On Thu, 14 Sep 2000 10:22:00 -0400 "Tom Rauch" <w8ji@contesting.com>
writes:
>
>> Marv:
>>
>> You're selling yourself short on your carrier resolution, I think!
>If the
>> DDS is clocked from the 196.608 MHz, I get 0.04578 Hz per lsb
>(196.608e6 /
>> (2^32)).
>>
>> But is this enough resolution for quality audio?
>>
>> Bruce R. Knox W8GN
>
>I want to know how the sidebands created by the switching
>amplifier will be filtered out, especially above 160 meters.
>73, Tom W8JI
>w8ji@contesting.com
>
Hi Tom,
I'm not quite sure which sidebands you are concerned with...
The note you replied to from Bruce concerns the FM mode. Since, I'm
not upsampling the FM modulation to match the DDS clock rate, there are
an infinite number of sidebands created in the process of making FM.
However, the system is operating above the requirements of Carson's rule
by greater than an order of magnitude so, they are created at a very low
level -- well within amateur specs.
The Switching Modulator will have a 4 pole filter with a 15KHz (or
thereabouts) cutoff frequency to keep it clean -- even on 160 meters.
There are other concerns with the power supply/modulator that may require
a linear post amplifier to make it totally right -- Though, I plan to try
it "bare" to start with.
The Class D RF power amp stage will be followed by narrow band
filtering plus a Pi-Net to match 20-200 ohms. This will reject harmonics
and other out of band spurious.
73,
Marv WC6W
*
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