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#1
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Hi folks,
I would have to ask a clarification regard to the TCXO. They are found with output TTL, HCMOS and CLIPPED SINE WAVE. It is just with respect to this last type that I would want to have elucidations on when it is convenient to employ it, which the advantages and the disadvantages and which the extension of spectral harmonicas. In synthesis, from the plan point of view, which are the motivations that they make to incline towards a Clipped oscillator? Anticipated thanks. 73's de IK6GQC Rocco |
#2
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From: Mario Bros on Jul 17, 4:34 pm
Hi folks, I would have to ask a clarification regard to the TCXO. They are found with output TTL, HCMOS and CLIPPED SINE WAVE. It is just with respect to this last type that I would want to have elucidations on when it is convenient to employ it, which the advantages and the disadvantages and which the extension of spectral harmonicas. The TTL and HCMOS outputs will have (generally) square waves which are rich in odd harmonic content. Clipped sine waves have less overall harmonics...but the harmonic content depends on the amount of clipping (presumably off the peak of the sine). In synthesis, from the plan point of view, which are the motivations that they make to incline towards a Clipped oscillator? Depends entirely on the designer's choice. If this is a purchased TCXO unit then a clipped sinewave output would probably come through a buffer stage directly from the internal oscillator output...which resembles a clipped sine wave. For the purchased unit, having that output is a convenience to the customer. TTL and HCMOS outputs would be made from internal digital devices, again a customer convenience to allow easy interface to external TTL or HCMOS circuitry. bit bit |
#3
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The clipped sinewave is the simplest output for a TCXO, it's usually just a
capacitor connected between the emitter of the oscillator and the output pin. You have to be careful what you connect to it, it's possible to throw the temp compensation out of tolerance. Most CMOS PLL chips have a reference frequency input that handles it well. If you're rolling your own PLL from SSI or MSI chips, you probable want to build up a buffer to make the clipped sinewave into TTL or HCMOS levels, or pay extra for a TCXO with the needed circuit built in. On Sun, 17 Jul 2005 20:34:25 GMT, "Mario Bros" wrote: Hi folks, I would have to ask a clarification regard to the TCXO. They are found with output TTL, HCMOS and CLIPPED SINE WAVE. It is just with respect to this last type that I would want to have elucidations on when it is convenient to employ it, which the advantages and the disadvantages and which the extension of spectral harmonicas. In synthesis, from the plan point of view, which are the motivations that they make to incline towards a Clipped oscillator? Anticipated thanks. 73's de IK6GQC Rocco |
#4
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Thanks for the answers.
I have one TCXO 40Mhz 1Vp-p 10K//15pf Clipped Sine Wave. My idea is of: 1) to gain 40Mhz Sine Wave through buffer with resonant circuit come to an agreement for 2nd the IF (40.455) 2) to gain 20Mhz with a divisor x2 for the 16F877 3) ..and last.. to gain before 120Mhz (40x3) and then 480Mhz with helical filter for clock the AD9954. Thoughts are possible? It is one bad solutions?? Thank you again. ha scritto nel messaggio ... The clipped sinewave is the simplest output for a TCXO, it's usually just a capacitor connected between the emitter of the oscillator and the output pin. You have to be careful what you connect to it, it's possible to throw the temp compensation out of tolerance. Most CMOS PLL chips have a reference frequency input that handles it well. If you're rolling your own PLL from SSI or MSI chips, you probable want to build up a buffer to make the clipped sinewave into TTL or HCMOS levels, or pay extra for a TCXO with the needed circuit built in. On Sun, 17 Jul 2005 20:34:25 GMT, "Mario Bros" wrote: Hi folks, I would have to ask a clarification regard to the TCXO. They are found with output TTL, HCMOS and CLIPPED SINE WAVE. It is just with respect to this last type that I would want to have elucidations on when it is convenient to employ it, which the advantages and the disadvantages and which the extension of spectral harmonicas. In synthesis, from the plan point of view, which are the motivations that they make to incline towards a Clipped oscillator? Anticipated thanks. 73's de IK6GQC Rocco |
#5
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It all sounds possible to me. It is not very different from some of
the schemes we use commercially to generate internal frequencies (in spectral analysis equipment). Your idea of good filtering at 480MHz (and also, I hope, at 120MHz) is a good one, to keep out other combinations of multiplication. The one other thing I would be careful about is the phase noise of the oscillator. The AD9954 should be capable of very good phase noise performance, but you MUST feed it a clean reference to keep the output clean. I have recently tested some TCXOs which are very stable and accurate, but whose phase noise is not as good as I would really like. It is important to feed the oscillator from a very clean supply, but that alone is not a guarantee of good phase noise. See http://www.techlib.com/electronics/finesse.html for a nice idea to make a very clean supply for circuits that draw essentially constant current. Cheers, Tom |
#6
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Hi Tom,
Thanks for the councils. An alternative could be an oscillator (480 Mhz) with 1/4 coax resonator and IC PLL TSA5511 I2C Bus clocked by 40Mhz/10 (4mhz)... I must think to us... Ciao IK6GQC Rocco "K7ITM" ha scritto nel messaggio oups.com... It all sounds possible to me. It is not very different from some of the schemes we use commercially to generate internal frequencies (in spectral analysis equipment). Your idea of good filtering at 480MHz (and also, I hope, at 120MHz) is a good one, to keep out other combinations of multiplication. The one other thing I would be careful about is the phase noise of the oscillator. The AD9954 should be capable of very good phase noise performance, but you MUST feed it a clean reference to keep the output clean. I have recently tested some TCXOs which are very stable and accurate, but whose phase noise is not as good as I would really like. It is important to feed the oscillator from a very clean supply, but that alone is not a guarantee of good phase noise. See http://www.techlib.com/electronics/finesse.html for a nice idea to make a very clean supply for circuits that draw essentially constant current. Cheers, Tom |
#7
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Hi Rocco,
I would use the multiplication. Within the loop bandwidth of the PLL, the phase noise will be determined by the crystal oscillator used as a reference PLUS the noise contributed by the PLL chip itself (the phase comparator and loop filter/amplifier), and outside the loop bandwidth, it will be determined by the VCO used in the PLL. The important thing, whether you use a PLL or multiplication, is to start with a reference which has low phase noise. In fact, your TCXO may have good phase noise...I don't know...I only know that some I've tested have not been as clean as I'd like. But I'm also setting pretty high standards for what I'm doing. If I were in your shoes, I would build the multiplier system just as you first described, and use the TCXO you have, and if it proves to be too noisy (phase noise), then look for a better oscillator. If you can make even a crude measurement of the oscillator's phase noise before you start, that would be good, too, just to know where you are starting. My comment in my earlier posting was just to make you aware to look at phase noise, not to change the basic way you are going about it. Cheers, Tom |
#8
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Hi Tom,
my TCXO is GTXO-531 GOLLEDGE-UK product. The parameters: Clipped Sine Wave 1.0V p-p Test Load 10k//10pF Phase noise ): -135dbc/Hz max To this point I have 2 possible ways: 1) TCXO+HelicalFilter480Mhz+BFR96 Buffer+HelicalFilter480Mhz 2) TCXO+Ampli-Multiplier x3(120 Mhz)+Ampli-Multiplier x4(480Mhz)+Helical Filter480Mhz Also here, perhaps it is the case to make some test. But, to the first impact, what you tasks? 73's de IK6GQC Rocco "K7ITM" ha scritto nel messaggio ups.com... Hi Rocco, I would use the multiplication. Within the loop bandwidth of the PLL, the phase noise will be determined by the crystal oscillator used as a reference PLUS the noise contributed by the PLL chip itself (the phase comparator and loop filter/amplifier), and outside the loop bandwidth, it will be determined by the VCO used in the PLL. The important thing, whether you use a PLL or multiplication, is to start with a reference which has low phase noise. In fact, your TCXO may have good phase noise...I don't know...I only know that some I've tested have not been as clean as I'd like. But I'm also setting pretty high standards for what I'm doing. If I were in your shoes, I would build the multiplier system just as you first described, and use the TCXO you have, and if it proves to be too noisy (phase noise), then look for a better oscillator. If you can make even a crude measurement of the oscillator's phase noise before you start, that would be good, too, just to know where you are starting. My comment in my earlier posting was just to make you aware to look at phase noise, not to change the basic way you are going about it. Cheers, Tom |
#9
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Hi Rocco,
The specified phase noise seems respectable to me. The ones I've been looking at have some very nice characteristics that I need, but phase noise about -115dBc/Hz at 1kHz offset. Though it might look like more work, I would use (2). If I used (1), I would have a buffer amplifier from the oscillator, driving something to generate rich harmonics...since you need an even harmonic, use an assymetrical harmonic generator. Square waves, for example, have extremely low even-order harmonics ("theoretically" zero). But with (2), you can use a square wave to get the 3rd harmonic, and not have to filter much 2nd or 4th, since there is little of either of those in your square wave. A relatively simple filter at 120MHz will suffice. Then the 480MHz helical must attenuate only signals 120MHz away, not signals 40MHz away like in (1). At least, that is my take on it. There are tradeoffs, always, and therefore you will see different designs to accomplish the same thing, and all will work. Cheers, Tom |
#10
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Ok Tom, just the second solution wants more job but creed also I that I am
the better . Thanks still for the good suggestions. 73's de IK6GQC Rocco "K7ITM" ha scritto nel messaggio oups.com... Hi Rocco, The specified phase noise seems respectable to me. The ones I've been looking at have some very nice characteristics that I need, but phase noise about -115dBc/Hz at 1kHz offset. Though it might look like more work, I would use (2). If I used (1), I would have a buffer amplifier from the oscillator, driving something to generate rich harmonics...since you need an even harmonic, use an assymetrical harmonic generator. Square waves, for example, have extremely low even-order harmonics ("theoretically" zero). But with (2), you can use a square wave to get the 3rd harmonic, and not have to filter much 2nd or 4th, since there is little of either of those in your square wave. A relatively simple filter at 120MHz will suffice. Then the 480MHz helical must attenuate only signals 120MHz away, not signals 40MHz away like in (1). At least, that is my take on it. There are tradeoffs, always, and therefore you will see different designs to accomplish the same thing, and all will work. Cheers, Tom |
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