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#11
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![]() "Alan Peake" wrote in message ... Hello all, I am trying to muliply a 72MHz crystal oscillator to 1296 MHz. My first approach was to just make a series of X2 or X3 transistor mulipliers to get the require X18 multiplication fact. However, I have found two pieces of equipment in the junk box which just use diode multipliers. One is an old Electrophone UHF CB radio which multiplies the crystal oscillator by 17 (I think, from memory)then uses helical resonators to filter the desired harmonic. The same approach is used in an old King aircraft transponder where the 138 MHz crystal is multiplied to 960 MHz with just a diode and uses the first two stages of an interdigital mixer to get rid of unwanted harmonics. So, the question is, which is the better approach? I just want a reasonably clean signal source to test a 1296 MHz down-converter. The diode approach seems simpler but is it likely to contain more spurious signals than a transistor multiplier chain? Alan VK2ADB Here's an interesting simple circuit to generate odd harmonics. "New Topology Multiplier Generates Odd Harmonics" http://www.wenzel.com/pdffiles1/pdfs/RFDesign2.pdf Mike |
#12
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![]() amdx wrote: Here's an interesting simple circuit to generate odd harmonics. "New Topology Multiplier Generates Odd Harmonics" http://www.wenzel.com/pdffiles1/pdfs/RFDesign2.pdf Mike Yeah, saw that. Trouble is that I need EVEN harmonics ![]() Anyway, there was some interesting info on snap diodes etc. Makes me think that the diode used in the gear I have, aren't SRDs as these seem to be mainly for microwave use. The other info I gleaned was that it's easier to get low phase noise with straight multiplication than it is with PLLs. Not that I have an urgent need for low phase noise - but it's interesting nonetheless. Alan |
#13
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In message , Alan Peake
writes amdx wrote: Here's an interesting simple circuit to generate odd harmonics. "New Topology Multiplier Generates Odd Harmonics" http://www.wenzel.com/pdffiles1/pdfs/RFDesign2.pdf Mike Yeah, saw that. Trouble is that I need EVEN harmonics ![]() If you want EVEN harmonics, then use the 'fullwave rectifier' circuit. This is essentially identical to what you use to rectify the mains, but it works at RF. One version is the simple bi-phase circuit, consisting of a ferrite input transformer (ratio 1 : 1+1) and two rectifying diodes. An alternative is the fullwave bridge version. I have used the former with great success to multiply by 2, 4 and even 6 but, of course, you get a lot less with the higher order harmonics. Also, the relative suppression of the even harmonics is less. Anyway, there was some interesting info on snap diodes etc. Makes me think that the diode used in the gear I have, aren't SRDs as these seem to be mainly for microwave use. I did get involved with a comb generator which used a single step recovery diode, driven at 6, 7 or 8MHz, to generate fairly equal amplitude harmonics (both odds and evens) to around 600MHz. The other info I gleaned was that it's easier to get low phase noise with straight multiplication than it is with PLLs. Not that I have an urgent need for low phase noise - but it's interesting nonetheless. Alan I didn't know that the "New Topology Multiplier Generates Odd Harmonics" circuit existed. It's every bit as simple as the fullwave rectifier circuit for generating even harmonic! -- Ian |
#14
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On Thu, 22 May 2008 22:49:01 +1000, Alan Peake
wrote: Yeah, saw that. Trouble is that I need EVEN harmonics ![]() A push-pull multiplier will suppress most odd harmonics. Paul OH3LWR |
#15
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On May 22, 5:49 am, Alan Peake wrote:
amdx wrote: Here's an interesting simple circuit to generate odd harmonics. "New Topology Multiplier Generates Odd Harmonics" http://www.wenzel.com/pdffiles1/pdfs/RFDesign2.pdf Mike Yeah, saw that. Trouble is that I need EVEN harmonics ![]() Anyway, there was some interesting info on snap diodes etc. Makes me think that the diode used in the gear I have, aren't SRDs as these seem to be mainly for microwave use. The other info I gleaned was that it's easier to get low phase noise with straight multiplication than it is with PLLs. Not that I have an urgent need for low phase noise - but it's interesting nonetheless. Alan Huh? I thought you wanted to multiply by 18. 18 = 3 * 3 * 2. If you were specific about what you want to do with the output, I missed it. If it's just low-level stuff (e.g., a signal to feed into the receiver input), you don't need much power and the multiplier can be very inefficient and still do what you want. As you note, phase noise will be better (possibly much better) than other ways of doing it, and it could be that the signal level will be a lot closer to what you want. You'll have to shield things pretty carefully to get down to microvolt levels from a PLL or other full-blown oscillator on 1.2GHz. There might already be enough 18th harmonic in the oscillator output to do what you need. I have some little very fast CMOS single gate chips that have square enough edges (under half a nanosecond rise time, unloaded) to generate quite a bit of harmonic content up that high. Though a PLL would work fine, you then need a way to set up the PLL chip, assuming you use one of the readily available programmable ones. I know for me, I could hack a decent analog multiplier with filters a lot quicker than I could a decent synthesizer. I had to do a x9 not long ago, and used a multiplication scheme. Also, I've seen small crystal oscillator modules from two different vendors that used a PLL to lock a 100MHz crystal to a lower frequency reference, and then used analog multiplication up to several times 100MHz--they are doing it for low phase noise. Cheers, Tom |
#16
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![]() K7ITM wrote: Yeah, saw that. Trouble is that I need EVEN harmonics ![]() Huh? I thought you wanted to multiply by 18. 18 = 3 * 3 * 2. Quite right. 18 is an even harmonic of course but I forgot about the 3*3*2 bit ![]() If you were specific about what you want to do with the output, I missed it. It's to test a 1296 down-converter basically but will try a CW transmitter later so a bit of grunt wouldn't go astray. ..... There might already be enough 18th harmonic in the oscillator output to do what you need. Could be. AT the moment I have a 2N5485 as the osc and a 2N918 as a doubler but the 2N918 runs out of puff well before 1296MHz. I have some BFR91s so I might put them into use. Cheers, Tom 73 de Alan |
#17
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On Fri, 23 May 2008, Alan Peake wrote:
K7ITM wrote: Yeah, saw that. Trouble is that I need EVEN harmonics ![]() Huh? I thought you wanted to multiply by 18. 18 = 3 * 3 * 2. Quite right. 18 is an even harmonic of course but I forgot about the 3*3*2 bit ![]() If you were specific about what you want to do with the output, I missed it. It's to test a 1296 down-converter basically but will try a CW transmitter later so a bit of grunt wouldn't go astray. But unless you need a signal to tune the converter to the right frequency, this is quite a bit simpler. So long as the converter is tuned to the right general frequency, merely something with good harmonics will do. The fact that they will be attenuated at the desired frequency is not a bad thing, since you won't want a strong signal for tuning the converter anyway. Your original post didn't give an end use, but it was easy to assume you needed a clean signal for a transmitter of local oscillator injection in a converter. You don't want unwanted signals there. But for a signal generator, having extra harmonics isn't really a problem (unless you can't be sure that your converter is tuned to the right frequency). The extra harmonics will just be ignored by the converter's tuned circuits. There was a time when a lot of cheap signal generators actually used harmonics for their upper ranges. Michael VE2BVW |
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