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#22
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#23
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Steven Swift wrote: Patrick Turner writes: Let me say a few words. 1. Many pages of RDH4 are devoted to AVC. The time constant for AVC application is very long, comprising of 1M and 0.047 uF, and measurement of bass distortions resulting from well applied AVC is low enough to be negligible. To work properly for fading, the AVC needs to be about 100ms. This causes significant distortion at bass audio. If a longer time constant is used since only local (non-fading) stations will be tuned, then you are right. You don't get fading on locals. 1M plus 0.047 uF typically used in AVC circuits has a TC = 47 ms signifcant bass distortions do not occur. The pole is at 3.37 Hz. 2. Nothing wrong with cathode bias, especially nowdays when cheap large value elcaps are plentiful, and we have better plastic caps. RDH4 speds a lot of time on cathode bias. Cathode bias is great, but do not bypass the resistor. The degenerative feedback will improve audio performance, but you lose gain. If the Gm of the tube is like that of 6AU6, or 6BX6, the gain is high enough to throw 6 dB away on current feedback from an unbypassed Rk. 3. Diode detectors are quite low distortion detectors even with very low voltages of 100 mV if there is a constant current trickeled through the crystal diode to keep them turned on with their forward conducting voltage. I gave details yesterday in another post of a detector which will change your views about diode detectors. Diodes can be used with DC shunt feedback around an RF opamp, and thd is negligible. I agree that this can be made mostly true using active filters and such, but a perfect diode, with perfect modulation has lots of distortion. I am willing to take a look at your analysis, but if you use Volterra series expansion, you simply can't prove that you'll get better than a few percent distortion. Its obvious from my tests of my detector that ths is minimal, and below the 1% level. Somewhere in my old grad school notes, I have a derivation done by Prof. Meyer (of Gray and Meyer, UC Berkeley) which shows the limits. I'll look for your other post. Better than a few percent is NOT possible with just an RC load (diagonal clipping) except for low modulation percentages. There is no diagonal clipping in a well set up diode detector used with a virtual CCS current flow from the C used to collect RF pulses. If the ripple voltage does not vary with signal strength, the detection is linear. 4. AC coupling is fine from an RC load fed by a diode. The impedance fed by the audio + RF ripple voltage should be high, like a cathode follower grid. 5. I have tried my radio with various speakers, and no trouble making full amplitude signals at 20 Hz. The LF pole is determined by the audio amp in the radio, but at the detector, the There is a discussion of speaker/cabinet resonances in RDH4 somewhere. Lots of distortion when you approach resonance. But that distortion isn't a problem due to the tuner. On which pages are RDH4 and Terman "dont's" spelled out? Each chapter has "crumbs" of knowledge in it. I have yet to find a nice do/don't do list anywhere. I may also be "integrating" Terman and Henney. I think the guy who started the thread should build his idea and try them on a few "beta" testers. He'll be able to sell enough to pay off his costs. Maybe pick a few channels in a few big markets (LA, New York, Chicago) to keep the work load down. Mr Noring has a huge backlog of R&D work ahead of him to come up with his prototype tuner. I'd rather build my own gear than wait for his solution. Patrick Turner. Steve. -- Steven D. Swift, , http://www.novatech-instr.com NOVATECH INSTRUMENTS, INC. P.O. Box 55997 206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA |
#24
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John Byrns wrote: In article , Patrick Turner wrote: Steven Swift wrote: If you are willing to live with about 5-10% THD, then you can use more common circuits. True, but onje doesn't have to live with 5-10%. linearize the IF amp and detector, and thd plummets. However, there are dozens of "Hi Fi" AM circuits published by the hobby magazines, tube vendors and kit makers. Have a look at them. The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a detector and audio amplifier. RDH4 has the circuit for the Selsted and Smith "infinite impedance " detector, where a 12AU7 performs as credible detector, and as a diode, but I think I'll stick with a germanium diode fet by a 12AU7 CF. The "Selsted and Smith" detector is not the same thing as the so called "infinite impedance" detector. The "infinite impedance" or "reflex" detector was designed by RCA, while the "Selsted and Smith" detector was designed by, well "Selsted and Smith", or at least "Selsted" who is still around, or was a year or two ago. The "Selsted and Smith" detector differs from the "infinite impedance" detector in that it has a diode in series with the grid, and also a diode load resistor. Yes, you are right, and I should have checked my RDH4. But the S&S detector pulls much less power from the tuned circuit.... There is no peak detection capacitor across the diode load, so the diode does not act as an ordinary diode peak detector, and the triode doesn't act as a cathode follower. The triode is the actual detector operating in a fashion similar to the "infinite impedance" detector, with the diode apparently serving to linearize the "infinite impedance" detector. The input impedance of the "Selsted and Smith" detector is not infinite due to the presence of the diode load resistor. True, but its higher than most other detectors. For that matter the input impedance of the so called "infinite impedance" detector is also not infinite, and can even have a negative resistance component which can cause stability problems. The negative resistance effect can occur when circuit conditions are right, similar to the conditions that can cause oscillation in cathode and emitter follower circuits if you aren't careful. I am happier with direct feed of the RF/IF signal to a CF, with following crystal diode and C, with nearly constant current discharge from the C. But does the RC-19 have enough tuned circuits to give over 70 dB rejection of signals which are 50 kHz away from the wanted station at any place on the band? The 6BA6 is a variable U tube, with a non linear transfer curve. There is nothing wrong with the 6BA6, it was specifically designed for this service and has very low odd order distortion which is all that matters since the even order distortion products can't get through the IFT. I hope I got that the right way around, if not it is explained in some detail in some of the old texts, I think "Radio Receiver Design" by Sturley is one that explains it. You only get in trouble if you try to run the tube at a very high signal level, simultaneously with a high AGC voltage applied for a large gain reduction. This is mainly a problem in the stage driving the diode detector, so it is best to avoid AGC on that stage, but in a minimal radio that is of course problematic. This is one of the many topics that the RDH4 gives short shrift. The 6BA6 is even usable as a gain control element in audio circuits where even order distortion does matter. IIRC the peak limiter at a radio station where I once worked used four 6BA6s in the audio path, where they were connected in push pull, presumably to cancel the even order nonlinearities which are inherent in the design of the tube. Indeed the PP connection of two 6BA6 would lead to cancelation of 2H in the thd. Not a bad idea for an RF/IF amp either. All distortion is bad. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#25
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Telamon wrote: In article , (John Byrns) wrote: Please delete rec.radio.shortwave from the news group header. I'll say one thing about this group currently posting: They use more complicated words than Bryant does! Thanks. -- Telamon Ventura, California |
#26
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John Byrns wrote: In article , (Steven Swift) wrote: Patrick Turner writes: 3. Diode detectors are quite low distortion detectors even with very low voltages of 100 mV if there is a constant current trickeled through the crystal diode to keep them turned on with their forward conducting voltage. I gave details yesterday in another post of a detector which will change your views about diode detectors. Diodes can be used with DC shunt feedback around an RF opamp, and thd is negligible. I agree that this can be made mostly true using active filters and such, but a perfect diode, with perfect modulation has lots of distortion. I am willing to take a look at your analysis, but if you use Volterra series expansion, you simply can't prove that you'll get better than a few percent distortion. Somewhere in my old grad school notes, I have a derivation done by Prof. Meyer (of Gray and Meyer, UC Berkeley) which shows the limits. I'll look for your other post. Better than a few percent is NOT possible with just an RC load (diagonal clipping) except for low modulation percentages. It would be interesting to see the derivation you speak of! It was my impression that if we had a "perfect diode" it could be used make a perfect envelope detector, with the exception of the "tangential clipping" problem that you mentioned. "Tangential clipping" is not just a function of the modulation level, but is also a function of the modulating frequency. As Patrick mentioned using a higher IF frequency will allow using a smaller peak hold capacitor which will reduce "tangential clipping". Also doubling the IF frequency by using a full wave detector will reduce the "tangential clipping". It is hard to believe that the distortion of a reasonably designed diode detector is anywhere near "a few percent", as simple diode detectors were used in the modulation monitors used by AM broadcast stations in times gone by, and they had to have distortion low enough to measure the system distortion at modulation percentages up to 100%. I will have to look up the specifications of a few. Of course there is the issue that negative peak clipping that is visible on a scope may represent only a small fraction of a percent distortion on an RMS basis. The "tangential clipping or diagonal clipping ocurs if the C value in the RC circuit after the diode is of too large a value for the voltage and frequency generated. You see the phenomena best when the AF modulation F = 10 kHz or more, and output voltage is a few volts. So it becomes more likely as F of audio rises. But the amount of audio declines with rising F. Patrick Turner. Regards, John Byrns Surf my web pages at, http://users.rcn.com/jbyrns/ |
#27
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In article ,
dxAce wrote: Telamon wrote: In article , (John Byrns) wrote: Please delete rec.radio.shortwave from the news group header. I'll say one thing about this group currently posting: They use more complicated words than Bryant does! I have sent a dozen responses to the other two newsgroups asking them to stop cross posting. I don't like playing netcop but I will report them if it continues. -- Telamon Ventura, California |
#28
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"Telamon" wrote in message ... In article , (John Byrns) wrote: Please delete rec.radio.shortwave from the news group header. Thanks. -- Telamon Ventura, California What's the problem? This is a radio related discussion. Frank Dresser |
#29
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"Telamon" wrote in message ... I have sent a dozen responses to the other two newsgroups asking them to stop cross posting. I don't like playing netcop but I will report them if it continues. -- Telamon Ventura, California Here's an excerpt from the rec.radio.shortwave charter: "This group is intended to be a place where ANY radio monitoring topic can and should be discussed. We are happy to hear from posters who listen to any part of the radio or microwave spectrum, from DC to daylight. We discuss topics of almost any kind, ranging from (but not limited to) shortwave broadcasting, DXing small or distant shortwave stations, utility and teletype monitoring, military eavesdropping, station schedules, QSLing sw broadcasters, spectrum usage, equipment design and modifications, antennas, receiver reviews and recommendations, and many more." "Despite what the newsgroup name might imply, we definitely DO NOT limit discussions to shortwave only. Any radio-related topics are welcome." http://www.faqs.org/faqs/radio/monitoring/introduction/ Frank Dresser |
#30
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In article ,
dxAce wrote: Telamon wrote: In article , (John Byrns) wrote: Please delete rec.radio.shortwave from the news group header. I'll say one thing about this group currently posting: They use more complicated words than Bryant does! Like just about any technical newsgroup it is a painful read. Half the time stuff is wrong and then other people try to correct mistakes or misunderstandings. You can sit there and watch the threads drift closer to and then away from being correct and it just drives me nuts. Worse sometime the correct answer is posted and others go on with the wrong ideas anyway. Really sucks. These groups generally have humongously long threads. Then there is always at least one retard to stir the crap. -- Telamon Ventura, California |
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