Hi Charles & Bruce
I'm not good at analog circuits. My circuit is modified from wenzel's, since RF pnp transistor is harder to get. I would like the front end works at 300MHz. My questions: 1) why the difference of DC bias of the 2 NPN matters? I thought only the frequency part is useful to a counter, amplitude information is useless right? 2) what's is the C4 in your circuit for? 3) If the noise is more important than the gain, what kind of transistor should I choose? The Ft near 300MHz ones(BFS17, 2SC9018) or Ft far beyond 300MHz ones(BFP420, BFP183,BFR93) ? Thanks 2014-12-26 4:31 GMT+08:00 Charles Steinmetz <csteinm...@yandex.com>: > In reply to Li Ang, Bruce wrote: > > The CLK1 input circuit produces an output incompatible with the 3.3V CMOS >> device it drives.A pair of pnp transistors in an otherwise similar circuit >> is capable of producing a 3.3V CMOS compatible output signal. >> Using independent voltage dividers to bias the transistor bases is a bad >> idea in that resistor tolerances may lead to a dc input offset of several >> tens of millivolts even with 1% resistors, >> > > I agree with Bruce. The circuit below avoids these problems. It is > generally known as a "Wenzel squarer" (after Charles Wenzel, who > popularized it -- see <http://www.wenzel.com/library/time-frequency- > articles/waveform-conversion-part-i-sine-to-square/>). I revised and > simulated a circuit I use all the time for 5v output to produce a 3v > output, but I did not build it, so some adjustment may be required. All > resistors should be metal film. The 1uF capacitors should be X7R, and the > 100nF and 10nF capacitors should be NP0/C0G. "Design center" for this > circuit is a 10MHz input at 1Vrms (not shown is the 50 ohm input resistor > that would terminate a 1Vrms, 50 ohm source). > > Note that the circuit needs some "overhead" voltage to bias the PNP > devices, so a 5v power supply is shown. Both this supply and the base > reference supply need to be quieter than the precision you expect from the > circuit. The decoupling shown should be significantly better than required > for your purposes here, but keep this in mind if you push on to > significantly higher resolution. > > I don't know what the highest frequency you expect to count is. The 3906s > are as fast as the 3904s you are using, and are fine at 10MHz and even up > toward 100MHz -- but at some point you would need faster transistors. The > MMBT5179 and BFT93 are two possibilities. Note that these faster > transistors also reduce the sloping of the top of the "square" wave output > (which is due to capacitive feedthrough of the B-E junction of the input > transistor). With less current available from this feedthrough (due to > lower junction capacitance), you will probably need to increase the output > resistor (R6) to achieve a full 3v output level if you use faster > transistors. [Note -- the attached simulated scope traces show the output > (Q2 collector) and reference (Q2 base). You want to keep Q2 out of > saturation, so the peak collector voltage needs to be no more positive than > the base voltage, as shown.] > > Note also that the input capacitor and the emitter coupling capacitor > limit the lowest frequency you can count. > > Finally, note that the circuit does not have a lot of gain, so the > low-signal limit is higher than on most commercial counters. A 0.2Vp-p > input produces a 0-2.6v, mostly sinusoidal output. By 1Vp-p input, it is > nicely square. I'd limit the input voltage to ~5Vp-p. If you need better > sensitivity, you can add a preamp. (If this were to be used as a > general-purpose counter, I'd design a limiting preamp for the input.) > > Best regards, > > Charles > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.