|[AMRadio] Broadcast Processors|
dennisgilliam at gmail.com
Tue Jun 14 13:04:21 EDT 2011
A short story about an AM transmitter:
Years ago I was CE for KZZP-AM in Mesa, AZ. The station had a Bauer 5kW,
and management wanted it to be 'louder'.
Replacing the old, single-band limiter, I installed a CRL set of three units
(AGC, EQ, and limiter), the usual setup.
Wow, what a difference that made! It still didn't please the program
director, so I got Ron Jones (the owner of Circuit Research Labs) to come
out and take a look. He said 'This transmitter has the worst bottom end
tilt I've ever seen'.
He went back and cooked up an experimental box we installed the next week,
and boy, did it sound really good & loud. Lots of mod plate glow, BTW. A
week later, the 4-000 modulators were flat. I didn't think that was too
bad, as they had been in service for some time.
I put in a new set, things were good again. Boy, those Eimacs were really
working for a living, I thought.
Two weeks later, they looked like old coffee pots, brown from ion burns and
totally without emission--again. I mentioned this to the PD, and that this
will cost some $. He said, 'Don't worry about it, we are going to make a
New tubes were ordered up, and in went my last spares. The same drill went
down, those tubes died in two weeks. More tubes installed. More great
Then the office manager asked me about the invoices she was getting from
Richardson Electronics for all those 4-1000's. I told her the story. She
went to the boss, and that started a loud argument between her and the PD
about the $$ out vs. the $$ in.
The big boss found for her, and the 'magic box' from CRL was retired and mod
tubes went back to lasting about a year or so.
The lesson of TANSTAAFL applied to transmitters.
On Tue, Jun 14, 2011 at 7:45 AM, Jim Tonne <Tonne at comcast.net> wrote:
> Regarding commercial broadcast processors for ham use.
> > The transmitter "Frequency response must be less than
> > 3dB down at 0.15Hz, and less than 0.1dB down at 9.5 kHz."
> That figure of 0.15 Hz may sound odd and arbitrary but is
> based on a direct-coupled modulator (i.e., flat down to DC)
> with a one-second time-constant on an input blocking
> capacitor and resistor.
> That figure of 0.15 Hz is sometimes specified in the contract
> between the manufacturer and the purchaser.
> Another way the low end is sometimes specified is that if
> the transmitter is modulated with a 50 Hz square wave that
> the tilt on the resultant modulation is to be less than 2%.
> Why this "nonsense" you may ask. It is because the
> commercial broadcast processors generally deliver an
> output waveform that is somewhat clipped, although it
> may be clipped only on transients. For them to control
> the modulation requires that the following audlio stages
> (the modulator) have such a flat response.
> Another area that is of concern in the transmitters is the
> insistence by the purchaser that the response be flat on
> the high end and yet have no overshoot on a 2 kHz
> squarewave. Can't do that and when the manufacturer
> sits down with the purchaser and explains the math,
> the purchaser comes away enlightened and happy.
> - Jim Tonne used to design BIG PWM rigs at CEC
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