Before the recent change in the IF sample circuit, the transfer gain was around -15 to -17 dB. In other words, a signal input to the antenna will appear at the IF sample port about 15 dB below the input level. This assumes the K3's preamplifier is off and, of course, that the K3 is tuned to within a couple hundred KHz of the input test signal.
My understanding of the modification (I have not made the change to my K3, but I have looked at the change in simulation) is that the transfer gain is now around -5 dB or so when measured into a 50 ohm termination. IF sample ports of commercial and military receivers tend to be of two types, and some receivers have both. If intended for panadapter use, the net transfer gain usually is around 0 dB, i.e., the output signal level at IF is approximately equal to the RF input signal level. The second type of IF output is used for external demodulation, and will often have considerable gain and is usually at a later IF stage and often will be behind the receiver's selective filtering. This type of IF sample is also usable to look at the signal envelope, but is close to worthless for wide band panadapter use. The K3, of course, has an IF sample of the first type. My Kenwood TS-940 has both, a panadapter usable high IF sample at 8850 KHz and the second type of IF output sample at 100 KHz. (These are from memory, so excuse me if I've made an error in the exact frequencies.) A typical oscilloscope will have a usable signal deflection at maximum gain in the range of a few millivolts per cm or graticule division. Let's say it's 5 mV/div, not an unusual value. An envelope deflection of 2 divisions (+/- 1 division) has a peak-to-peak voltage therefore of 10 mV, or an RMS value of 3.5 mV. If 50uV = S9, and if the net transfer gain of the K3 to the IF is -5 dB (it will probably be better than this since the oscilloscope has a high Z input) then it will require an input signal level of 42 dB over S9 to yield 2 divisions peak-to-peak deflection. Hence, it's not at all surprising that one will not see an envelope displayed when an oscilloscope is connected to the K3's IF sample port with normal antenna signal levels. Spectrum analyzers or panadapters, of course, are designed to be much more sensitive, responding to signal levels in the 1 uV range or less. Jack K8ZOA www.cliftonlaboratories.com Duncan Carter wrote: > Don Nelson wrote: > >> Ken K3IU wrote: >> >> >>> G'morning: >>> >>> Tomorrow I am going to make this mod on the K3 of a friend. To try to >>> see what difference it makes, I though I'd look at the IF signal on a >>> scope before and after. I have looked at the output on a modified K3 >>> (mine) and it looks like about 2.5 mV (P-P). >>> >>> Does that look about the right order of magnitude??? >>> >>> Thanks... >>> >>> Ken K3IU >>> ______________________________________________________________ >>> Elecraft mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/elecraft >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:Elecraft@mailman.qth.net >>> >>> This list hosted by: http://www.qsl.net >>> Please help support this email list: http://www.qsl.net/donate.html >>> >>> >>> >>> >> Ken, >> >> I have already made this modification to my K3. But I have not tested >> it. So your email triggered a curiosity to check it. Since the test gear >> is in the basement and the K3 is here in the shack, I thought of >> connecting my FT817 and listening to the IF output. The IF center >> frequency is 8.215 MHz more or less and so you can listen above the IF >> and below the IF to what stations are transmitting. The FT817 was chosen >> because it is highly portable and tunes to 8.215 MHz easily. You could >> do an A/B test on signal level. There were a lot of signals on 20m and >> so I had a lot to hear. If you put the K3 on say 14.350 MHz, for >> example, then a signal at 14.325 MHz is going to be 25 KHz above the >> 8.215 MHz IF or 8.240 MHz out of the IF output. This is because the IF >> signal is inverted and goes up in frequency as you go down, in this case >> down from14.350 MHz. The signals are going to be LSB for the same >> reason. You could tune for WWV at 10 MHz to have a stable signal for the >> before/after test. >> >> By the way, this is an odd and awkward way to add a sub receiver to the K3. >> >> Don, N0YE >> >> > .I see about the the same on my new K3 that had the mod factory > installed. I had expected to be able to look at IF noise with my > oscilloscope like I do on my FT-101E but the level is too low and > there's some pickup of other signals away from the IF so you need some > gain and some selectivity ahead of the scope. I suppose I'll get an 8 > MHz crystal for the 101 because I'm really curious about how well the K3 > noise blanker works on different noises that I'm used to seeing on the 101. > > I did a bit of blanker experimenting last night and sent this note to a > friend W5JAW, who is very experienced in blanker design, as I am. > > "I'm accustomed to looking at the IF noise on my FT-101E with an > oscilloscope but the IF out level on the K3 is at such a low level that > it's hard to see if you don't have big signals. I've seen a few such > big signals but only with the blankers off. There one that even my > FT-101E can handle at any gain level, a series of five pulses that may > be a control signal sent over the AC lines every few minutes, that I > don't see on either radio if the blankers are on. The FT-101 can handle > other noises but some are really level sensitive and it doesn't have an > external control except for the 20 dB attenuator and my trusty alignment > tool. > > I just did a check on 30 meters which is usually my worst band for > noise; tonight so far, the noise is not too bad. To test, I set the K3 > in AM and looked at the audio out spectrum on a spectrum analyzer. > With the blanker off, I see a harmonic series of 120 Hz. On the > oscilloscope, I don't see any sharp spikes so I suspect this noise may > be generated by neighborhood TV sets. If my neighbor who moved to > Boston six weeks ago were still here, I'd have a severe example for > testing but he and his nasty tv set are gone. With the dsp blanking, at > the extreme end of the IF WIDE range, I can lower the higher order > harmonics, 4th order and higher. If I add the IF DSP at the more > extreme end of it's ranges, E3-7, the 120 Hz harmonic series completely > disappears. I haven't tried this with any big signals in the ham bands; > apparently my neighbors who are phone men are too discouraged by the > lousy conditions. The only big signals that I've heard are on AM > broadcast. If I add the Noise Reduction, the interference really goes > away. With the limited or no gain ahead of the k3 blanker, the IF > blanker may need fairly large pulses to trigger blanking. " or not. > > 73, Dunc, W5DC > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
