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[AMPS] G2DAF

To: <amps@contesting.com>
Subject: [AMPS] G2DAF
From: measures@vc.net (Rich Measures)
Date: Tue, 3 Nov 1998 03:53:57 -0800
>>>
>>>Why not use a storage spectrum analyzer to look at occupied bandwidth 
>>and
>>>IM over a significant time period while modulating with speech or 
>>pulsed
>>>white noise?  Wouldn't that be a better worst-case test?
>>>
>
>>RE:  pulsed noise:  Only if the pulses are spaced far enough apart to 
>>cause the screen potential to decay to near zero volts between pulses.
>
>Implied in my question, but not expressed, sorry.  Broadcasters sometimes
>use pulsed noise as a substitute test signal for "program audio".  It's
>handy for occupied BW and other measurements.  The pulsing is an
>important part of it all, since xmtrs bounce and overshoot with
>modulation sometimes causing carrier shift and thereby IMD...  (You don't
>need a carrier there for the IM to happen).  Anything that wiggles the
>screen V will wiggle the output and that may cause IM.  At least, so it
>would seem...
>
yes.  The exception is the supercathode circuit where the grid is 
"disconnected" by connecting it to the cathode.  This causes the 
(grounded) screen to act like a control grid when the cathode is driven.  

>>RE:  speech modulation:  I have listened with stereo headphones using two 
>>double-filter receivers.  The total IMD in the window above or below the 
>>DAF amps. was no better than 23db down. 
>
>I guess for mere mortals in the real world  without spectrum analyzers
>that's a pretty good test!  Just a little difficult to quantify and
>duplicate.  

Dual receive is nice because it allows one to confirm that the splatter 
is coming from the person speaking on the fundamental.  However, if only 
one receiver is available:  offset the RIT c. 3.8KHz, switch the RIT off 
and on, and the relative splatter level can be semi-accurately measured  
on the S-meter.  However,  since we have seen everything from 2db to 6db 
per "S-unit" on commercial transceivers,  an accurate decibel calibration 
chart of the S-meter must be made beforehand. 

>It ought to be simple to test this design on the bench with
>modern test gear and settle this pretty quick.  Has anyone done this?
>(Not discounting your 2 rcvr tests here.)

Measuring the IMD of speech with a distortion analyzer is not very exact. 
 The tests my two friends did on DAFs were 2-tone.  In my opinion, the 
best test is what relative level of splatter an amplifier delivers on the 
air.  
>.........
>It sure seems intuitive that screen V needs to be fairly constant, but
>I've been surprised before.
>
I have never seen a tetrode or pentode whose screen does not follow the 
3/2 power rule on its characteristic curves.  
-  later, Gary



Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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