1 GHz synthesizer
in [Design]
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From: Joerg on 24 Feb 2010 19:11 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. > If it's regular X7R ceramics in 0402, 0603 or even 0805 it'll be ok at 1GHz. No need to swap those out. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 24 Feb 2010 19:21 Fred Bartoli wrote: > Joerg a �crit : [...] >> 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... > I did. It's not that bad. Basically you bend it downwards a bit and then rotate it slowly until you hit close to zero. If the noise shoots up way more than 2x after soldering it on snip pieces off. If you can't get anywhere near zero it needs to go on the other phase. Another trick I learned way back in my early ham radio days is the poor man's variable capacitor. If you need just a smidgen of signal injection and the source is really close by (less than 1cm) use two wires and twist tightly several times. Then snip off a millimeter at a time until "right". Trick #2 did not come without pain. Had to twist some more on my first one, reached in, forgot about the fact that 300V plate voltage was riding on one side, snipped end touched index finger ... *BZZZT* [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 24 Feb 2010 19:32 On Wed, 24 Feb 2010 16:21:58 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Fred Bartoli wrote: >> Joerg a �crit : > >[...] > >>> 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... >> > >I did. It's not that bad. Basically you bend it downwards a bit and then >rotate it slowly until you hit close to zero. If the noise shoots up way >more than 2x after soldering it on snip pieces off. If you can't get >anywhere near zero it needs to go on the other phase. Called "neutralizing". I saw it in early textbooks, but it didn't really register until I had my first Raytheon radio with CK760's and IF cans with a neutralizing winding. > >Another trick I learned way back in my early ham radio days is the poor >man's variable capacitor. If you need just a smidgen of signal injection >and the source is really close by (less than 1cm) use two wires and >twist tightly several times. Then snip off a millimeter at a time until >"right". Called a "gimmick"... a mainstay of Mad Man Muntz' IF alignment procedure ;-) > >Trick #2 did not come without pain. Had to twist some more on my first >one, reached in, forgot about the fact that 300V plate voltage was >riding on one side, snipped end touched index finger ... *BZZZT* > >[...] *BZZZT*, then WHAP when you jerk !-) ...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.
From: Fred Bartoli on 24 Feb 2010 19:40 Joerg a �crit : > Fred Bartoli wrote: >> Joerg a �crit : > > [...] > >>> 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... >> > > I did. It's not that bad. Basically you bend it downwards a bit and then > rotate it slowly until you hit close to zero. If the noise shoots up way > more than 2x after soldering it on snip pieces off. If you can't get > anywhere near zero it needs to go on the other phase. > > Another trick I learned way back in my early ham radio days is the poor > man's variable capacitor. If you need just a smidgen of signal injection > and the source is really close by (less than 1cm) use two wires and > twist tightly several times. Then snip off a millimeter at a time until > "right". > Yep, knew that one. One related I used/invented? where I needed really super fine cap adjustment (to GND) was: Have two surface pads located on top side, located half an inch to one inch apart. One unconnected, the other being your cap terminal. Then solder a small bare wire between the pads, so that it makes a flat inverted U, something like 5mm above the GND plane. And voil�, you have your really fine trim cap. Just bent the wire up and down with a small plastic tool while using the PCB as fulcrum and you can easily adjust your cap to the few fF. Thought of this while thinking about the way some chineese tuners coils are hand adjusted. It made wonders once in prod at tweaking HF CMRR. > Trick #2 did not come without pain. Had to twist some more on my first > one, reached in, forgot about the fact that 300V plate voltage was > riding on one side, snipped end touched index finger ... *BZZZT* > -- Thanks, Fred.
From: Joerg on 24 Feb 2010 20:19
Fred Bartoli wrote: > Joerg a �crit : >> Fred Bartoli wrote: >>> Joerg a �crit : >> >> [...] >> >>>> 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... >>> >> >> I did. It's not that bad. Basically you bend it downwards a bit and >> then rotate it slowly until you hit close to zero. If the noise shoots >> up way more than 2x after soldering it on snip pieces off. If you >> can't get anywhere near zero it needs to go on the other phase. >> >> Another trick I learned way back in my early ham radio days is the >> poor man's variable capacitor. If you need just a smidgen of signal >> injection and the source is really close by (less than 1cm) use two >> wires and twist tightly several times. Then snip off a millimeter at a >> time until "right". >> > > Yep, knew that one. > One related I used/invented? where I needed really super fine cap > adjustment (to GND) was: > Have two surface pads located on top side, located half an inch to one > inch apart. One unconnected, the other being your cap terminal. > Then solder a small bare wire between the pads, so that it makes a flat > inverted U, something like 5mm above the GND plane. > And voil�, you have your really fine trim cap. Just bent the wire up and > down with a small plastic tool while using the PCB as fulcrum and you > can easily adjust your cap to the few fF. > This is actually how some RF mixer modules are aligned in production. By pushing and bending those wire stubs. > Thought of this while thinking about the way some chineese tuners coils > are hand adjusted. It made wonders once in prod at tweaking HF CMRR. > 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 :-) [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |