On Sun, Jan 10, 2010 at 11:55 AM, Michael Jones <mi...@scsitoolbox.com> wrote: > Hello All, > > > > I'm wondering what is the relationship between the AGC-T setting and the pre > amp setting?
They are different, way different. You use them for completely different things. Probably the first thing to do is to read the article that was written on the topic of AGC Threshold (AGC-T) and placed in the knowledge base at: http://kc.flex-radio.com/KnowledgebaseArticle50153.aspx That covers how best to use the AGC-T control. > For instance, is changing the preamp from "off" to "Pre1" doing the same > thing as raising the AGC-T setting some set amount? No. > What is the recommendation for improving copy of a weak signal? Is it to > turn up the Pre amp setting, or too turn up AGC-T? It depends on the level of atmospheric noise being received. > >From input from the list here I have gotten very wary of ever turning up the > AGC-T setting past 70 or 75 - maybe that's just an unfounded fear - LOL! Definitely. I am trying to think of when I ever set AGC-T below about 80 but then, I don't seem to use it the same way as others since I want the noise to be audible. Think of the AGC Threshold (AGC-T) control more of an RF gain control. You can use it to turn down the sensitivity of the receiver. The preamp is able to actually increase the sensitivity of the receiver. You need to understand two key things: 1. the relationship between the internal noise of the receiver and the external noise being picked up by the antenna; 2. the job of AGC to reduce the gain of the receiver so that very strong signals appear to be about the same level as a weak signal. What is not always obvious is the relationship between the two and that is probably why you are asking this question. Perhaps the first thing to do to understand all this is to understand the relationship between the receiver's internal noise (noise figure) and noise picked up by the antenna. This is easy to do if you try a simple experiment. Connect your receiver to a dummy load, turn off the preamp, and adjust the panadaptor to show -145dBm on the bottom. Select 20m USB and set the filter to 2.7kHz. At this point the S-meter should be showing about -110 dBm. That represents that equivalent noise power of the receiver. Basically, until a signal reaches that power level you aren't going to hear it. Now turn on the preamp. Notice that the noise power level drops to about -123 dBm. This is the new noise floor. The preamp's noise now dominates the system noise as opposed to the noise in the rest of the receiver chain. This means that the receiver can now hear a 2.7 kHz wide signal that is at -123 dBm, about a 13dB improvement over the receiver without the preamp. Notice also that we haven't even touched the AGC-threshold control. One more thing before we go on, try changing the filter width. You will notice that when you double the filter bandwidth the S-meter shows a 3dB increase in noise. That is because twice the bandwidth admits twice the noise power (given equal noise power per Hz). You may also notice that the noise level reported by the panadaptor is much less than the noise reported by the S-meter. That is because the panadaptor use a series of very narrow filters (FFT bins) to make up the measurement. If you set the filter width to the same as the FFT bin width then both the panadaptor and the S-meter would show the same noise power. Turn the preamp back off. Switch from the dummy load to the antenna. You may notice that the noise level does not increase very much. (You can look at the panadaptor if you like.) If it doesn't increase at all that means that the noise generated by the receiver is greater than the noise and signal being picked up by the antenna. It also means that you might not hear a signal right at the noise level. Now repeat the same test with the preamp on. Notice that there is a noticeable increase in noise floor when you connect and disconnect the antenna. Now you know that the system noise is being dominated by received noise. Your receiver will now hear any signal that is hearable. On my Flex 5000 with the noise at my location on 20m, the noise level typically rises about 3dB when I connect the antenna with the preamp turned off. That means that the noise contributed by the antenna is about equal to the noise in the receiver. I would probably get a little improvement from the preamp (but not much). Once I get to 15m and above, the preamp is needed in order to hear the minimum discernible signal. > I've notice that changing the preamp setting does not change the levels I > see on the panadapter, and I think I understand from this list that that is > proper behavior because the panadapter is showing the absolute input level > of the signals before any preamp gain. Exactly. -- 73 de Brian, WB6RQN/J79BPL _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/ Message delivered to arch...@mail-archive.com
