caused by higher than 50 ohm nominal loads do not 
> cause the power output to "fold back".    At the same time, a higher PA 
> current caused by a lower than 50 ohm nominal load will be controlled by 
> reduced driver power thus lower drive to the PA hence PA current is 
> controlled.  In any event, the available power to the PA remains as a 
> constant and again does not fold back.  In either case, the volts remain 
> constant from the power supply and the current is controlled to a max value 
> thus the power input remains the same regardless of load.
> 
> Yes, the input of the low pass filter and SWR at that point is one that we 
> don't see.  In some cases the PA transistor combined Z is something near 0.9 
> ohms.  Thus the low pass filter has a job transforming this very low Z of 
> 0.9 ohms to 50 ohms nominal.  Its transfer characteristic will vary with 
> load and more specifically the reactive components of the load.

In all designs I've seen the low pass filter isn't doing any impedance
transformation, its all in the PA output transformer with 1 primary turn
and a bunch of secondary turns. In single ended VHF PAs sometimes there
is a tuned three element T matching circuit (values taken from a
Motorola applications note), but still the bulk of the harmonic
filtering is usually done with a 50 ohm in and out low pass filter.
> 
> Now regarding SWR, I have encountered cases where a component in the low 
> pass filter has failed more specifically as a result of trying to operate a 
> 75M antenna on 160M without any type of external matching network.  Those 
> voltage values can get nasty and can exceed the component ratings in the low 
> pass filter.

Smokesville!
> 
> All in all, err on the side of caution and double check antennas and loads 
> at a reduced power level.  And most definitely I agree with Jerry on the 
> fuses and the brands.  All fuses are not created equal, regardless of the 
> printing on the case or package.  My solution is a good quality switching 
> power supply with both current limiting and over voltage protection.  For a 
> play on words, when all else fails, let it be the fuse that fails.
> 
> 73
> Bob, K4TAX
> 
> 
In many solid state applications, a failed fuse has proven to be a
reliable indicator that the "protected" device has failed and so has
blown the fuse. Often the thermal time constant of the power device,
especially when the transistor goes into an avalanche mode from excess
collector voltage, is shorter than the fastest of fuses, like those made
especially for solid state. TenTec PAs are relatively unique in being
able to handle a great deal of SWR without frying instantly and so a
simple DC current protecting is adequate whether fuse, fast circuit
breaker, or power overcurrent shut down. In any of these the protection
works in tens or 100s of milliseconds while without TenTec's circuit and
thermal care the lifetime of the over voltaged RF power transistor may
be half a cycle at 30 MHz, something like 17 nanoseconds. DON'T EXPECT
OTHER BRANDS OF SOLID STATE POWER AMPLIFIERS TO BE SO RUGGED!!!
-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer