[time-nuts] schematics of frequency counter

[Li Ang]

Hi Charles,
   In my circuit, the VCC is 5v. I've noticed my bias and emitter resistor
is something need to be changed. I will play with the resistors and see if
it improves. Thanks.

2014-12-27 6:42 GMT+08:00 Charles Steinmetz <csteinmetz at yandex.com>:

> Li Ang wrote:
>
>  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?
>>
>
> You want the circuit to switch near the mid-point of the input sine wave,
> and at exactly the same place every time.  How you bias the transistors
> determines how well this is accomplished.
>
> You also want the output to switch fast and cleanly between a low voltage
> very near 0v ("ground") to a high voltage very near 3v (Vcc, logic high).
> An NPN cannot do that, biased the way that you have them connected (the
> emitter of the output transistor Q301 can only pull the output down to a
> little less than 1v due to R315, which may sort of work but is not a proper
> way to run 3v logic).  This operation also saturates Q301, which is bad for
> performance.  See simulated results below.
>
> In order for an NPN to provide a useful output for 3v logic, (i) its
> emitter must be grounded, and (ii) it must either be run into saturation or
> use a Baker clamp.  Running the transistor into saturation must be avoided,
> particularly if you want to reach 300MHz, and a Baker clamp raises the
> "logic low" output voltage to >0.5v (not a good thing with 3v logic).  So,
> it is very much better to use a PNP differential pair.  For a 300MHz
> circuit, I would use BFT93 (and even that barely gets you to 300MHz).
>
>  2) what's is the C4 in your circuit for?
>>
>
> C4 makes Q1 and Q2 a differential (emitter-coupled) pair at RF
> frequencies, but not at DC.  So, the circuit has no gain at DC and
> therefore the DC errors between Q1 and Q2 cause much less output error than
> they would if the emitters were connected directly together.
>
>  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) ?
>>
>
> Far beyond.  The Ft is the frequency where a transistor completely runs
> out of gain.  You want to operate at a much lower frequency where the
> transistor still has substantial gain, particularly with fast RF
> transistors, which generally have much lower DC hfe than general-purpose
> transistors like 3904 and 3906.  Note that the simulation of the circuit
> you published (simulated results below) barely works at even 20MHz.  As I
> noted above, even the BFT93 barely gets you to 300MHz with a 1Vrms input.
>
> Best regards,
>
> Charles
>
>
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