1 GHz synthesizer
in [Design]
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From: John Larkin on 24 Feb 2010 21:40 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. John
From: Joerg on 24 Feb 2010 21:58 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Michael A. Terrell on 24 Feb 2010 23:29 Joerg wrote: > > On coils that's been done since a long time. Pushing the windings > together or if you want to go in the other direction insert the little > flat wooden stick and twist it so the turns spread again. But a really > experienced tuning tech never has to go backwards :-) You want real fun? Try to align a 'Tubular Filter' without a network analyzer. -- Greed is the root of all eBay.
From: Fred Bartoli on 25 Feb 2010 04:21 Joerg a �crit : > John Larkin wrote: >> On Wed, 24 Feb 2010 21:50:07 -0000, "Andrew Holme" <ah(a)nospam.com> >> wrote: >> ...... >>>> >>> 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. > Or maybe, if the reference input is a square signal, let it pick the 10MHz fifth harmonic... With a high enough Q you'll probably have enough signal to almost not notice. -- Thanks, Fred.
From: John Larkin on 25 Feb 2010 12:37
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. John |