25MHz 2dBm sine wave to logic conversion

to

MSA0404

struggling

Is there a good low-noise solution involving a broadband transformer e.g. wound on a ferrite bead? How about ditching the MMIC and going for a broadband step-up transformer, with load resistor, followed by the HCT schmitt trigger?

There are plenty of comparators that'll handle the task. For example, in the older high-voltage realm, there's the LT1016 and the TL3016. I also like the MAX903, which I'm using for a similar job at 80MHz.

Or you could bias an unbuffered CMOS inverter into the linear region, and ac-couple the signal to it. A second inverter will sharpen the risetime.

Use a fast comparator or, even better, a 5-volt LVDS line receiver.

For the LVDS, offset your sig to +2.5 volts and apply to the + input. Feedback the output to the - input through a slowish R-C lowpass. That will servo the input threshold such as to make a 50% output duty cycle output. Shouldn't need the MMIC. Expect under 10 ps RMS jitter if your input's that good.

Actually, you could probably get away with just applying your sig to one input and grounding the other. The LVDS receivers seem to work fine comparing around ground.

National, Fairchild, TI, ONsemi make LVDS receivers. They are fast and cheap.

John

Only the power necessary to operate at 25MHz. A 250mV signal is more than enough to drive the inverter out of class A.

Yes, we're putting in some cool stuff, like actual measured MOSFET gate-voltage to drain-current transfer characteristics for the N- and P-channel FETs in the unbuffered CMOS logic ICs, both for older HC types and for the newer low-voltage CMOS processes. Want to know the class-A current for different voltages? Read it off the plots.

With respect to my class-A argument, I stand corrected. As I mentioned, we've taken considerable data, but I was going from memory in making my remark. Looking at my supply-shoot-through current curves, I see they're more like those shown by Philips. However, note that the 11mA current we measured at 5V is only 60mW, which isn't that toasty. :>) Also, we measured 2mA at 3V and Philips shows 0.5mA at 2V, so the staggered-voltage trick could be used to reduce the supply current.

Yes, it would be nice, but it's beyond what we'll be able to do with our limited time and more importantly, limited sample set.

The feedback resistor value isn't critical, if it's not too small.

No. You should establish a jitter spec and a way to measure it, so you don't over-specify and reject an other-wise good solution. That said, the jitter is easily understood by analyzing the noise density of the amplifier performing the limiting function. The CMOS MOSFETs are running at fairly-high currents (we observed 11mA for an U04 at 5V), which does lower the FET's e_n, but perhaps not as low as a BJT running at say 500uA in a high-speed comparator.

Yes, I was going to mention that one too.

No need to broadband with differential pair, and Schmitt trigger is the last component you want to use for low jitter output. Lowest noise would be 50 ohm input single transistor CE with reasonably high impedance inductive load terminated in 2.5V ( a simple variant of broadbanded video CE amp) driving logic buffer.

Hello Winfield,

Ah, my kind of stuff. It will burn a bit of quiescent power though since he wants to reach 5V. 74HCU04 and the like can get pretty toasty.

Alternatively, Andrew could use a BFS17 or two to jazz up the amplitude and then run it into an inverter.

BTW, some info on "not so digital" uses of HCU and other chips would be nice for the next edition of AoE. And no, I am trying hard not to ask again when it'll hit the book stores.

Regards, Joerg

The hcU04's tranconductance g_m = 30mS at 5V supply, so its gain at frequency f is G = g_m Xc = g_m / (2pi f Cnode), which is not too shabby. For example, G = 19 at 25MHz into a 10pF node capacitance. Andrew's 250mV signal (125mV peak) has a zero-crossing slew rate of S = 2pi f A = 20V/us. According to the Philips' datasheet, the U04 inverter's 25MHz gain of ~20, which would amplify this to 400V/us, implying a 10ns risetime for the middle 4V of the swing. A second inverter stage would further sharpen this up.

Hello Winfield,

Not really for the unbuffered devices. Look at figure 8:

That is great. One piece of info that would be nice is the min and max specs on class A current. Data sheets like the one above only state typical values and that is not quite enough information for a design that goes into production.

Regards, Joerg

Hello Winfield,

Well, one of the newfangled TSSOP packs can get a bit hot. 2V would work but the 74HCU04 becomes really slow then, unless you take newer faster TI low voltage logic. More $$$ though.

I'd probably opt for a discrete design in this case.

No, I didn't mean sampling at your lab. That's hard to do because you may not be able to obtain the process extremes. But you could try to cajole the process mins and maxes out of a manufacturer with good specsmanship (but a sluggish website...) such as Philips. Your name carries a lot more clout with it while us regular guys will usually be turned down. Tried many times. Even the notion of the potential truck loads in sales volume or a letter to the big brass doesn't sway them :-(

Thing is, the minute you are facing a regulated design history path, and nowadays that's nearly all of my designs, the absence of hard data on min and max ratings is usually the end of the road.

Regards, Joerg

Hello Andrew,

Solderless breadboards give me the goose pimples. Wait until you see it perform on a real board ;-)

The 74HCU04 is unbuffered, meaning it doesn't have much gain. So the slopes will be shallower than with an 74HC04. Jitter and noise should be fine, unless your application is really critical in which case I'd use discrete fast transistors. With logic it greatly depends on the quality of VCC and ground. Use at least one small ceramic SMT 0.01uF really close to the VCC pin, and a solid ground plane.

Much cheaper, too. Also, you can "rent out" the remaining inverters in there.

Regards, Joerg

Thanks Win, and everyone else, for the suggestions.

I've got an MM74HCU04N stuffed in a solderless breadboard - with all the strays that entails - and it puts my old circuit to shame. Experimenting with different feedback resistors, I'm seeing 80% rise times below 4ns on my

I've also got an AD8561 comparator, which is billed as an "upgrade for LT1016 designs", but I prefer the HCU solution.

Hello Winfield,

Yes, if you don't need to get too close to the rails with the output of this amp it is indeed not too shabby. The beauty of such solutions is that the 2nd stage costs next to nothing because it has six inverters. Or if it has to be priced out it would be 1/6th of 10 cents ;-)

Regards, Joerg