1 GHz synthesizer
in [Design]
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From: John Larkin on 24 Feb 2010 17:47 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. > >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? > > > TACI (That's A Complex Issue). If you have 1 GHz superimposed on your 50 MHz, a fast comparator will follow the composite waveform, and make edge jitter, but a slower comparator will partially lowpass filter the 1 GHz out... depending on a lot of things, like where inside the comparator the bandwidth really limits. If the comparator is really, really slow, the output edge rate will slow down, and that can cause jitter too. Wildly guessing, I'd think that the National thing is too fast, Something with more like a 3 ns response might be about right. That makes the Fairchild LVDS a little fast even. But if you plop a tuned circuit right at the input of a fast comparator, that should kill most of the 1 GHz, and a lot of other crud. Both the LMH and the FIN parts will work at a common mode voltage of zero, so a single-ended tuned network based on ground, right at the comparator input, could bandpass the 50 MHz nicely, attenuate the 1G a lot, and give you some voltage gain for free. Ground is a great thing to use in RF circuits. If I were doing this, I'd maybe breadboard the front-end circuit on a bit of copperclad, connect a couple of signal generators, and check the jitter with a sampling scope. Trying without/with a tuned circuit would be interesting. John
From: Joerg on 24 Feb 2010 18:16 Andrew Holme 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. ... <ouieeeeeeeeee ...> (sound of alarm siren) If there is no ground plane in that area that's you first problem, needs to be fixed. If there is one and you just don't have a via to it place copper tape to the next via so you get a part of a plane there. If too crowed put that on the other side and drill a hole. Say a prayer before drilling :-) If the comparator is well bypassed, which I hope it is, then you cold also solder caps from inputs to its VDD and VEE. If not well bypassed then that needs to be accomplished first. As John said transformers have plenty of leakage inductance. This picks up RF and you may have to shield the transformer. > ... 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. > Add a little snippet of copper tape to bridge the 10mm with a fairly wide swath of copper. > 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? > In the RF world that is usually a recipe for desaster. A ground split is like roulette. You win on one number only, on black nothing happens, on red you lose ;-) > Any other suggestions? > Yes, can you add make-shift shield cans? Shallow shield tops of a few millimeters height can be bought but when in a rush at a client I sometimes cut up a butter cookie can. The contents are dutifully eaten by the crew :-) Another more nasty trick that borders on cheating is to solder a 2-3cm snippet of wire to one side of the transformer and bend it around. Try both sides. Then try to find a minimum in the spurs by bending. It acts as an antenna and the goal is to make it feed 180 degrees against the 1GHz that leaks in through other paths. Typically one would use a longer than anticipated snippet and when the reaction upon bending is too gross cut a chunk off and see whether it affords more neutralization control. Bending needs to be done by plastic tweezers. Or in a pinch wooden chop sticks, always a good excuse to have lunch at an Asian place. Make sure a black cat crosses the lawn and is seen by others when you do this so when it works you will be awarded guru status :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Fred Bartoli on 24 Feb 2010 18:51 Joerg a �crit : > Andrew Holme 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. ... > > > <ouieeeeeeeeee ...> (sound of alarm siren) > > If there is no ground plane in that area that's you first problem, needs > to be fixed. If there is one and you just don't have a via to it place > copper tape to the next via so you get a part of a plane there. If too > crowed put that on the other side and drill a hole. Say a prayer before > drilling :-) > > If the comparator is well bypassed, which I hope it is, then you cold > also solder caps from inputs to its VDD and VEE. If not well bypassed > then that needs to be accomplished first. > > As John said transformers have plenty of leakage inductance. This picks > up RF and you may have to shield the transformer. > >> ... >> 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. >> > > Add a little snippet of copper tape to bridge the 10mm with a fairly > wide swath of copper. > > >> 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? >> > > In the RF world that is usually a recipe for desaster. A ground split is > like roulette. You win on one number only, on black nothing happens, on > red you lose ;-) > > >> Any other suggestions? >> > > Yes, can you add make-shift shield cans? Shallow shield tops of a few > millimeters height can be bought but when in a rush at a client I > sometimes cut up a butter cookie can. The contents are dutifully eaten > by the crew :-) > > Another more nasty trick that borders on cheating is to solder a 2-3cm > snippet of wire to one side of the transformer and bend it around. Try > both sides. Then try to find a minimum in the spurs by bending. It acts > as an antenna and the goal is to make it feed 180 degrees against the > 1GHz that leaks in through other paths. Typically one would use a longer > than anticipated snippet and when the reaction upon bending is too gross > cut a chunk off and see whether it affords more neutralization control. > Bending needs to be done by plastic tweezers. Or in a pinch wooden chop > sticks, always a good excuse to have lunch at an Asian place. > Did you really use that one? Might be a bear to find the right position... > Make sure a black cat crosses the lawn and is seen by others when you do > this so when it works you will be awarded guru status :-) > Better to bring one with you. I'll add one to the box next time I'll go to a customer... -- Thanks, Fred.
From: Andrew Holme on 24 Feb 2010 19:00 "Joerg" <invalid(a)invalid.invalid> wrote in message news:7ulqb2F719U1(a)mid.individual.net... > Andrew Holme 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. >> ... > > > <ouieeeeeeeeee ...> (sound of alarm siren) > > If there is no ground plane in that area that's you first problem, needs > to be fixed. If there is one and you just don't have a via to it place > copper tape to the next via so you get a part of a plane there. If too > crowed put that on the other side and drill a hole. Say a prayer before > drilling :-) > > If the comparator is well bypassed, which I hope it is, then you cold also > solder caps from inputs to its VDD and VEE. If not well bypassed then that > needs to be accomplished first. I have a continuous ground plane on the bottom but no ground vias near the comparator because it only has VEE -5V, VCCI 3.3V and VCCO 2.5V. There are vias to 100nF decouplers on the bottom and I could easily put caps from these vias to the inputs. I didn't do this earlier because I wasn't sure if the SRF of the 100nF would be effective against 1 GHz; but I could maybe replace them with 10nF. > > As John said transformers have plenty of leakage inductance. This picks up > RF and you may have to shield the transformer. > >> ... 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. >> > > Add a little snippet of copper tape to bridge the 10mm with a fairly wide > swath of copper. > > >> 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? >> > > In the RF world that is usually a recipe for desaster. A ground split is > like roulette. You win on one number only, on black nothing happens, on > red you lose ;-) > Thanks for putting me straight on that one! > >> Any other suggestions? >> > > Yes, can you add make-shift shield cans? Shallow shield tops of a few > millimeters height can be bought but when in a rush at a client I > sometimes cut up a butter cookie can. The contents are dutifully eaten by > the crew :-) > > Another more nasty trick that borders on cheating is to solder a 2-3cm > snippet of wire to one side of the transformer and bend it around. Try > both sides. Then try to find a minimum in the spurs by bending. It acts as > an antenna and the goal is to make it feed 180 degrees against the 1GHz > that leaks in through other paths. Typically one would use a longer than > anticipated snippet and when the reaction upon bending is too gross cut a > chunk off and see whether it affords more neutralization control. Bending > needs to be done by plastic tweezers. Or in a pinch wooden chop sticks, > always a good excuse to have lunch at an Asian place. > > Make sure a black cat crosses the lawn and is seen by others when you do > this so when it works you will be awarded guru status :-) > > -- > Regards, Joerg > > http://www.analogconsultants.com/ > > "gmail" domain blocked because of excessive spam. > Use another domain or send PM.
From: Phil Hobbs on 24 Feb 2010 19:04
On 2/24/2010 7:00 PM, Andrew Holme wrote: > "Joerg"<invalid(a)invalid.invalid> wrote in message > news:7ulqb2F719U1(a)mid.individual.net... >> Andrew Holme 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. >>> ... >> >> >> <ouieeeeeeeeee ...> (sound of alarm siren) >> >> If there is no ground plane in that area that's you first problem, needs >> to be fixed. If there is one and you just don't have a via to it place >> copper tape to the next via so you get a part of a plane there. If too >> crowed put that on the other side and drill a hole. Say a prayer before >> drilling :-) >> >> If the comparator is well bypassed, which I hope it is, then you cold also >> solder caps from inputs to its VDD and VEE. If not well bypassed then that >> needs to be accomplished first. > > > > I have a continuous ground plane on the bottom but no ground vias near the > comparator because it only has VEE -5V, VCCI 3.3V and VCCO 2.5V. There are > vias to 100nF decouplers on the bottom and I could easily put caps from > these vias to the inputs. I didn't do this earlier because I wasn't sure if > the SRF of the 100nF would be effective against 1 GHz; but I could maybe > replace them with 10nF. > > > >> >> As John said transformers have plenty of leakage inductance. This picks up >> RF and you may have to shield the transformer. >> >>> ... 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. >>> >> >> Add a little snippet of copper tape to bridge the 10mm with a fairly wide >> swath of copper. >> >> >>> 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? >>> >> >> In the RF world that is usually a recipe for desaster. A ground split is >> like roulette. You win on one number only, on black nothing happens, on >> red you lose ;-) >> > > > Thanks for putting me straight on that one! > > > >> >>> Any other suggestions? >>> >> >> Yes, can you add make-shift shield cans? Shallow shield tops of a few >> millimeters height can be bought but when in a rush at a client I >> sometimes cut up a butter cookie can. The contents are dutifully eaten by >> the crew :-) >> >> Another more nasty trick that borders on cheating is to solder a 2-3cm >> snippet of wire to one side of the transformer and bend it around. Try >> both sides. Then try to find a minimum in the spurs by bending. It acts as >> an antenna and the goal is to make it feed 180 degrees against the 1GHz >> that leaks in through other paths. Typically one would use a longer than >> anticipated snippet and when the reaction upon bending is too gross cut a >> chunk off and see whether it affords more neutralization control. Bending >> needs to be done by plastic tweezers. Or in a pinch wooden chop sticks, >> always a good excuse to have lunch at an Asian place. >> >> Make sure a black cat crosses the lawn and is seen by others when you do >> this so when it works you will be awarded guru status :-) >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ >> >> "gmail" domain blocked because of excessive spam. >> Use another domain or send PM. > > One other thing is that comparators and other triggering gizmos don't have infinite input-output isolation, and power supply rejection is also far from infinite at high frequencies. I'd try putting chokes in the comparator's supplies if other suggestions don't work. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs at electrooptical dot net http://electrooptical.net |