At 07:34 PM 5/4/07, Don Wilhelm wrote:
All of what you say is true *EXCEPT* for one item - and that is the AGC.
Don you are absolutely correct. The reason
I never considered it is that I have never experienced it
with my Orion. I seldom operate with strong signals
spaced much closer than 500 Hz. The most severe
case of this may be in the CQ 160 CW Contest where
it's not uncommon for S9+20 to 30 signals to be spaced
every 500 Hz. With typical key click bandwidths, very
few people will try to get closer than 500 Hz. Given
500 Hz signal spacing and given a 500 Hz BW filter,
the filter easily knocks them down to a reasonable
level, since the filter BW is +/- 250 Hz and the signals
are +/- 500 Hz.
Orion uses a two-stage AGC system, one is
analog and ahead of the DSP. It has has the main
function of preventing over-driving the DSP stage.
"ADC overload can never be allowed to occur because when that
happens, signals are irrevocably corrupted. Signals that are larger
than the full-scale range of the ADC must force reduction of gain in
the analog section of the receiver. So both analog and digital AGCs
are used in the Orion. Resort to analog AGC only need be made when
signals inside the roofing bandwidth exceed about S-9 plus 30 dB. At
that point, sensitivity is reduced but what we are discovering is
that phase-noise performance-- as determined by reciprocal-mixing
measurements-- already limits what you can hear. So performance is
phase-noise limited and not DSP limited. Even so, the Orion gives you
the option of kicking in a crystal filter, preventing movement of the
analog AGC and the sensitivity reduction described above."
http://www.doug-smith.net/orion.htm
As stated, this analog AGC does not activate
until signals reach the S9+35 range. The second AGC is
digital and internal to the DSP (which works very well).
"As the resolution and speed of ADC hardware improve, digital radio
receivers become less dependent on analog AGC to meet dynamic-range
requirements. State-of-the-art 24-bit IF-DSP converters, such as
those used in the Ten-Tec Orion, produce about 100 dB of dynamic
range. That means a receiver can handle signals from the noise floor
to almost 40 dB over S9 without analog AGC. Above that level, analog
AGC is still necessary to maintain the linearity of analog circuits
and to prevent overload in the ADC hardware."
http://www.doug-smith.net/dspdynamics.htm
I haven't seen much about the K3 AGC but I
hope it uses a similar same technique as Orion. I have
NEVER heard any AGC pumping in Orion, even under
severe situations like the CQ 160 CW Contest which
I consider the ultimate test for any receiver. I guess
that's why it did not occur to me to mention it. Maybe
one of the Elecraft folk will tell us more abour the K3
AGC.
73, Bill W4ZV
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