1 GHz synthesizer

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.

1. 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

ps - if you spin the board, replace that $5 comparator with an 80-cent FIN1101K8X and tune its 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?

Don't you just love those Fairchild LVDS parts?

You should... I was a major player in that design team in 2001 ;-) ...Jim Thompson

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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

(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.

Add a little snippet of copper tape to bridge the 10mm with a fairly wide swath of copper.

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 ;-)

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 :-)

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Did you really use that one? Might be a bear to find the right position...

Better to bring one with you. I'll add one to the box next time I'll go to a customer...

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Thanks,
Fred.

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.

Thanks for putting me straight on that one!

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

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If it's regular X7R ceramics in 0402, 0603 or even 0805 it'll be ok at

1GHz. No need to swap those out.

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[...]

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*

[...]

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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.

Called a "gimmick"... a mainstay of Mad Man Muntz' IF alignment procedure ;-)

*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.

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 voilà, 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.

--
Thanks,
Fred.

This is actually how some RF mixer modules are aligned in production. By pushing and bending those wire stubs.

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 :-)

[...]

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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

Or build a filter with two peaks, one at 10MHz and the other at 50MHz. They are far enough apart, should work.

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You want real fun? Try to align a 'Tubular Filter' without a network analyzer.

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.....

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.

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