Hi Ed:
I run a sweep rate of about 15 - 20 CPS with the width expansion
as far as it will go so that the sweep is over scanned by about 200
percent. This rate is slow enough to see a complete audio waveform down
to as low as 50-60 cps without double scanning. Yep, those bright spots
in the middle are definitely signs of over modulation. If you hold the
microphone at a constant 1/2 thumb distance and watch the scope you we
eventually develop the speech that keeps the level more constant else
invest money or time in a compressor/limiter to help keep the level
constant. If you have another audio amplifier that you could run the
microphone into, then you could incorporate a optical coupling unit to
monitor the output of the second amplifier and use the control
resistance or photo side of the optical coupler to control the
amplification factor of the second amp. Then all you have to do is
sample the corrected constant audio level from some where in the second
amplifier to feed into the transmitter. Here's a little hint on the
stock DX100s. The audio sounds a lot better when they are only load up
to 100 watts input or about 75 watts output and then reducing the audio
gain accordingly.
Good Luck
John, WA5BXO
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Edward B Richards
Sent: Wednesday, April 14, 2004 7:03 PM
To: [email protected]
Subject: [AMRadio] Modulation Question
Hi Guys;
I am new to this list so please forgive me if my questions are dumb or
redundant. I just rebuilt a much abused Heathkit DX-100B. As far as I
can
tell it is operating OK into a dummy load. I have not put it on the air
yet. For one thing, I don't have a 75 meter antenna up yet. I have a
mobile ant I will install as soon as I have time. I do have a MFJ G5RV
jr
that is good for 40 meters and higher. I have not heard any AM on
anything but 75 meters. Is there activity on 40 and / or 20 meters?
I modified a scope to serve as a modulation monitor. I noticed 2 things
that puzzle me.
1. It is hard to tell modulation percent with voice as the waveform
keeps
changing and unless I use a sine wave I can't sync it to get a good
envelope pattern. Any trick to this?
2. Using the mod mon I see that it is very easy to over-modulate. If I
keep the modulation down so I don't see short bright lines in the center
of the screen (carrier disappearing, I think) , the average modulation
is
around 50% except for occasional peaks. Is this normal? would a
compression amplifier help? Thank you.
73, Ed Richards K6UUZ
_______________________________________________
AMRadio mailing list
[email protected]
http://mailman.qth.net/mailman/listinfo/amradio
