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Paul Burridge wrote in message . ..
Exactly how much power do you need? Only enough to feed another inverter gate. Egad, Paul! You've been wasting this much net bandwidth just to drive another HC gate?? All you need is a filter/matching circuit that steps up the voltage. This is DOG SIMPLE! See below. Exactly how "clean" (free from other harmonics) must it be? Preferably filthy. It's another multiplier (this time only 3X, thank God!) Then you need a clean enough input that you'll get the desired output purity. "Filthy" is likely NOT the right answer and will just get you into further trouble. But fortunately, "clean" is simple, and "really clean" isn't at all difficult. Try this: square wave output -- I don't recall your exact freq; I used 3.7MHz -- from HC gate, feeds 4.58pF capacitor (make at least that one tuneable). Other end of cap feeds 20uH inductor, Qu=200. Other end of that inductor connects to next gate input, and net 18.6pF of capacitance to ground: say 15pF cap plus 3.6pF of gate input capacitance. For DC bias, gate input to ground = 22kohms; gate input to Vcc = 47kohms. That keeps the gate in a valid logic state when there's no excitation. Assuming the gate's RF input resistance at 18MHz is at least 2.5kohms, you should get a voltage gain at the fifth harmonic of about 15dB, which will be ample to drive the gate input. The available current from the filter is low enough that the gate's input protection diodes should clamp things nicely at the rails. Be sure to use a gate that has input protection, or else add low-capacitance, fast diodes externally. Gain at the third and seventh is down 20dB or so from that. If it needs to be cleaner than that, you can add a second resonator. The gate biasing suggested may result in an output duty cycle significantly different from 50%. If you will always have 3.7MHz drive, you can bias the input more in the center of its range, or even rearrange the circuit a bit and use a feedback resistor from output to input to set the DC bias. The gate's input impedance is then much lower, but you don't need much voltage to drive it. Don't use that trick with a Schmitt trigger input, though. 69 turns of 36AWG (0.125mm) wire, spaced 2 wire diameters c-c, on an 0.375" former, should give you about 20uH at Qu=200 and first parallel SRF about 50MHz, but you should be able to make it more compact using something like a T-50-2 powdered iron core. |
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BTW, I just built the suggested circuit, on one of those old white
plug-boards which is NOT a great idea at RF. It worked fine. Easy to tune just with a scope. Performance was as I expected: full -0.6V to +5.6V or so swing at 18MHz at the input of the "18MHz" gate, and an 18MHz rectangular wave at its output. 74HC04 hex inverter; 3 stages as a simple RC oscillator to generate the 3.7MHz input. Duty cycle of that 3.7MHz was fairly close to 50%, but definitely not right on. The performance tells me that the 'HC04 really does have a fairly high input impedance at 18MHz, and requires under a milliwatt to drive--perhaps well under. There's AMPLE fifth harmonic power in that 3.7MHz square wave out of an 'HC04, and you just need to filter and do whatever impedance transformation is appropriate to get the desired result. Incomplete filtering of the non-fifth harmonics causes variation in the output pulse width (in a cycle that repeats every cycle of the 3.7MHz input), and frankly if I wanted a pretty pure 15th harmonic at the final output, I'd use a bit better filter to get a cleaner fifth. But try the simple one first, to convince yourself that it's possible. It's a pretty "hack" circuit, in that I just made educated guesses at the 'HC04 input and output impedances at 18MHz, and at a reasonable load resistance for an 'HC04. It could probably be optimized a bit, but it DOES work as described. Inductor Qu: The loaded Q of the circuit I described depends on the output impedance of an 'HC04 inverter, and the input impedance of the same with the bias resistors included. It should be running somewhere around 20-25, I believe. That determines how well the third and seventh are rejected. The unloaded Q of the inductor determines how much fifth-harmonic power is lost in the filter. You can stand some loss, but you don't want to waste too much. If the loaded Q is 25 and the inductor has an unloaded Q of 50, then you will waste half the power in the inductor. That probably would still work, but it would be better if the Q was more like 100, as a minimum. Based on what I've been hearing so far, I'm not very confident that you will be able to tell the Qu of your junque-box inductors, especially since you need to know it at the operating frequency. Qu of a lot of manufactured inductors is quite low, especially at non-optimal frequencies. If you picked a ~20uH inductor designed for use in a switching power supply at 100kHz, it could well be awful at 18MHz. It only takes a couple minutes to wind up an air-core one, so that's what I'd suggest. The one I used was 49 turns on a T-94-2 powdered iron core. Cheers, Tom Paul Burridge wrote in message . .. On 18 Mar 2004 10:16:53 -0800, (Tom Bruhns) wrote: Paul Burridge wrote in message . .. Exactly how much power do you need? Only enough to feed another inverter gate. Egad, Paul! You've been wasting this much net bandwidth just to drive another HC gate?? All you need is a filter/matching circuit that steps up the voltage. This is DOG SIMPLE! See below. Thanks, Tom. I'll give your suggestion a try. Just one question: is the unloaded Q of 200 you specify critical? I probably have a 20uH factory-made inductor laying about in my parts stash but won't know it's Q without measuring it. Need I bother? |
BTW, I just built the suggested circuit, on one of those old white
plug-boards which is NOT a great idea at RF. It worked fine. Easy to tune just with a scope. Performance was as I expected: full -0.6V to +5.6V or so swing at 18MHz at the input of the "18MHz" gate, and an 18MHz rectangular wave at its output. 74HC04 hex inverter; 3 stages as a simple RC oscillator to generate the 3.7MHz input. Duty cycle of that 3.7MHz was fairly close to 50%, but definitely not right on. The performance tells me that the 'HC04 really does have a fairly high input impedance at 18MHz, and requires under a milliwatt to drive--perhaps well under. There's AMPLE fifth harmonic power in that 3.7MHz square wave out of an 'HC04, and you just need to filter and do whatever impedance transformation is appropriate to get the desired result. Incomplete filtering of the non-fifth harmonics causes variation in the output pulse width (in a cycle that repeats every cycle of the 3.7MHz input), and frankly if I wanted a pretty pure 15th harmonic at the final output, I'd use a bit better filter to get a cleaner fifth. But try the simple one first, to convince yourself that it's possible. It's a pretty "hack" circuit, in that I just made educated guesses at the 'HC04 input and output impedances at 18MHz, and at a reasonable load resistance for an 'HC04. It could probably be optimized a bit, but it DOES work as described. Inductor Qu: The loaded Q of the circuit I described depends on the output impedance of an 'HC04 inverter, and the input impedance of the same with the bias resistors included. It should be running somewhere around 20-25, I believe. That determines how well the third and seventh are rejected. The unloaded Q of the inductor determines how much fifth-harmonic power is lost in the filter. You can stand some loss, but you don't want to waste too much. If the loaded Q is 25 and the inductor has an unloaded Q of 50, then you will waste half the power in the inductor. That probably would still work, but it would be better if the Q was more like 100, as a minimum. Based on what I've been hearing so far, I'm not very confident that you will be able to tell the Qu of your junque-box inductors, especially since you need to know it at the operating frequency. Qu of a lot of manufactured inductors is quite low, especially at non-optimal frequencies. If you picked a ~20uH inductor designed for use in a switching power supply at 100kHz, it could well be awful at 18MHz. It only takes a couple minutes to wind up an air-core one, so that's what I'd suggest. The one I used was 49 turns on a T-94-2 powdered iron core. Cheers, Tom Paul Burridge wrote in message . .. On 18 Mar 2004 10:16:53 -0800, (Tom Bruhns) wrote: Paul Burridge wrote in message . .. Exactly how much power do you need? Only enough to feed another inverter gate. Egad, Paul! You've been wasting this much net bandwidth just to drive another HC gate?? All you need is a filter/matching circuit that steps up the voltage. This is DOG SIMPLE! See below. Thanks, Tom. I'll give your suggestion a try. Just one question: is the unloaded Q of 200 you specify critical? I probably have a 20uH factory-made inductor laying about in my parts stash but won't know it's Q without measuring it. Need I bother? |
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I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. W4ZCB |
I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. W4ZCB |
On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson"
wrote: I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. I was told 40Mhz is "practically DC" too. I guess it depends on where you're coming from. Actually I've dumped the factory inductor as suggested by Tom and wound-up a large, air core job on 15mm plastic water pipe. It's made a big difference. I'm happy to report I've now got the 5th! Could be a little cleaner but who cares? Tom made a big deal out of the importance of high-Q so it was the obvious thing to try. Fortunately, it's worked. God knows how I'm going to squeeze this monster coil onto the board, though! :-| Can I infer from this experience that SMD inductors of over a few uH are a waste of time? -- The BBC: Licensed at public expense to spread lies. |
On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson"
wrote: I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. I was told 40Mhz is "practically DC" too. I guess it depends on where you're coming from. Actually I've dumped the factory inductor as suggested by Tom and wound-up a large, air core job on 15mm plastic water pipe. It's made a big difference. I'm happy to report I've now got the 5th! Could be a little cleaner but who cares? Tom made a big deal out of the importance of high-Q so it was the obvious thing to try. Fortunately, it's worked. God knows how I'm going to squeeze this monster coil onto the board, though! :-| Can I infer from this experience that SMD inductors of over a few uH are a waste of time? -- The BBC: Licensed at public expense to spread lies. |
Harold E. Johnson wrote,
I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. W4ZCB The BBC couldn't do it unless all the leads were bent to the right. 73, Tom Donaly, KA6RUH |
Harold E. Johnson wrote,
I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. W4ZCB The BBC couldn't do it unless all the leads were bent to the right. 73, Tom Donaly, KA6RUH |
I think you can get adequate Q if you use a powdered iron toroid. I'm
not going to take the time to look; there are some Q curves on the Amidon website. But I think that a T-37-2 core might work OK. I could also have a look at what's possible with a lower Q coil, but don't have the time right now. You could do that with Spice or with RFSim99, too. If you lower the "output" capacitor, you will raise the voltage delivered to the 18MHz gate input, given that the LC is resonant at the desired output freq. A note on the effects of inadequate filtering... Of course, the result is harmonics other than just the fifth in the output. But what does that mean after you've squared things up by running the signal through another gate? How does a waveform which spends all its time at either 5V or 0V have multiple frequencies? It does it by having different high and low times on each cycle of the five output cycles that happen for each input cycle. And if you want a pretty pure 15th harmonic (3rd harmonic of the 5th harmonic), you are probably going to want the cycles to be uniform, and close to 50% duty cycle. (They WILL have plenty of third harmonic...the rise and fall times I observed yesterday on 'HC04s were about 4 nanoseconds, and that will get you up above 100MHz...) That will take more filtering. If your inductor Q is only moderate, you still have a chance if you build a higher order filter, instead of trying to do it with a single pole (really single pole pair) filter such as I suggested. You can do it with a single pole but it has to be quite high loaded Q, and the coil unloaded Q must be higher still. But you can make the multi-pole filter wider, maybe two or three MHz wide, and get by with lower Q inductors and still have reasonably low loss and adequate rejection of the other harmonics. Yeah, the little moulded SMT inductors aren't very high Q. They are intended more for power supply filtering and choking, but you can use them in relatively wideband filters too. Now that you've seen that it can be done, I hope you'll be inspired to see what you can do to make it better. Cheers, Tom Paul Burridge wrote in message . .. On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson" wrote: I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. I was told 40Mhz is "practically DC" too. I guess it depends on where you're coming from. Actually I've dumped the factory inductor as suggested by Tom and wound-up a large, air core job on 15mm plastic water pipe. It's made a big difference. I'm happy to report I've now got the 5th! Could be a little cleaner but who cares? Tom made a big deal out of the importance of high-Q so it was the obvious thing to try. Fortunately, it's worked. God knows how I'm going to squeeze this monster coil onto the board, though! :-| Can I infer from this experience that SMD inductors of over a few uH are a waste of time? |
I think you can get adequate Q if you use a powdered iron toroid. I'm
not going to take the time to look; there are some Q curves on the Amidon website. But I think that a T-37-2 core might work OK. I could also have a look at what's possible with a lower Q coil, but don't have the time right now. You could do that with Spice or with RFSim99, too. If you lower the "output" capacitor, you will raise the voltage delivered to the 18MHz gate input, given that the LC is resonant at the desired output freq. A note on the effects of inadequate filtering... Of course, the result is harmonics other than just the fifth in the output. But what does that mean after you've squared things up by running the signal through another gate? How does a waveform which spends all its time at either 5V or 0V have multiple frequencies? It does it by having different high and low times on each cycle of the five output cycles that happen for each input cycle. And if you want a pretty pure 15th harmonic (3rd harmonic of the 5th harmonic), you are probably going to want the cycles to be uniform, and close to 50% duty cycle. (They WILL have plenty of third harmonic...the rise and fall times I observed yesterday on 'HC04s were about 4 nanoseconds, and that will get you up above 100MHz...) That will take more filtering. If your inductor Q is only moderate, you still have a chance if you build a higher order filter, instead of trying to do it with a single pole (really single pole pair) filter such as I suggested. You can do it with a single pole but it has to be quite high loaded Q, and the coil unloaded Q must be higher still. But you can make the multi-pole filter wider, maybe two or three MHz wide, and get by with lower Q inductors and still have reasonably low loss and adequate rejection of the other harmonics. Yeah, the little moulded SMT inductors aren't very high Q. They are intended more for power supply filtering and choking, but you can use them in relatively wideband filters too. Now that you've seen that it can be done, I hope you'll be inspired to see what you can do to make it better. Cheers, Tom Paul Burridge wrote in message . .. On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson" wrote: I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. I was told 40Mhz is "practically DC" too. I guess it depends on where you're coming from. Actually I've dumped the factory inductor as suggested by Tom and wound-up a large, air core job on 15mm plastic water pipe. It's made a big difference. I'm happy to report I've now got the 5th! Could be a little cleaner but who cares? Tom made a big deal out of the importance of high-Q so it was the obvious thing to try. Fortunately, it's worked. God knows how I'm going to squeeze this monster coil onto the board, though! :-| Can I infer from this experience that SMD inductors of over a few uH are a waste of time? |
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Paul,
If you are trying to keep things small, have you considered one of the high speed versions of the 4046 PLL/OSC? Some manufacturers spec these up to 20 MHz. No coils. If you go with the LC, and you have any fixed capacitors in parallel with the inductor, use decent capacitors, like mica or RF approved ceramic. I have seen cheap ceramic caps meant for bypassing just not work in applications like yours. Tam "Paul Burridge" wrote in message ... On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson" wrote: I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. I was told 40Mhz is "practically DC" too. I guess it depends on where you're coming from. Actually I've dumped the factory inductor as suggested by Tom and wound-up a large, air core job on 15mm plastic water pipe. It's made a big difference. I'm happy to report I've now got the 5th! Could be a little cleaner but who cares? Tom made a big deal out of the importance of high-Q so it was the obvious thing to try. Fortunately, it's worked. God knows how I'm going to squeeze this monster coil onto the board, though! :-| Can I infer from this experience that SMD inductors of over a few uH are a waste of time? -- The BBC: Licensed at public expense to spread lies. |
Paul,
If you are trying to keep things small, have you considered one of the high speed versions of the 4046 PLL/OSC? Some manufacturers spec these up to 20 MHz. No coils. If you go with the LC, and you have any fixed capacitors in parallel with the inductor, use decent capacitors, like mica or RF approved ceramic. I have seen cheap ceramic caps meant for bypassing just not work in applications like yours. Tam "Paul Burridge" wrote in message ... On Fri, 19 Mar 2004 17:46:43 GMT, "Harold E. Johnson" wrote: I don't think you can get away with short cuts at RF, especially if you're a non-expert. 3 MHz is practically DC. Why don't you ask one of your friends at the BBC to build it for you. I was told 40Mhz is "practically DC" too. I guess it depends on where you're coming from. Actually I've dumped the factory inductor as suggested by Tom and wound-up a large, air core job on 15mm plastic water pipe. It's made a big difference. I'm happy to report I've now got the 5th! Could be a little cleaner but who cares? Tom made a big deal out of the importance of high-Q so it was the obvious thing to try. Fortunately, it's worked. God knows how I'm going to squeeze this monster coil onto the board, though! :-| Can I infer from this experience that SMD inductors of over a few uH are a waste of time? -- The BBC: Licensed at public expense to spread lies. |
On Sat, 20 Mar 2004 10:13:22 -0500, "Tam/WB2TT"
wrote: Paul, If you are trying to keep things small, have you considered one of the high speed versions of the 4046 PLL/OSC? Some manufacturers spec these up to 20 MHz. No coils. If you go with the LC, and you have any fixed capacitors in parallel with the inductor, use decent capacitors, like mica or RF approved ceramic. I have seen cheap ceramic caps meant for bypassing just not work in applications like yours. That's a point well worth remembering, Tam: it's not just coils that exhibit a Q-factor and just as much harm can be caused by using lossy capactors in tuing applications, too. The PLL suggestion is a good one, but I'm trying to keep things simple for this fairly straightforward project. It's nice to be able to build everything just from what's in the junkbox. -- The BBC: Licensed at public expense to spread lies. |
On Sat, 20 Mar 2004 10:13:22 -0500, "Tam/WB2TT"
wrote: Paul, If you are trying to keep things small, have you considered one of the high speed versions of the 4046 PLL/OSC? Some manufacturers spec these up to 20 MHz. No coils. If you go with the LC, and you have any fixed capacitors in parallel with the inductor, use decent capacitors, like mica or RF approved ceramic. I have seen cheap ceramic caps meant for bypassing just not work in applications like yours. That's a point well worth remembering, Tam: it's not just coils that exhibit a Q-factor and just as much harm can be caused by using lossy capactors in tuing applications, too. The PLL suggestion is a good one, but I'm trying to keep things simple for this fairly straightforward project. It's nice to be able to build everything just from what's in the junkbox. -- The BBC: Licensed at public expense to spread lies. |
On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge
posted this: Can I infer from this experience that SMD inductors of over a few uH are a waste of time? ANY part used to perform a function which it is not suited for is a waste of time. It would be interesting to know which SMD inductors you used that seemed to eat up the 40 MHz so effectively. I'd be willing to bet that a quick look at the spec sheet would show why that particular part was a bad choice for that particular job. Jim |
On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge
posted this: Can I infer from this experience that SMD inductors of over a few uH are a waste of time? ANY part used to perform a function which it is not suited for is a waste of time. It would be interesting to know which SMD inductors you used that seemed to eat up the 40 MHz so effectively. I'd be willing to bet that a quick look at the spec sheet would show why that particular part was a bad choice for that particular job. Jim |
On Sun, 21 Mar 2004 19:17:38 GMT, James Meyer
wrote: On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge posted this: Can I infer from this experience that SMD inductors of over a few uH are a waste of time? ANY part used to perform a function which it is not suited for is a waste of time. It would be interesting to know which SMD inductors you used that seemed to eat up the 40 MHz so effectively. Eh? I've *never* used SMD inductors! -- The BBC: Licensed at public expense to spread lies. |
On Sun, 21 Mar 2004 19:17:38 GMT, James Meyer
wrote: On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge posted this: Can I infer from this experience that SMD inductors of over a few uH are a waste of time? ANY part used to perform a function which it is not suited for is a waste of time. It would be interesting to know which SMD inductors you used that seemed to eat up the 40 MHz so effectively. Eh? I've *never* used SMD inductors! -- The BBC: Licensed at public expense to spread lies. |
Paul,
I probably should have included more details. A fellow I was working with was trying to extract a clock signal from a synchronous data stream, and was getting nowhere. We swept the frequency back and forth to be sure he was tuned to resonance - he was. Changed the ceramic cap to mica, and everything worked like a charm. We never analyzed why the ceramic did not work, but I suspect it was because of the capacitance vs. applied voltage dependence. If the cap had 6VDC on it, and he had a few mv of RF, I expect it would have worked. Instead, he had 0 bias, and a couple of Volts p-p signal. BTW, somebody mentioned powdered iron toroids. Sounds like a good idea. Tam |
Paul,
I probably should have included more details. A fellow I was working with was trying to extract a clock signal from a synchronous data stream, and was getting nowhere. We swept the frequency back and forth to be sure he was tuned to resonance - he was. Changed the ceramic cap to mica, and everything worked like a charm. We never analyzed why the ceramic did not work, but I suspect it was because of the capacitance vs. applied voltage dependence. If the cap had 6VDC on it, and he had a few mv of RF, I expect it would have worked. Instead, he had 0 bias, and a couple of Volts p-p signal. BTW, somebody mentioned powdered iron toroids. Sounds like a good idea. Tam |
On Sun, 21 Mar 2004 22:43:17 +0000, Paul Burridge
posted this: On Sun, 21 Mar 2004 19:17:38 GMT, James Meyer wrote: On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge posted this: Can I infer from this experience that SMD inductors of over a few uH are a waste of time? ANY part used to perform a function which it is not suited for is a waste of time. It would be interesting to know which SMD inductors you used that seemed to eat up the 40 MHz so effectively. Eh? I've *never* used SMD inductors! Sorry. When you said "this experience" I thought you meant "my experience". Carry on. Jim |
On Sun, 21 Mar 2004 22:43:17 +0000, Paul Burridge
posted this: On Sun, 21 Mar 2004 19:17:38 GMT, James Meyer wrote: On Fri, 19 Mar 2004 19:40:01 +0000, Paul Burridge posted this: Can I infer from this experience that SMD inductors of over a few uH are a waste of time? ANY part used to perform a function which it is not suited for is a waste of time. It would be interesting to know which SMD inductors you used that seemed to eat up the 40 MHz so effectively. Eh? I've *never* used SMD inductors! Sorry. When you said "this experience" I thought you meant "my experience". Carry on. Jim |
On Sun, 21 Mar 2004 20:58:07 -0500, "Tam/WB2TT"
wrote: Paul, I probably should have included more details. A fellow I was working with was trying to extract a clock signal from a synchronous data stream, and was getting nowhere. We swept the frequency back and forth to be sure he was tuned to resonance - he was. Changed the ceramic cap to mica, and everything worked like a charm. We never analyzed why the ceramic did not work, but I suspect it was because of the capacitance vs. applied voltage dependence. If the cap had 6VDC on it, and he had a few mv of RF, I expect it would have worked. Instead, he had 0 bias, and a couple of Volts p-p signal. Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the black tipped ones, either. You can't beat silver mica but they're a bit hard to find and expensive. BTW, somebody mentioned powdered iron toroids. Sounds like a good idea. Indeed. I'm looking into it. -- The BBC: Licensed at public expense to spread lies. |
On Sun, 21 Mar 2004 20:58:07 -0500, "Tam/WB2TT"
wrote: Paul, I probably should have included more details. A fellow I was working with was trying to extract a clock signal from a synchronous data stream, and was getting nowhere. We swept the frequency back and forth to be sure he was tuned to resonance - he was. Changed the ceramic cap to mica, and everything worked like a charm. We never analyzed why the ceramic did not work, but I suspect it was because of the capacitance vs. applied voltage dependence. If the cap had 6VDC on it, and he had a few mv of RF, I expect it would have worked. Instead, he had 0 bias, and a couple of Volts p-p signal. Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the black tipped ones, either. You can't beat silver mica but they're a bit hard to find and expensive. BTW, somebody mentioned powdered iron toroids. Sounds like a good idea. Indeed. I'm looking into it. -- The BBC: Licensed at public expense to spread lies. |
I tried to "reply", but your inbox was full!
....[snip].... Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge. Hey Doc. Should that quote be attributed to you or what? Yes, at least mostly, 'Way back when email first started (with long strings of !...!...! in the addresses), I had "Three boxes..." (altho I don't recall just which three). Shortly thereafter, someone suggested a fourth box and someone else the fifth, and I was lucky enough to be able to condense all five boxes into the one line I've used ever since. --Myron. -- Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
I tried to "reply", but your inbox was full!
....[snip].... Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge. Hey Doc. Should that quote be attributed to you or what? Yes, at least mostly, 'Way back when email first started (with long strings of !...!...! in the addresses), I had "Three boxes..." (altho I don't recall just which three). Shortly thereafter, someone suggested a fourth box and someone else the fifth, and I was lucky enough to be able to condense all five boxes into the one line I've used ever since. --Myron. -- Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
On 22 Mar 2004 13:06:44 -0600, mcalhoun wrote:
I tried to "reply", but your inbox was full! Uh, it used to be ".invalid" but until I gat this blasted new newsreader to allow me to use that, I dunno. Maybe example.org. Maybe a sneakemail addy I can throw away when it gets spammed to hell so at least some people can backchannel me for the real addy if I think they need it. ....[snip].... Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge. Hey Doc. Should that quote be attributed to you or what? Yes, at least mostly, 'Way back when email first started (with long strings of !...!...! in the addresses), I had "Three boxes..." (altho I don't recall just which three). Shortly thereafter, someone suggested a fourth box and someone else the fifth, and I was lucky enough to be able to condense all five boxes into the one line I've used ever since. --Myron. Ok. Duly noted. Maybe Myron Calhoun, et. al. ? -- Best Regards, Mike |
On 22 Mar 2004 13:06:44 -0600, mcalhoun wrote:
I tried to "reply", but your inbox was full! Uh, it used to be ".invalid" but until I gat this blasted new newsreader to allow me to use that, I dunno. Maybe example.org. Maybe a sneakemail addy I can throw away when it gets spammed to hell so at least some people can backchannel me for the real addy if I think they need it. ....[snip].... Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge. Hey Doc. Should that quote be attributed to you or what? Yes, at least mostly, 'Way back when email first started (with long strings of !...!...! in the addresses), I had "Three boxes..." (altho I don't recall just which three). Shortly thereafter, someone suggested a fourth box and someone else the fifth, and I was lucky enough to be able to condense all five boxes into the one line I've used ever since. --Myron. Ok. Duly noted. Maybe Myron Calhoun, et. al. ? -- Best Regards, Mike |
Paul Burridge wrote:
Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the black tipped ones, either. You can't beat silver mica but they're a bit hard to find and expensive. I strongly disagree with this. I've successfully used ceramic capacitors many times for both high and low Q tuned circuits from HF to UHF. Years ago, I found that NPO ceramics were decidedly superior to silver micas for temperature stability, so I use them exclusively for VFO tank circuits. You might have come across some bad parts. Or perhaps you don't realize that many different types of ceramic are used for making capacitors. Three general classes are most common. The "general purpose" class (something of a misnomer) is a very high-k ceramic used for capacitors of Z5U, Y5V and similar types. Those are very good for bypassing because of their small physical size, but terrible for nearly anything else. They're microphonic, hygroscopic, piezoelectric, and highly temperature and voltage dependent. Another class is used for X7R and related types, sometimes called "stable". These are much more stable in all respects, but are physically larger due to the lower dielectric constant (k) of the ceramic. They're suitable for a wider variety of uses, but still not for high Q tuned circuits. They should be used in low Q circuits only after evaluating the potential effects of temperature and voltage dependence, at the least. The third common class is used for making capacitors with near-zero temperature coefficients, such as C0G (formerly and still often called NPO). These *are* an excellent choice for tuned circuit applications, and are often better than silver mica (whose temperature coefficient and Q are variable and unpredictable). They're sometimes identified with a black dot -- on "dog bone" capacitors, it's on one end. Many years ago I published an article describing a stable, VFO controlled QRP transceiver which used NPO ceramic capacitors for the oscillator tank. Scattered feedback indicated that some people had gotten "NPO" capacitors from less-than-reliable vendors and were experiencing an objectionable amount of frequency drift. That didn't happen with name-brand parts. So there are some flakey capacitors out there. If you need to depend on the quality, get name brand parts from a reputable vendor. And you'll find that NPO ceramics are a very good choice for tuned circuits. Roy Lewallen, W7EL |
Paul Burridge wrote:
Yes, ceramics are *hopeless* for tuned circuits; I wouldn't trust the black tipped ones, either. You can't beat silver mica but they're a bit hard to find and expensive. I strongly disagree with this. I've successfully used ceramic capacitors many times for both high and low Q tuned circuits from HF to UHF. Years ago, I found that NPO ceramics were decidedly superior to silver micas for temperature stability, so I use them exclusively for VFO tank circuits. You might have come across some bad parts. Or perhaps you don't realize that many different types of ceramic are used for making capacitors. Three general classes are most common. The "general purpose" class (something of a misnomer) is a very high-k ceramic used for capacitors of Z5U, Y5V and similar types. Those are very good for bypassing because of their small physical size, but terrible for nearly anything else. They're microphonic, hygroscopic, piezoelectric, and highly temperature and voltage dependent. Another class is used for X7R and related types, sometimes called "stable". These are much more stable in all respects, but are physically larger due to the lower dielectric constant (k) of the ceramic. They're suitable for a wider variety of uses, but still not for high Q tuned circuits. They should be used in low Q circuits only after evaluating the potential effects of temperature and voltage dependence, at the least. The third common class is used for making capacitors with near-zero temperature coefficients, such as C0G (formerly and still often called NPO). These *are* an excellent choice for tuned circuit applications, and are often better than silver mica (whose temperature coefficient and Q are variable and unpredictable). They're sometimes identified with a black dot -- on "dog bone" capacitors, it's on one end. Many years ago I published an article describing a stable, VFO controlled QRP transceiver which used NPO ceramic capacitors for the oscillator tank. Scattered feedback indicated that some people had gotten "NPO" capacitors from less-than-reliable vendors and were experiencing an objectionable amount of frequency drift. That didn't happen with name-brand parts. So there are some flakey capacitors out there. If you need to depend on the quality, get name brand parts from a reputable vendor. And you'll find that NPO ceramics are a very good choice for tuned circuits. Roy Lewallen, W7EL |
Roy,
We were talking about bypass type ceramics. See the 3/20 10:13AM posting. BTW, I think with SM you are pretty much forced into using ceramics. Tam/WB2TT |
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