Bill,
I did ask Wayne about the AGC (with respect to the roofing filter
selection) and he indicated that the AGC was developed in hardware
before the A/D conversion and there is additional AGC action created in
the DSP. As I understand it, it is similar to that which you describe
in the Orion.
Yes, from a receiver performance standpoint, only the A/D overload
problem counts (and it is critical), but from an operating (AGC pumping)
standpoint, there can be undesired performance in the presence of strong
signals that pass through the roofing filter even though they are
filtered out by the DSP because they will reduce the overall gain of the
receiver. Quite apart from the receiver specs, it does show up during
operation and the AGC pumping can be a real problem for weak signal
reception regardless of the total dynamic range of the receiver.
We both await the real details on the K3 AGC (and other things too).
73,
Don W3FPR
Bill Tippett wrote:
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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