k2 spurious
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hi, i see 2 spurious left and right from the 3,5mhz main carrier, output 10w,
on 2,1 mhz and 4,9 mhz -40 and -38 dbm with -30dbm attenuation i wonder i anyone could tell me the correct level for the attenuation and maybe why this signals are there, not so simple as i stated of course,and what is the purpose of the binocular toroid in the power amp section. carlos on6cn -- mvg carlos popelier regards carlos popelier ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Carlos,
Do you have the internal frequency counter probe plugged into TP2? That is one possible cause, remove it and retest. CW or SSB mode? All spurious signals should be greater than 43 dB below the carrier level - they cannot be expressed meaningfully in dBm, only with respect to the carrier. The binocular toroid is a transformer to match the impedance of the PA collectors to 50 ohms, which is the input impedance of the Low Pass Filters. 73, Don W3FPR On 10/25/2020 7:40 AM, carlos popelier wrote: > hi, i see 2 spurious left and right from the 3,5mhz main carrier, output 10w, > on 2,1 mhz and 4,9 mhz -40 and -38 dbm with -30dbm attenuation > i wonder i anyone could tell me the correct level for the attenuation > and maybe why this signals are there, not so simple as i stated of > course,and what is the purpose of the binocular toroid in the power > amp section. > carlos on6cn > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Surely the binocular transformer is converting the input impedance of
the filter to the optimum load line for the PA transistors. Typically that would mean such that the the voltage swing was just short of the full supply rail for the worst case complex impedance, at the worst case acceptable SWR, for maximum power, and with that power. Also that transformer provides negative feedback. I'd expect the actual PA collector impedance to be a lot higher than the impedance reflected through the transformer, as transistors and FETs are, to a first approximation, current sources, although the negative feedback would actually make the reverse termination impedance much lower than that from the transformer. A hint that is the case is the option for different winding ratio if you are always going to use a lower maximum power. Maximising the voltage swing increases the efficiency. The alternative ratio doesn't transform the filter input impedance down so much. I mention this as there is a common misconception that the "maximum power transfer theorem" actually applies in such cases, when applying it would produce less than 50% efficiency. -- David Woolley Owner K2 06123 On 25/10/2020 18:17, Don Wilhelm wrote: > > The binocular toroid is a transformer to match the impedance of the PA > collectors to 50 ohms, which is the input impedance of the Low Pass > Filters. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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David and all,
I am a little late chiming in here, but all David's points are correct except for a caution on one. The alternate winding ratio for T4 should never be used with the KSB2 or the KPA100 installed. It is OK for power in CW kept below 5 watts. If the PA is operated above 5 watts, it becomes more and more inefficient as the power is increased. I have repaired several for buyers of a used K2 who had problems developing full output and observed that the current draw at 10 watts was quite high, sometimes causing HiCur messages. 73, Don W3FPR On 10/26/2020 8:43 PM, David Woolley wrote: > > I'd expect the actual PA collector impedance to be a lot higher than the > impedance reflected through the transformer, as transistors and FETs > are, to a first approximation, current sources, although the negative > feedback would actually make the reverse termination impedance much > lower than that from the transformer. > > A hint that is the case is the option for different winding ratio if you > are always going to use a lower maximum power. Maximising the voltage > swing increases the efficiency. The alternative ratio doesn't transform > the filter input impedance down so much. > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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