[time-nuts] schematics of frequency counter

[Li Ang]

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 <csteinmetz at 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
>
>
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