1 GHz synthesizer
in [Design]
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From: Andrew Holme on 24 Feb 2010 15:16 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
From: John Larkin on 24 Feb 2010 16:18 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
From: John Larkin on 24 Feb 2010 16:22 ps - if you spin the board, replace that $5 comparator with an 80-cent FIN1101K8X and tune its input. John
From: Andrew Holme on 24 Feb 2010 16:50 "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. I wanted it to take the -10 dBm 50 MHz STD output from my spectrum analyzer. I'm currently needing to boost this by 20dB to push the spurs down. Can you elaborate on why a slower comparator might result in less phase noise? I can see how it might be less susceptible to 1 GHz and therefore lower the spurs; but wouldn't it have more random jitter?
From: Jim Thompson on 24 Feb 2010 16:53
On Wed, 24 Feb 2010 13:22:20 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > > >ps - if you spin the board, replace that $5 comparator with an 80-cent >FIN1101K8X and tune its input. > >John > Don't you just love those Fairchild LVDS parts? You should... I was a major player in that design team in 2001 ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |