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#1
March 13th 04, 07:00 PM
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In article , Paul Burridge
writes: On 13 Mar 2004 07:33:15 -0800, (Tim Shoppa) wrote: Fifth harmonic frequency multipliers do exist, but it's usually much easier to double and triple your way to the final frequency if possible. (You just discovered this, I think!) Yeah, but trying to get the 5th is hardly asking for the moon... In a way, it IS. Fifth harmonic of an infinitely sharp transition rectangular waveform is still low in energy compared to the fundamental. Chances are that a single stage using an active device as a quintupler will NOT be successful, transistor or tube (valve). Let's get to some specifics on this problem - 1. Let us know what you are using to determine whether or not a 5th harmonic exists. The lack of indication may be due to whatever it is (not a spectrum analyzer) being used. 2. Describe the multiplier stage in more detail and include an approximate level and impedance/admittance of the RF source. That would include supply rails and biasing. 3. Describe whatever is being used to select the 5th harmonic and inhibit the fundamental and other harmonics. There's lots of energy at many different frequencies floating around there and you only want one frequency. 4. Review again with us the output drive level requirements so we can get a handle on that. 5. If you are using an oscilloscope to measure the fundamental waveform, estimate the actual risetime/falltime based on the rise/fall times limits of the oscilloscope. That yields some basic data that can be applied to a Fourier series to determine the level of 5th harmonic energy you have to work with. [that will also reveal the approximate frequency limits of the scope] The lack of even harmonics is typical of push-pull stages ... if you are messing around with CMOS gates, you might try using a TTL gate (which pulls low much stronger than it pulls high) or an open collector TTL gate, both with smmallish (100-200 ohm) pull-up resistors for doubling. I've a reasonably fast Schmitt I'm going to stick in there in place of the 74HC04 before I resort to anything fancy (same pin-out). Why not do a x3 followed by a x2 to get 17.2 MHz out of 2.866 MHz? Because I don't have a rock lying about for that fundamental! Hint: A mixed 2x and 3x = 5x out if filtered to pass that. No lying- around rock needed. [not an optimum solution] Note: As already pointed out, a single PLL IC can do the job in the same PCB footprint. When frustration hits a peak, it's time to sit back away from the problem and do an objective review of what is the overall task and what you've accomplished so far and what you know about certain circuits fundamentals. General problem descriptions only result in general solutions without quantitative values needed for a specific application. I've found that NON-linear circuits (a multiplier stage is definietly in the non-linear category) take rather more development time than a linear circuit. There's lots of different things going on in a multiplier circuit and those have to be considered for the whole. Hopefully some supreme being here will spot a problem with the traces I've now posted... Try as we might in the depths of our frustrations, supreme beings tend not to intervene in us humans' petty affairs. :-) Len Anderson retired (from regular hours) electronic engineer person |
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#2
March 14th 04, 12:35 AM
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#3
March 14th 04, 12:35 AM
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