>>>
>>>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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