1 GHz synthesizer
in [Design]
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From: JosephKK on 26 Feb 2010 22:43 On Thu, 25 Feb 2010 19:44:17 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Feb 25, 1:45 pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 25 Feb 2010 10:07:22 -0800 (PST), George Herold >> >> >> >> >> >> <ggher...(a)gmail.com> wrote: >> >On Feb 25, 12:37 pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Wed, 24 Feb 2010 18:58:36 -0800, Joerg <inva...(a)invalid.invalid> >> >> wrote: >> >> >> >John Larkin wrote: >> >> >> On Wed, 24 Feb 2010 21:50:07 -0000, "Andrew Holme" <a...(a)nospam.com> >> >> >> wrote: >> >> >> >>> "John Larkin" <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote in message >> >> >>>news:u05bo5hknlma4qpda45muddb5tl5slkud2(a)4ax.com... >> >> >>>> On Wed, 24 Feb 2010 20:16:15 -0000, "Andrew Holme" <a...(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- Hide quoted text - >> >> >> - Show quoted text - >> >> >Hmm wouldn't a Q = ~5 high pass filter at 10 MHz followed by a Q = ~5 >> >Low pass filter at 50 MHz do it? Of course it would be relativly flat >> >in between. >> >> >(Or is that too simple?) >> >> >George H. >> >> That might work. Post a schematic! >> >> The ideal clock-input-reference filters have narrowband responses >> around the input frequency, as narrow as parts tolerances allow, to >> reject as much non-ref crud as possible. You want to be operating on >> the flat part of the frequency response curve, so's not to convert any >> amplitude or temperature effects into phase shift... so too much Q is >> bad too. That suggests higher-order filters in extreme cases. >> >> At 10 MHz, we usually use a simple R-L-C with a Q around 5. That also >> allows ugly inputs, like square waves that ring and whatever. >> >> John- Hide quoted text - >> >> - Show quoted text - > >That might work. Post a schematic! > >Ouch, sorry John, I was speaking theoretically. I never designed any >RF filters. Ive only been mucking about at audio frequencies where >with opamps it would be dead easy. > >George H. Here is a deal, do it for 1 kHz and 5 kHz instead and let others scale it. |