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#21
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I usually cut PC board material with tin snips. A hacksaw also works,
but it makes a lot of irritating dust. In the schematic you mention, the "a" and "b" capacitors form a voltage divider to control the amount of signal that's fed back to the active device. If there's too much (a/b ratio too large), the waveform will be distorted and the temperature drift will probably increase. If there's too little, the oscillator will be slow to start or won't start reliably or at all. I don't want to answer your question about calculating the oscillation frequency without some detailed thought, which I don't have time for just now. But I believe you're doing the right calculation. Note that the oscillator output is in parallel with the "b" capacitor, so any capacitive loading on the oscillator will lower the frequency. The RFC and your inductor will both present some stray C. It's nearly impossible to tell how much C your inductor will contribute. There are a couple of ways to measure it, but if you've got the other capacitances under control, you've already made yourself a test fixture for determining inductor capacitance (assuming you know its inductance) -- it's the oscillator. Roy Lewallen, W7EL Jock Cooper wrote: Roy Lewallen writes: When you get _Experimental Methods. . _, look at the very first few pages -- "Getting Started". You'll find a nice drawing and some other information about "ugly construction". Incidentally, the method was first described and the term coined in the article "The Ugly Weekender" by Roger and Wes Hayward, in August 1981 QST. If you can get hold of a copy of this article, it has more about the method, as well as some other good information. If and when you do eventually decide to get a scope -- I've been surprised to see Tek 465 scopes going on eBay for about $200. That's an awful lot of bang for the buck. Roy Lewallen, W7EL Well last night I took some double sided copper clab board and cut off a small square. (By the way, what is the best way to cut this stuff?) I rebuilt the oscillator on there in 'ugly' style. And it was in fact quite ugly. It looked like something I would have done as a kid. But it worked! Now I'm sold on it. Hopefully I can get some pointers on improving my technique from the book. Now I have a new question though. The circuit is a Clapp based on the MPF102 JFET. When I compute the frequency range, it is close to the measured range but not the same; but there appears to be some capacitance that I can't account for, and its value seems higher than would be 'stray'. The circuit I'm using is basically like the one at http://www.electronics-tutorials.com...scillators.htm in figure 2, but without the extra parallel stuff on the variable cap. So I calculate the cap value by adding the var cap value in series with the caps labelled on that website as cfb-a and cfb-b. (By the way, what happens if those two aren't the same value?) Do any of the other capacitors in the circuit factor in? How much capacitance could my 'pill bottle inductor' be contributing? I was careful, but not extra careful when winding it. |
#22
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I usually cut PC board material with tin snips. A hacksaw also works,
but it makes a lot of irritating dust. In the schematic you mention, the "a" and "b" capacitors form a voltage divider to control the amount of signal that's fed back to the active device. If there's too much (a/b ratio too large), the waveform will be distorted and the temperature drift will probably increase. If there's too little, the oscillator will be slow to start or won't start reliably or at all. I don't want to answer your question about calculating the oscillation frequency without some detailed thought, which I don't have time for just now. But I believe you're doing the right calculation. Note that the oscillator output is in parallel with the "b" capacitor, so any capacitive loading on the oscillator will lower the frequency. The RFC and your inductor will both present some stray C. It's nearly impossible to tell how much C your inductor will contribute. There are a couple of ways to measure it, but if you've got the other capacitances under control, you've already made yourself a test fixture for determining inductor capacitance (assuming you know its inductance) -- it's the oscillator. Roy Lewallen, W7EL Jock Cooper wrote: Roy Lewallen writes: When you get _Experimental Methods. . _, look at the very first few pages -- "Getting Started". You'll find a nice drawing and some other information about "ugly construction". Incidentally, the method was first described and the term coined in the article "The Ugly Weekender" by Roger and Wes Hayward, in August 1981 QST. If you can get hold of a copy of this article, it has more about the method, as well as some other good information. If and when you do eventually decide to get a scope -- I've been surprised to see Tek 465 scopes going on eBay for about $200. That's an awful lot of bang for the buck. Roy Lewallen, W7EL Well last night I took some double sided copper clab board and cut off a small square. (By the way, what is the best way to cut this stuff?) I rebuilt the oscillator on there in 'ugly' style. And it was in fact quite ugly. It looked like something I would have done as a kid. But it worked! Now I'm sold on it. Hopefully I can get some pointers on improving my technique from the book. Now I have a new question though. The circuit is a Clapp based on the MPF102 JFET. When I compute the frequency range, it is close to the measured range but not the same; but there appears to be some capacitance that I can't account for, and its value seems higher than would be 'stray'. The circuit I'm using is basically like the one at http://www.electronics-tutorials.com...scillators.htm in figure 2, but without the extra parallel stuff on the variable cap. So I calculate the cap value by adding the var cap value in series with the caps labelled on that website as cfb-a and cfb-b. (By the way, what happens if those two aren't the same value?) Do any of the other capacitors in the circuit factor in? How much capacitance could my 'pill bottle inductor' be contributing? I was careful, but not extra careful when winding it. |
#23
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Jock Cooper wrote: I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Oh, that's easy. You're *trying* to build an oscillator; give up on that goal and choose an easier one: try to build an amplifier. I mean, really *want* to build a stable amplifier. Voila! An oscillator!! Dana K6JQ |
#24
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Jock Cooper wrote: I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Oh, that's easy. You're *trying* to build an oscillator; give up on that goal and choose an easier one: try to build an amplifier. I mean, really *want* to build a stable amplifier. Voila! An oscillator!! Dana K6JQ |
#25
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I tried numerous circuits and numerous types (Hartley, Colpitts etc) I
had found but I couldn't get any of them to start up. Oh, that's easy. You're *trying* to build an oscillator; give up on that goal and choose an easier one: try to build an amplifier. I mean, really *want* to build a stable amplifier. Voila! An oscillator!! I once built a power supply that oscillated above 150 MHz. Unfortunately, I could never drop it into the 2-meter band. --Myron. -- Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
#26
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I tried numerous circuits and numerous types (Hartley, Colpitts etc) I
had found but I couldn't get any of them to start up. Oh, that's easy. You're *trying* to build an oscillator; give up on that goal and choose an easier one: try to build an amplifier. I mean, really *want* to build a stable amplifier. Voila! An oscillator!! I once built a power supply that oscillated above 150 MHz. Unfortunately, I could never drop it into the 2-meter band. --Myron. -- Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |
#27
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mcalhoun wrote: I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Oh, that's easy. You're *trying* to build an oscillator; give up on that goal and choose an easier one: try to build an amplifier. I mean, really *want* to build a stable amplifier. Voila! An oscillator!! I once built a power supply that oscillated above 150 MHz. Unfortunately, I could never drop it into the 2-meter band. Heh. Of course a DC power supply is indistinguishable for a 0Hz oscillator under the best of circumstances, right? But, 150MHz. That's pretty special. What were you using for a pass element(s) ? Dana |
#28
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mcalhoun wrote: I tried numerous circuits and numerous types (Hartley, Colpitts etc) I had found but I couldn't get any of them to start up. Oh, that's easy. You're *trying* to build an oscillator; give up on that goal and choose an easier one: try to build an amplifier. I mean, really *want* to build a stable amplifier. Voila! An oscillator!! I once built a power supply that oscillated above 150 MHz. Unfortunately, I could never drop it into the 2-meter band. Heh. Of course a DC power supply is indistinguishable for a 0Hz oscillator under the best of circumstances, right? But, 150MHz. That's pretty special. What were you using for a pass element(s) ? Dana |
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