1 GHz synthesizer
in [Design]
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From: Joerg on 25 Feb 2010 12:44 John Larkin wrote: > On Wed, 24 Feb 2010 18:58:36 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Wed, 24 Feb 2010 21:50:07 -0000, "Andrew Holme" <ah(a)nospam.com> >>> wrote: >>> >>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >>>> news:u05bo5hknlma4qpda45muddb5tl5slkud2(a)4ax.com... >>>>> On Wed, 24 Feb 2010 20:16:15 -0000, "Andrew Holme" <ah(a)nospam.com> >>>>> wrote: >>>>> >>>>>> I'm developing a 1 GHz fractional-N synthesizer. The reference input is a >>>>>> 50 MHz sine wave which I'm converting to LVDS as follows: >>>>>> >>>>>> 1. 50 MHz sine wave arrives at board mounted SMA in 50-ohm system >>>>>> 2. Converted to 100 ohms differential using Mini-Circuits ADT2-1T >>>>>> transformer >>>>>> 3. Carried 10mm along pair of closely-spaced traces to 100-ohm 0402 >>>>>> resistor >>>>>> adjacent to differential input of LMH7324 high-speed comparator. >>>>>> >>>>>> I think some 1 GHz from my VCO (3 inches away) is getting into the (3.84 >>>>>> Gbps rated) LMH7324 comparator input and modulating the zero-crossings. >>>>>> Result: integer boundary spurs when the VCO frequency is set very close to >>>>>> an integer multiple of the reference. >>>>>> >>>>>> Reducing reference drive level increases integer boundary spurs. >>>>>> Increasing reference drive level reduces integer boundary spurs (quite >>>>>> encouraging reduction is possible). >>>>>> Touching one side of the transformer secondary with the metal blade of a >>>>>> plastic-handled scalpel increases integer boundary spurs. >>>>>> >>>>>> Presumably, the 1 GHz enters the comparator as a common-mode signal. I >>>>>> would like to try two small caps from the comparator inputs to ground. >>>>>> Unfortunately, there are no convenient grounds to be had in that area. >>>>>> The >>>>>> transformer secondary centre-tap is grounded and I could easily put small >>>>>> (10pF?) caps across the secondary windings; but that's 10mm away. I will >>>>>> just have to try a few different things and maybe drill some holes through >>>>>> to the continuous copper ground plane on the bottom. Annoyingly, there >>>>>> are >>>>>> some decouplers on the bottom just under the 0402 100-ohm. >>>>>> >>>>>> How/why is the 1 GHz leaking all over the board like this? >>>>>> >>>>>> Should I have split my ground plane between reference frequency and VCO >>>>>> frequency areas of the board? >>>>>> >>>>>> Any other suggestions? >>>>>> >>>>>> TIA >>>>>> >>>>> 1. The transformer may be doing more harm than good. Its leakage >>>>> inductance could be increasing the impedance downstream, and the >>>>> transformer is likely not perfectly balanced. >>>>> >>>>> 2. A tuned circuit right at the comparator input is nice. That would >>>>> improve 1 GHz rejection, and could give an amplitude boost, too. It >>>>> would improve all sorts of rejection... radio stations, cell phones, >>>>> whatever. >>>>> >>>>> 3. A slower comparator might paradoxically result in less phase noise. >>>>> >>>>> 4. Caps from the transformer outputs to ground will probably help. >>>>> Make them big enough to contribute some decent lowpass filtering, and >>>>> maybe seesaw their values to trim CMRR. Better yet, replace the >>>>> wideband transformer with something that resonates. >>>>> >>>>> >>>>> 1 GHz does get around. Splitting the planes may not help and could >>>>> make things worse. >>>>> >>>>> What's your target voltage range for the 50 MHz input? >>>>> >>>>> John >>>>> >>>> Thanks for the suggestions. Using a tuned circuit sounds good. I was >>>> planning to support 10 or 50 MHz reference inputs; but I could drop that >>>> requirement and stick to one frequency. >>> How about a 10 MHz tuned circuit driving one input of the comparator, >>> and a 50 MHz network driving the other? You could do all that with 5 >>> or 6 parts and get some voltage gain too. >>> >> Or build a filter with two peaks, one at 10MHz and the other at 50MHz. >> They are far enough apart, should work. > > I suppose I could design a network that would have two peaks and some > voltage gain at both, but it would hurt my head. > The Zeitgeist pub would have some soothing medicine for that :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: George Herold on 25 Feb 2010 13:07 On Feb 25, 12:37 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 24 Feb 2010 18:58:36 -0800, Joerg <inva...(a)invalid.invalid> > wrote: > > > > > > >John Larkin wrote: > >> On Wed, 24 Feb 2010 21:50:07 -0000, "Andrew Holme" <a...(a)nospam.com> > >> wrote: > > >>> "John Larkin" <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in message > >>>news:u05bo5hknlma4qpda45muddb5tl5slkud2(a)4ax.com... > >>>> On Wed, 24 Feb 2010 20:16:15 -0000, "Andrew Holme" <a...(a)nospam.com> > >>>> wrote: > > >>>>> I'm developing a 1 GHz fractional-N synthesizer. The reference input is a > >>>>> 50 MHz sine wave which I'm converting to LVDS as follows: > > >>>>> 1. 50 MHz sine wave arrives at board mounted SMA in 50-ohm system > >>>>> 2. Converted to 100 ohms differential using Mini-Circuits ADT2-1T > >>>>> transformer > >>>>> 3. Carried 10mm along pair of closely-spaced traces to 100-ohm 0402 > >>>>> resistor > >>>>> adjacent to differential input of LMH7324 high-speed comparator. > > >>>>> I think some 1 GHz from my VCO (3 inches away) is getting into the (3.84 > >>>>> Gbps rated) LMH7324 comparator input and modulating the zero-crossings. > >>>>> Result: integer boundary spurs when the VCO frequency is set very close to > >>>>> an integer multiple of the reference. > > >>>>> Reducing reference drive level increases integer boundary spurs. > >>>>> Increasing reference drive level reduces integer boundary spurs (quite > >>>>> encouraging reduction is possible). > >>>>> Touching one side of the transformer secondary with the metal blade of a > >>>>> plastic-handled scalpel increases integer boundary spurs. > > >>>>> Presumably, the 1 GHz enters the comparator as a common-mode signal.. I > >>>>> would like to try two small caps from the comparator inputs to ground. > >>>>> Unfortunately, there are no convenient grounds to be had in that area. > >>>>> The > >>>>> transformer secondary centre-tap is grounded and I could easily put small > >>>>> (10pF?) caps across the secondary windings; but that's 10mm away.. I will > >>>>> just have to try a few different things and maybe drill some holes through > >>>>> to the continuous copper ground plane on the bottom. Annoyingly, there > >>>>> are > >>>>> some decouplers on the bottom just under the 0402 100-ohm. > > >>>>> How/why is the 1 GHz leaking all over the board like this? > > >>>>> Should I have split my ground plane between reference frequency and VCO > >>>>> frequency areas of the board? > > >>>>> Any other suggestions? > > >>>>> TIA > > >>>> 1. The transformer may be doing more harm than good. Its leakage > >>>> inductance could be increasing the impedance downstream, and the > >>>> transformer is likely not perfectly balanced. > > >>>> 2. A tuned circuit right at the comparator input is nice. That would > >>>> improve 1 GHz rejection, and could give an amplitude boost, too. It > >>>> would improve all sorts of rejection... radio stations, cell phones, > >>>> whatever. > > >>>> 3. A slower comparator might paradoxically result in less phase noise. > > >>>> 4. Caps from the transformer outputs to ground will probably help. > >>>> Make them big enough to contribute some decent lowpass filtering, and > >>>> maybe seesaw their values to trim CMRR. Better yet, replace the > >>>> wideband transformer with something that resonates. > > >>>> 1 GHz does get around. Splitting the planes may not help and could > >>>> make things worse. > > >>>> What's your target voltage range for the 50 MHz input? > > >>>> John > > >>> Thanks for the suggestions. Using a tuned circuit sounds good. I was > >>> planning to support 10 or 50 MHz reference inputs; but I could drop that > >>> requirement and stick to one frequency. > > >> How about a 10 MHz tuned circuit driving one input of the comparator, > >> and a 50 MHz network driving the other? You could do all that with 5 > >> or 6 parts and get some voltage gain too. > > >Or build a filter with two peaks, one at 10MHz and the other at 50MHz. > >They are far enough apart, should work. > > I suppose I could design a network that would have two peaks and some > voltage gain at both, but it would hurt my head. > > John- Hide quoted text - > > - Show quoted text - Hmm wouldn't a Q = ~5 high pass filter at 10 MHz followed by a Q = ~5 Low pass filter at 50 MHz do it? Of course it would be relativly flat in between. (Or is that too simple?) George H.
From: John Larkin on 25 Feb 2010 13:45 On Thu, 25 Feb 2010 10:07:22 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Feb 25, 12:37�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 24 Feb 2010 18:58:36 -0800, Joerg <inva...(a)invalid.invalid> >> wrote: >> >> >> >> >> >> >John Larkin wrote: >> >> On Wed, 24 Feb 2010 21:50:07 -0000, "Andrew Holme" <a...(a)nospam.com> >> >> wrote: >> >> >>> "John Larkin" <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in message >> >>>news:u05bo5hknlma4qpda45muddb5tl5slkud2(a)4ax.com... >> >>>> On Wed, 24 Feb 2010 20:16:15 -0000, "Andrew Holme" <a...(a)nospam.com> >> >>>> wrote: >> >> >>>>> I'm developing a 1 GHz fractional-N synthesizer. �The reference input is a >> >>>>> 50 MHz sine wave which I'm converting to LVDS as follows: >> >> >>>>> 1. 50 MHz sine wave arrives at board mounted SMA in 50-ohm system >> >>>>> 2. Converted to 100 ohms differential using Mini-Circuits ADT2-1T >> >>>>> transformer >> >>>>> 3. Carried 10mm along pair of closely-spaced traces to 100-ohm 0402 >> >>>>> resistor >> >>>>> adjacent to differential input of LMH7324 high-speed comparator. >> >> >>>>> I think some 1 GHz from my VCO (3 inches away) is getting into the (3.84 >> >>>>> Gbps rated) LMH7324 comparator input and modulating the zero-crossings. >> >>>>> Result: integer boundary spurs when the VCO frequency is set very close to >> >>>>> an integer multiple of the reference. >> >> >>>>> Reducing reference drive level increases integer boundary spurs. >> >>>>> Increasing reference drive level reduces integer boundary spurs (quite >> >>>>> encouraging reduction is possible). >> >>>>> Touching one side of the transformer secondary with the metal blade of a >> >>>>> plastic-handled scalpel increases integer boundary spurs. >> >> >>>>> Presumably, the 1 GHz enters the comparator as a common-mode signal. �I >> >>>>> would like to try two small caps from the comparator inputs to ground. >> >>>>> Unfortunately, there are no convenient grounds to be had in that area. >> >>>>> The >> >>>>> transformer secondary centre-tap is grounded and I could easily put small >> >>>>> (10pF?) caps across the secondary windings; �but that's 10mm away. �I will >> >>>>> just have to try a few different things and maybe drill some holes through >> >>>>> to the continuous copper ground plane on the bottom. �Annoyingly, there >> >>>>> are >> >>>>> some decouplers on the bottom just under the 0402 100-ohm. >> >> >>>>> How/why is the 1 GHz leaking all over the board like this? >> >> >>>>> Should I have split my ground plane between reference frequency and VCO >> >>>>> frequency areas of the board? >> >> >>>>> Any other suggestions? >> >> >>>>> TIA >> >> >>>> 1. The transformer may be doing more harm than good. Its leakage >> >>>> inductance could be increasing the impedance downstream, and the >> >>>> transformer is likely not perfectly balanced. >> >> >>>> 2. A tuned circuit right at the comparator input is nice. That would >> >>>> improve 1 GHz rejection, and could give an amplitude boost, too. It >> >>>> would improve all sorts of rejection... radio stations, cell phones, >> >>>> whatever. >> >> >>>> 3. A slower comparator might paradoxically result in less phase noise. >> >> >>>> 4. Caps from the transformer outputs to ground will probably help. >> >>>> Make them big enough to contribute some decent lowpass filtering, and >> >>>> maybe seesaw their values to trim CMRR. Better yet, replace the >> >>>> wideband transformer with something that resonates. >> >> >>>> 1 GHz does get around. Splitting the planes may not help and could >> >>>> make things worse. >> >> >>>> What's your target voltage range for the 50 MHz input? >> >> >>>> John >> >> >>> Thanks for the suggestions. �Using a tuned circuit sounds good. �I was >> >>> planning to support 10 or 50 MHz reference inputs; but I could drop that >> >>> requirement and stick to one frequency. >> >> >> How about a 10 MHz tuned circuit driving one input of the comparator, >> >> and a 50 MHz network driving the other? You could do all that with 5 >> >> or 6 parts and get some voltage gain too. >> >> >Or build a filter with two peaks, one at 10MHz and the other at 50MHz. >> >They are far enough apart, should work. >> >> I suppose I could design a network that would have two peaks and some >> voltage gain at both, but it would hurt my head. >> >> John- Hide quoted text - >> >> - Show quoted text - > >Hmm wouldn't a Q = ~5 high pass filter at 10 MHz followed by a Q = ~5 >Low pass filter at 50 MHz do it? Of course it would be relativly flat >in between. > >(Or is that too simple?) > >George H. That might work. Post a schematic! The ideal clock-input-reference filters have narrowband responses around the input frequency, as narrow as parts tolerances allow, to reject as much non-ref crud as possible. You want to be operating on the flat part of the frequency response curve, so's not to convert any amplitude or temperature effects into phase shift... so too much Q is bad too. That suggests higher-order filters in extreme cases. At 10 MHz, we usually use a simple R-L-C with a Q around 5. That also allows ugly inputs, like square waves that ring and whatever. John
From: osr on 25 Feb 2010 14:00 > > You want real fun? Try to align a 'Tubular Filter' without a network > analyzer. > > -- > Greed is the root of all eBa That DOES NOT need to be the five hours it usually takes... On odd element microwave ones, short the Inner element. Adjust the outer ones for SWR as desired, open up the inner one. Dishal's method.... The guy wrote a brilliant paper on it.. Steve
From: osr on 25 Feb 2010 14:09
On Feb 25, 2:00 pm, o...(a)uakron.edu wrote: > > You want real fun? Try to align a 'Tubular Filter' without a network > > analyzer. > > > -- > > Greed is the root of all eBa > > That DOES NOT need to be the five hours it usually takes... > > On odd element microwave ones, short the Inner element. Adjust the > outer ones for SWR as desired, open up the inner one. > > Dishal's method.... The guy wrote a brilliant paper on it.. > > Steve Ah found it: Dishal, M., Alignment and Adjustment of Synchronously Tuned Multiple- Resonant- Circuit Filters, Elec. Commun., pp. 154-164, June, 1952. Also see Hayward, Introduction to Radio Frequency Design, ARRL, 1994, pp95-101. Steve |