And yet I find everything works better when I pot down on the RF gain... hmmm > Date: Fri, 9 Jul 2010 20:16:37 -0700 > From: [email protected] > To: [email protected] > CC: [email protected] > Subject: Re: [Elecraft] Regarding the K3 and high QRN levels > > > > > > I'm sorry, but you can't have it both ways. > > > > "RF" (i-f) gain as implemented in the K3 operates on the post-filter > (8 MHz) i-f amplifier. The control voltage is derived at least two > (perhaps three) ways. > > > > The post 2nd mixer i-f amplifier (15 KHz) output is detected and if a > threshold > is reached, so-called hardware AGC is applied to the post-filter > amplifier. The manual i-f gain control is derived from an encoder, > processed by the DSP and summed to the same post-filter amplifier. If I > remember what Lyle told me the last time I saw him, there is also a > contribution to this control voltage from the signal processing DSP. I > could be mistaken on this last point, but no matter, for the sake of this > discussion it's immaterial. > > > > So as far as the 8 MHz if amplifier is concerned, it can't tell manual r-f > gain > from hardware AGC. So when you worry about AGC degrading SNR (it does) a > reduction in i-f gain does exactly the same thing! > > > > In fact, as I demonstrated to Lyle over a year ago, before the "R-F gain > calibration" routine, my K3 suffered noticeable SNR degradation even on > S9+50 dB signals, with the slightest engagement of the "r-f" gain > control. This is because of the wacky gain control characteristics of the > FET(s), which can have Gm variations of 2:1, device-to-device. > > > > I submit that a modern DSP radio with all of the "smarts" this > suggests, should not even need an r-f (or i-f) gain control. I've used > spectrum analyzers for over 30 years (even before computer control) that > slaved > the input attenuator and the i-f gain control to maximize dynamic range > without > overloading the front-end mixer. If you uncoupled the controls, the test for > overload was to change the input attenuator 10 dB and see if the display > changed 10 dB. If it was less than 10 dB then the mixer was in > compression. > > > > If this trivial calculation can't be made in a DSP radio, then I want an > analog > receiver back. > > > > To be sure, AGC development isn't trivial, particularly with the latency that > seems to be associated with DSP. (I'm not a digital guy, so I'm guessing > here) But even in analog receivers, group delay is an issue with > control loop stability. In a homebrew received I did in the 70's I had to > pick off i-f for AGC detection before the tail-end-of-the-if-crystal filter to > stabilize the loop. But with attack time set by i-f and decay time set by > audio, it worked flawlessly. > > > > I know it can be done digitally, if my TS-870 is any measure. I never touch > the manual gain control on that > radio (or my K3 for that matter.) If a > human has to intervene, there is something wrong with the ACG design. > > > > Wes > > --- On Fri, 7/9/10, Guy Olinger K2AV <[email protected]> wrote: > AGC is instant and dynamic, and does not vary RF gain, PRE/ATT settings which > are static unless the user changes them. RF gain, PRE, and ATT result in > numeric "advice" to the CPU, rather than being in direct control of a circuit > or device as in an analog receiver. The CPU in turn drives the actual > circuit devices from it's many outputs. There are many uses for this > indirect linking. > > > At issue is that the main smart AGC is digital signal processing, and is > *AFTER* the analog to digital conversion (ADC). The RF gain (which is really > IF gain), PRE and ATT are before the ADC, and improperly setting those can > squeeze the noise into the high numerical range in the conversion or even > engage the defensive hardware AGC which is better off not being engaged. The > defensive AGC is analog, has no smarts, and unfortunately must reduce > signal-to-noise in doing its job of preventing ADC input overload. > > > Those who understand how the rig works will throttle back the pre-digital > controls to where noise is at most moderately loud to obtain best operation > of digital features. > > What I am suggesting is that in an "AUTO" novice mode a slow (rate of human > turning the RF gain) throttling back can be set (and then left alone) by the > radio for those who are not into the theory and do not recognize the source > of their complaints as being caused/worsened by their misadjustment of RF > gain, PRE and ATT. > > > Leaving PRE on and RF gain at max on all bands regardless causes the digital > AGC to increase or reduce the noise to the same level as the wanted signal in > pauses. This reduces the effectiveness of the NR algorithms. > > > Those who ride their own RF gains would not be using this. > > 73, Guy. > > > > > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html _________________________________________________________________ Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_1 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected]
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