Sorry for writing too much lately, I usually am a "lurker".
Question: I have no experience with "IMD meters" that are designed around
measuring the IMD products of PSK AFSK inputs. I get the idea of how they
might work (famous last words) but I am curious as to how they would
compare to conventional designs using two tone tests where the audio sources
are carefully isolated, and the measurement devices have the attenuator set
to remain in the linear range without putting the products down into the
noise floor.
My attempts have always resulted in: "Ok, this is better, and this is far
worse", but I have never been satisfied that I have something like a TOI
number that I can quote. The premise that I can purchase something like
this that gives accurate results with a simple AFSK input is attractive, but
I have questions as to its accuracy.
A lot has been said here lately regarding IMD and how ALC can influence it,
etc . My limited experience measuring it has led me to doubt my own
results, at least with ham radio devices.
Following advice is great. Measuring it and seeing it with your own eyes is
better.
What are the experts best suggestions in this regard?
Thanks.
Mark
----- Original Message -----
From: "Kevin" <kstover@ac0h.net>
To: <amps@contesting.com>
Sent: Tuesday, January 03, 2017 7:13 PM
Subject: Re: [Amps] FCC Denies Expert Linears' Request for Waiver of
Jim,
My TS-520 does not have a "power control" like my K3. My fathers TS-450SAT
does have a power control. I used to use the 520 on PSK31 and MFSK. I'd
tune it for maximum smoke on SSB and then feed it negative ALC voltage
from a 9V battery through a pot to control output power. Get it right down
to 25W and the IMD readings from my IMD meter would be -35 or better. I
did AFSK RTTY the same way.
I assume the reason it worked so well was because the ALC voltage did not
vary with the transmitted signal. Otherwise it was a royal PITA messing
with the drive control, carrier control, and mic gain.
On 1/3/2017 4:55 PM, Jim Thomson wrote:
Date: Tue, 3 Jan 2017 13:31:37 -0700
From: MU 4CX250B <4cx250b@miamioh.edu>
To: jim@audiosystemsgroup.com
Cc: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] FCC Denies Expert Linears' Request for Waiver of
15 dB Rule
I remember that with the Drake C-Line, some ops would short the T4XC
ALC line, which would increase the power output to 175W. Most vacuum
tube transmitters I am familiar with did not have a power adjustment.
Even the mighty solid state Collins KWM-380 lacks a power adjustment.
73,
Jim w8zr
## I owned several new C lines.... and you wont get any 175 watts out of
any of them
with the ALC shorted out. All that shorting out the ALC line did...was
increase the talk
power a whole bunch on ssb. Pep out stayed the same. Average went way
up.
## For ALC between the xcvr and amplifier in question to even work
correctly, the
time constant in the xcvrs alc has to match the time constant in the
amps alc circuit.
The drake L4B wont work with the Drake TR7. Ditto with a drake L7 and
a drake
T4XC. An ALC cap has to be swapped out to a different value in both
those amps.
## T4XC + L4B require no cap changes, alc is matched for time
constants....
ditto with TR7 + L7.
## If the amp in question doesnt have adjustable ALC, it still wont
work properly.
Both the L7 and L4B have adjustable alc. There is NO power output
control on the drake
T4X series, nor the TR3 /4 series. The adjustable ALC on the drake amps
was the only way
to tweak the po of the exciter.
## there were several of the kenwood SS xcvrs, like the TS-450 I
believe that had no PO control.
They ran flat out.... so again, if driving amp, the alc line to the amp
had to be installed..and the amp had
to have adjustable alc.
## back then using a xcvr that put out 130 watts pep, with an amp that
required 100w of drive would work
fine, with the alc line installed and tweaked correctly. The problems
started when some bozo tried to use his
swan 500-700 CX to drive a SB-200, or 8877, then try and rely on the
alc connection between em..to keep things
under control. Didnt work.
## even on the older SS xcvrs that had no PO control, you could still
control the output on ssb mode, by inserting
a low value of dc voltage on the alc buss on the back of the xcvr. -9
vdc from a battery, through a 50k pot, tweaked
correctly works just fine. Then you can dial up exactly how much pep out
u want on ssb mode. You have already
developed the ALC voltage. Scream all u want, but you wont exceed
say.... 65 watts, or whatever its tweaked to.
## another method is to use limiters and distortion cancelled clippers in
the audio chain, results are the same.
Jim VE7RF
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--
R. Kevin Stover
AC0H
ARRL
FISTS #11993
SKCC #215
NAQCC #3441
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