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On Sat, 13 Mar 2004 11:50:13 +0000, Paul Burridge
wrote: On Sat, 13 Mar 2004 10:00:52 +1000, Tony wrote: The 5th harmonic should be only 14dB below the fundamental, although it will drop fairly quickly as the sides of the input square wave deviate from vertical. Does the 3.44MHz have a 50% duty cycle? Not quite, no. Why would that make any difference? As the duty cycle deviates from 50%, the even harmonics start to appear, so you need a better filter to keep them out. John |
In article ,
John Larkin wrote: [....] As the duty cycle deviates from 50%, the even harmonics start to appear, so you need a better filter to keep them out. Also as you get nearer the 20-80 duty cycle the amplitude of the 5th harmonic decreases. -- -- forging knowledge |
In article ,
John Larkin wrote: [....] As the duty cycle deviates from 50%, the even harmonics start to appear, so you need a better filter to keep them out. Also as you get nearer the 20-80 duty cycle the amplitude of the 5th harmonic decreases. -- -- forging knowledge |
On Sat, 13 Mar 2004 10:00:52 +1000, Tony wrote:
The 5th harmonic should be only 14dB below the fundamental, although it will drop fairly quickly as the sides of the input square wave deviate from vertical. Does the 3.44MHz have a 50% duty cycle? Okay, I've now tweaked the osc. to get as near to 50% as possible. Alas, still no sign of any 5th present in the multiplier's output. Here's a shot of the (fundamental) output from the inverters. I can't see any real problem with why it shouldn't be good for a reasonable comb of harmonics, but our experts may know better. BTW, settings were 2V/div. and 0.1uS/div. http://www.burridge8333.fsbusiness.co.uk/trace.gif This other trace was snapped at the base of the transistor stage that does the multiplying. All there is (circuitry-wise) between this trace and the last one is a 330 ohm series resistor and a 47pF cap. I'd have expected to see some clamping action due to the b/e junction, but the waveshape seems very odd - but does concur with the Spice simulation. Is there anything amiss, here? (Setting here is 0.5V/div) http://www.burridge8333.fsbusiness.co.uk/trace2.gif Still not a sniff of a fifth! :-( Currently building Reg's 17.2Mhz BPF to see if that can expose it. -- The BBC: Licensed at public expense to spread lies. |
On Sat, 13 Mar 2004 10:00:52 +1000, Tony wrote:
The 5th harmonic should be only 14dB below the fundamental, although it will drop fairly quickly as the sides of the input square wave deviate from vertical. Does the 3.44MHz have a 50% duty cycle? Okay, I've now tweaked the osc. to get as near to 50% as possible. Alas, still no sign of any 5th present in the multiplier's output. Here's a shot of the (fundamental) output from the inverters. I can't see any real problem with why it shouldn't be good for a reasonable comb of harmonics, but our experts may know better. BTW, settings were 2V/div. and 0.1uS/div. http://www.burridge8333.fsbusiness.co.uk/trace.gif This other trace was snapped at the base of the transistor stage that does the multiplying. All there is (circuitry-wise) between this trace and the last one is a 330 ohm series resistor and a 47pF cap. I'd have expected to see some clamping action due to the b/e junction, but the waveshape seems very odd - but does concur with the Spice simulation. Is there anything amiss, here? (Setting here is 0.5V/div) http://www.burridge8333.fsbusiness.co.uk/trace2.gif Still not a sniff of a fifth! :-( Currently building Reg's 17.2Mhz BPF to see if that can expose it. -- The BBC: Licensed at public expense to spread lies. |
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John Larkin ) writes:
On Fri, 12 Mar 2004 15:32:23 +0000, Ian Bell wrote: In RF circles, the 'normal' way to do this would be a simple Class C amplifier with a collector load tuned to the fifth harmonic. In calls C, conduction only occurs for a small fraction of a cycle which produces a correspondingly higher proportion of higher harmonics than a square wave. But if you want to filter the 5th, it's mighty handy not to have nuch 4th or 6th around. But one of the problems Paul seems to be having, on an ongoing basis, is the lack of a historical perspective. He's going the digital route because that's where he comes from, and therefore he extends the idea. But anyone of us who have been around for a while, or even has just spent time with older books and magazines, knows that RF multipliers was done all the time before digital circuits had made much inroad. WItness the thread about VXOs a while back. He started with a digital oscillator, again because that's all he knows, and then sets out to pull it as much as possible. But this too has been done before, and doing some research in older ham magazines would have shown the problems and solutions. Maybe "oldtimers" are biased towards what they grew up with. But I see time after time in the sci.electronics.* hierarchy people wanting to do RF and see it as simply extending audio or digital concepts they already know. So they suddenly need a 5MHz oscillator, and wonder why they aren't having success with a function generator type IC, where the needed frequency is at it's upper limit. "How can I make an active filter at 10MHz" is not usually someone who has a specific need for an active filter at that frequency, but from someone who is used to active filters and has yet to see their limitations, or maybe more importantly, are unaware of what's normally done at RF. A coil is pretty bulky at 1KHz, but at 10MHz it's downright easy to wind. And I do find it interesting, to see fairly complicated answers for problems that were solved a long time ago, simply by coming from a different angle. Michael VE2BVW |
John Larkin ) writes:
On Fri, 12 Mar 2004 15:32:23 +0000, Ian Bell wrote: In RF circles, the 'normal' way to do this would be a simple Class C amplifier with a collector load tuned to the fifth harmonic. In calls C, conduction only occurs for a small fraction of a cycle which produces a correspondingly higher proportion of higher harmonics than a square wave. But if you want to filter the 5th, it's mighty handy not to have nuch 4th or 6th around. But one of the problems Paul seems to be having, on an ongoing basis, is the lack of a historical perspective. He's going the digital route because that's where he comes from, and therefore he extends the idea. But anyone of us who have been around for a while, or even has just spent time with older books and magazines, knows that RF multipliers was done all the time before digital circuits had made much inroad. WItness the thread about VXOs a while back. He started with a digital oscillator, again because that's all he knows, and then sets out to pull it as much as possible. But this too has been done before, and doing some research in older ham magazines would have shown the problems and solutions. Maybe "oldtimers" are biased towards what they grew up with. But I see time after time in the sci.electronics.* hierarchy people wanting to do RF and see it as simply extending audio or digital concepts they already know. So they suddenly need a 5MHz oscillator, and wonder why they aren't having success with a function generator type IC, where the needed frequency is at it's upper limit. "How can I make an active filter at 10MHz" is not usually someone who has a specific need for an active filter at that frequency, but from someone who is used to active filters and has yet to see their limitations, or maybe more importantly, are unaware of what's normally done at RF. A coil is pretty bulky at 1KHz, but at 10MHz it's downright easy to wind. And I do find it interesting, to see fairly complicated answers for problems that were solved a long time ago, simply by coming from a different angle. Michael VE2BVW |
budgie wrote in message . ..
On Fri, 12 Mar 2004 15:32:23 +0000, Ian Bell wrote: Paul Burridge wrote: In RF circles, the 'normal' way to do this would be a simple Class C amplifier with a collector load tuned to the fifth harmonic. In calls C, conduction only occurs for a small fraction of a cycle which produces a correspondingly higher proportion of higher harmonics than a square wave. I've been waiting for someone to post this. I would only add "The drive level, and the bais point, will vary the amount of fifth (or whichever) you will see." It's as common as noses in RF, as Ian pointed out. Just look at the average two-way radio prior to frequency synthesisers. Crystal freqs were multiplied this way in transmitter chains and for receive injection, although use of fifth wasn't especially common because you normally had enough design control to use the more efficient *2, *3 or *4. 'Tune for smoke' isn't an option for most new products, which have to be manufactured without hands. Better to pick a suitable duty cycle (or more likely a conduction time period in a digital circuit), that has an efficient 5th harmonic component, including delays, at low power levels. http://www.wenzel.com/pdffiles/choose.pdf RL |
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