[AMRadio] Frequency Response


Jim Candela jcandela at prodigy.net
Sat Sep 25 11:27:06 EDT 2004


 
I have always admired what Don has done with his modulation system. That 3400 hertz brick wall filter is a rarity unless one uses high technology active circuitry such as active op-amps, switched capacitor filters, dsp, etc.  I believe Don has a passive module that does a great job. I wonder where he got it?
 
Also consider your individual voice characteristics, microphone, and speech polarity when building a modulation system. 
Jim WD5JKO
 


Donald Chester <k4kyv at hotmail.com> wrote:


>What is a good ballpark response that I should use for operating AM in the
>amateur bands.
>

I would shoot for minimum of 100~ to 5000~.

According to the UTC catalogue (United Transformer Co.), to keep phase shift 
distortion to a minimum, the response capability of the transmitter should 
run one octave above and one octave below the actual response used, so a 
transmitter with 50~ to 10,000~ capability would sound better even with 
restricted response of the audio actually used.

Of course, that would require broadcast quality components. I would suggest 
running response tests on the transformers in the rig, and then make sure 
the coupling components in the rest of the audio chain allow at least the 
response of the most restricted transformer. If you feel you need to 
further restrict the response in any way, it should be done right at the 
microphone preamp. If you limit your overall response to 300-3000~ you 
might just as well run SSB.

I have been able to get satisfactory results with a very sharp, brickwall 
lowpass audio filter that cuts off at 3400~, in combination with a rising 
response boost in the mic preamp beginning at 800~ and peaking at about 9 dB 
from 2000~ to the cutoff frequency of the filter, while the low frequency 
response remains flat down to about 40~.

Don k4kyv


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