Dohn, N8EWY, asked: Isn't the position of the AF gain (volume?) dependent on the position of the RF gain knob? As I turn the "RF gain" knob clockwise, the AF gain (volume) increases. So it seems that the position of the AF gain knob is variable with respect to the RF gain.
Just to get another thread going about this, I would ask where you best position the RF gain knob when operating. ---------------------------------------------------------- That's right Dohn. Most communications receivers have two gain controls: an audio gain that varies the amount of amplification in the audio stages and an "RF" gain that varies the amount of amplification before the RF signal is converted to audio. If you turn the RF gain down, you reduce the amount of audio being delivered to the audio amplifiers by the product detector, so you have to turn the AF gain up to maintain the same audio level. Going waaaaaay back to when I was first building receivers, that RF gain actually controlled the gain of the RF amplifier (preamplifier in modern jargon) at the input to the receiver as well as the gain of the intermediate frequency (i.f.) amplifiers ahead of the detector. Nowadays in state-of-the-art receivers like the K2 we control the gain at the input by switching the rf amplifier (preamp)into our out of the signal path or we can add an attenuator (ATT) at the input to reduce the signal gain reaching the receiver. The RF gain now controls only the gain of the i.f. amplifier, but the term "RF Gain" is still used to describe that control (after all, the intermediate frequency is still an RF signal). When AGC is enabled, it controls the "RF" gain (gain of the intermediate frequency amplifier) automatically. For the AGC to have full range of control, you want the RF gain fully CW (maximum gain). Turning it down over-rides the AGC, limiting the maximum gain available. If a signal is strong enough to cause the AGC to try to reduce the gain below the level you've set with the RF gain, it will do so, but the AGC cannot increase the gain above what you have set with the RF gain. Watch the S-meter behavior as you adjust the RF gain and you'll see how strong a signal is needed to activate the AGC for any setting. It has to be a signal stronger than that indicated on the "S-meter" bargraph. So the normal position for the RF gain is fully CW if you have AGC on. If you want to use the RF gain to control the loudness of signals in your speaker or phones, first turn the RF gain all the way CCW (minimum gain) and turn off the Preamp, then adjust your AF gain up until you hear background noise in the speaker, then back it off a bit so that noise won't be objectionable. In the K2, there's a fair amount of signal leak through the I.F. amplifier even with the RF gain at minimum, so you might need to disconnect the antenna to note the ideal position for the AF gain. Once you've established that, you can reset it to approximately the same spot in the future. It's not critical. Now use the RF gain control to control the level of signals. I do that a lot and turn the AGC off because it makes listening much, much nicer to my ear. With the AGC off, the background noise stays down at the same level even though a signal may not be present. With AGC, the gain is increased when a signal is not present, bringing up the noise level to an unnaturally high level. That might help one hear a weak signal calling, but to me it's just a nuisance in normal operating. I prefer to adjust the RF gain manually when listening for a 'weak one'. A related question that comes up here from time to time is "When is it important to use the Preamplifier?" If the K2 received only the lower frequency bands such as 160 meters, 80 meters and perhaps 40 meters, the preamplifier would be completely unnecessary. Your K2 would receive the weakest signal possible without it. That's because the noise picked up by your antenna will override any noise produced by the amplifiers inside the K2. Atmospheric noise levels drop dramatically on the higher frequencies. Depending upon your antenna, at some point between 7 and 30 MHz the internal noise in the K2 may start to compete with weak signals. That's when you use your preamp. The preamp is a low noise amplifier that boosts the weak signal so it will be stronger than any noise produced in the K2. And that leads to the next question, "Why would I turn the preamp off? Signals are louder with it on." Every receiver will overload in the presence of a strong enough signal, even the K2. In an ideal receiver, all of the selectivity would be right where the antenna connects to isolate the signal you want to hear from all the other signals on the bands before it is amplified. If you need only listen to one frequency, that's practical to do. But, in a receiver like the K2 that we expect to tune all over the band and even switch bands from one end of the HF spectrum to the other, the incoming signals are first converted to a fixed, intermediate frequency where the filter suppresses all the signals except the one we want to hear. That works well, allowing us to use one filter system tuned to the intermediate frequency in a receiver that covers a wide range of frequencies, but it also causes a problem. Strong signals can overload the stages in the receiver ahead of the filter! The results of overload can take many forms. Commonly, you might hear "phantom" signals QRMing other signals or the band might simply sound noisy, as if there's something creating lots of QRN in the area (there is, your receiver!). Often you'll hear both, depending upon the frequency. The designers built the K2 to handle the biggest signals possible, but there's still limits to what they can do. If you use your preamp on bands where it's not needed, you are amplifying a broad spectrum of signals, increasing the chances a strong signal that you don't want to copy will overload the receiver. That's why the attenuator is provided as well. On some bands, the noise picked up by the antenna is high enough the dynamic range (range of signal strengths your receiver can handle without overload) can be improved by reducing the strength of all the signals coming into the antenna jack. That's when you use the attenuator. Here's a simple way to see if you need the preamplifier. Turn the preamplifier off. Tune to a quiet frequency (no signals) on the band you're using, set your filter to the *narrowest* setting you use, then turn up the RF/AF gains until you hear the background QRN. Now disconnect your antenna. If the noise level drops, you do not need the preamplifier. The band QRN (which dropped out when you disconnected the antenna)is setting the limit on the weakest signal you can hear. The only way you can improve the receiver sensitivity is to use a narrower filter(which removes more of the noise from around the signal frequency)or reduce the noise coming in from the antenna in some way. The noise blanker helps a lot with specific types of QRN, but it, too can *cause* QRN with its switching action. That's why you should only turn it on *if* it improves the reception. Unless you have a very poor antenna, the preamp will do you no good from 1.8 through at least 7 MHz. Often it's not needed on 14 MHz either. The preamp is usually a help up near 30 MHz and possibly as low as 14 MHz, depending upon your antenna and whether you live in an area with very low QRN levels. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. 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