[Orion] Question about noise floor

[Bill Tippett <btippett@alum.mit.edu>]

N1EU wrote:
The digital AGC does not amplify - a signal will emerge with the maximum 
gain if the digital AGC applies no compression to it.

        Barry, I share your confusion about how this works.  Note the
following in KF6DX's article just above and below Figure 1:

At input levels below actuation of analog AGC, digital AGC is solely 
responsible for leveling output signals. DSP peak-detects IF signals 
falling within the desired passband and adjusts a digital gain-control 
factor to maintain constant peak output. Fig 1 is a simplified block 
diagram of this system, which is identical to that of a traditional analog AGC.

Figure 1 omitted

In digital AGC systems, it is relatively easy to provide a variable 
threshold or "knee." Input signals below the threshold do not actuate the 
digital AGC and are not compressed. At thresholds well above the receiver 
noise floor, a receiver therefore gets quiet when only puny input signals 
are present in the passband. At thresholds near the noise floor, all 
signals are boosted to meet the output-level criterion. The net effect of a 
variable threshold is very much like that of an IF gain control.

http://www.doug-smith.net/digitalagc.htm

        I have also noticed some strange effects which seem
to be a function of bandwidth.  The AGC seems to work differently
depending on the CW bandwidth selected.  It's almost as though the
AGC detector measures the total POWER in the bandwidth rather than
a voltage LEVEL, and then adjusts for constant noise power output.

        If I ever go to visit Ten-Tec, there are questions I probably
need to ask with the radio in front of me and hopefully have someone
like Doug answer "Why does it do this?"  I may call Scott or Doug
sometime and try to do this over the phone since it is very difficult
to describe without having an Orion with real antennas in front of you.

                                        73,  Bill  W4ZV 

_______________________________________________
Orion mailing list
Orion@contesting.com
http://lists.contesting.com/mailman/listinfo/orion