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"Jock Cooper" wrote in message ... Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. [tale of woe deleted] I'd start again with a simple Hartley oscillator using a tapped toroid (you don't need a choke). Get the basic oscillator working (a scope helps) then add a buffer stage. Forget about getting the oscillator and front end tuning to track properly at this stage, use separate capacitors. When you get the thing working properly you can sort out the tracking. 73, Leon -- Leon Heller, G1HSM http://www.geocities.com/leon_heller |
"Jock Cooper" wrote in message ... Hi everyone, I've been interested in radios and how they work since I was a kid -- I had a crystal radio kit and a RF-2200. Recently I started building radios and have been having a blast. Please keep in mind that I am very newbie. [tale of woe deleted] I'd start again with a simple Hartley oscillator using a tapped toroid (you don't need a choke). Get the basic oscillator working (a scope helps) then add a buffer stage. Forget about getting the oscillator and front end tuning to track properly at this stage, use separate capacitors. When you get the thing working properly you can sort out the tracking. 73, Leon -- Leon Heller, G1HSM http://www.geocities.com/leon_heller |
Hi Jock,
there is nothing you should apologize for. My recommendations: 1/ Do not use breadboards for RF. They are perfect for AF, good for digital citcuits, but RF circuits sometimes work, more often not. And keep the groundplane as big as possible. The "ugly", "rats-nest", "dead bug" or "manhattan" style really seem to be far the best for RF prototyping. Look at http://w1.859.telia.com/~u85920178/blocks/deadbug_0.htm for more info. 2/ Get an oscilloscope covering at least three times your frequency range. An add-on unit for your PC can turn it into an oscillscope and spectrum analyzer for a fair price. 3/ Use the necessary tuning capacitors and their configuration to keep the oscillator and input frequency near to each other at more than one point. The configuration and formulae can be found in many books. 4/ Try the oscillator from http://w1.859.telia.com/~u85920178/blocks/osc7m00.htm ,it should work reliably in a wide frequency band. BR from Ivan OK1SIP |
Hi Jock,
there is nothing you should apologize for. My recommendations: 1/ Do not use breadboards for RF. They are perfect for AF, good for digital citcuits, but RF circuits sometimes work, more often not. And keep the groundplane as big as possible. The "ugly", "rats-nest", "dead bug" or "manhattan" style really seem to be far the best for RF prototyping. Look at http://w1.859.telia.com/~u85920178/blocks/deadbug_0.htm for more info. 2/ Get an oscilloscope covering at least three times your frequency range. An add-on unit for your PC can turn it into an oscillscope and spectrum analyzer for a fair price. 3/ Use the necessary tuning capacitors and their configuration to keep the oscillator and input frequency near to each other at more than one point. The configuration and formulae can be found in many books. 4/ Try the oscillator from http://w1.859.telia.com/~u85920178/blocks/osc7m00.htm ,it should work reliably in a wide frequency band. BR from Ivan OK1SIP |
Jock Cooper writes:
Hi everyone, [snip] Thanks to everyone for the great and helpful suggestions. This sure is a friendly group. Since I just recently started all this tinkering, my junkbox is a little low in some areas, notably NP0 capacitors.. I have a few in values from 3.9 up to 47 and a few 100s but not else. So I've used various other types when higher values are called for.. I'm planning on picking up an assortment soon. Also I have some copper clad non drilled pc boards, I'm going to try to rebuild the oscillator (and radio) using the 'ugly' method; although I'm a little afraid I'll make myself a tangled mess of useless and stuck together parts. The main reason I use breadboards is so that I can take parts in and out of the circuit quickly (ie I'm at the stage where I'm curious about what happens when I take such and such part out, or change its value etc). I'd love to get a oscilloscope but my 'play' budget is mostly spent for awhile. So I've been making due with an LCR and a little cheapo DMM. I did order the book _Experimental Methods in RF Design_ .. I've also got a lot of good info from an old (well 1998) ARRL handbook I got on Ebay. Jock |
Jock Cooper writes:
Hi everyone, [snip] Thanks to everyone for the great and helpful suggestions. This sure is a friendly group. Since I just recently started all this tinkering, my junkbox is a little low in some areas, notably NP0 capacitors.. I have a few in values from 3.9 up to 47 and a few 100s but not else. So I've used various other types when higher values are called for.. I'm planning on picking up an assortment soon. Also I have some copper clad non drilled pc boards, I'm going to try to rebuild the oscillator (and radio) using the 'ugly' method; although I'm a little afraid I'll make myself a tangled mess of useless and stuck together parts. The main reason I use breadboards is so that I can take parts in and out of the circuit quickly (ie I'm at the stage where I'm curious about what happens when I take such and such part out, or change its value etc). I'd love to get a oscilloscope but my 'play' budget is mostly spent for awhile. So I've been making due with an LCR and a little cheapo DMM. I did order the book _Experimental Methods in RF Design_ .. I've also got a lot of good info from an old (well 1998) ARRL handbook I got on Ebay. Jock |
You can series and parallel any number of capacitors to get the right
value. A given oscillator type works best within some range of reactance values for L and C. If you go too far outside the ideal range, stability will suffer, noise might increase, and if too extreme, it might not oscillate. The ideal range depends on the oscillator design. That being said, you can often vary L and C quite a great deal and still get adequate operation. Real inductors are, in general, a lot more different from their theoretical ideal counterparts than capacitors are. Real inductors have considerable loss and stray capacitance, as well as being temperature sensitive. So you'll often find that a circuit works quite a bit differently if you substitute an inductor for one with different physical construction. For a VFO tank, I use only toroids wound on type 6 powdered iron material. (As I mentioned earlier, type 7 is probably as good, but I've never used it.) For VFO tank capacitors, I use only NPO ceramic. The combination produces low drift, dominated by the inductor. If desired, the drift can be compensated by replacing part of the C with negative temperature coefficient parts. Most other types of L and C have substantially greater temperature sensitivity. You might get lucky and have poor parts drift in opposite directions and more-or-less cancel, but that's poor practice and hard to duplicate. Better to design for minimal inherent drift, then compensate what's left if necessary. Toroids have the big advantage of producing a relatively small external field. That makes them much less sensitive to mounting, the influence of nearby objects, and microphonics. Jock Cooper wrote: . . . I wasn't sure if this was even OK to do (if it is too lossy or someting), I figured I could experiment around with the result and find out :) But maybe now I can just ask.. how much mucking around can I do by adding in parallel and serial caps to tweak the variable cap range? Also, Can I just find any old values of L and C that when plugged in would give me the F I'm looking for? Or are there only certain values of L and C that are appropriate for certain ranges of Freq. . . . Roy Lewallen, W7EL |
You can series and parallel any number of capacitors to get the right
value. A given oscillator type works best within some range of reactance values for L and C. If you go too far outside the ideal range, stability will suffer, noise might increase, and if too extreme, it might not oscillate. The ideal range depends on the oscillator design. That being said, you can often vary L and C quite a great deal and still get adequate operation. Real inductors are, in general, a lot more different from their theoretical ideal counterparts than capacitors are. Real inductors have considerable loss and stray capacitance, as well as being temperature sensitive. So you'll often find that a circuit works quite a bit differently if you substitute an inductor for one with different physical construction. For a VFO tank, I use only toroids wound on type 6 powdered iron material. (As I mentioned earlier, type 7 is probably as good, but I've never used it.) For VFO tank capacitors, I use only NPO ceramic. The combination produces low drift, dominated by the inductor. If desired, the drift can be compensated by replacing part of the C with negative temperature coefficient parts. Most other types of L and C have substantially greater temperature sensitivity. You might get lucky and have poor parts drift in opposite directions and more-or-less cancel, but that's poor practice and hard to duplicate. Better to design for minimal inherent drift, then compensate what's left if necessary. Toroids have the big advantage of producing a relatively small external field. That makes them much less sensitive to mounting, the influence of nearby objects, and microphonics. Jock Cooper wrote: . . . I wasn't sure if this was even OK to do (if it is too lossy or someting), I figured I could experiment around with the result and find out :) But maybe now I can just ask.. how much mucking around can I do by adding in parallel and serial caps to tweak the variable cap range? Also, Can I just find any old values of L and C that when plugged in would give me the F I'm looking for? Or are there only certain values of L and C that are appropriate for certain ranges of Freq. . . . Roy Lewallen, W7EL |
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. |
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. |
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