[time-nuts] schematics of frequency counter

[Magnus Danielson]

Hi,

On 12/26/2014 02:38 PM, Li Ang wrote:
> 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?

For time-interval measurements, offset errors can translate to time 
errors. HP filed a patent for a calibration devise and compensation 
routines that aimed to separate offset errors from time offsets, such 
that proper compensation can be done. This was done for the HP5370A in 
mind. The calibrator consists of RF phase-splitters and RF-relays such 
that one can attempt different polarities on the inputs.

Naturally, these offset can depend on temperature, and you want to make 
sure that temperature stability of your input and input offset is low 
enough not to completely spoil your measurements.

So, I agree that you would like to make sure that both sides of the 
differential pair should have a common voltage bias source, to make sure 
that the nominal current splitting between the transistors is about the 
same and thus the nominal offsets of the I/V for the NP-junctions 
balance each other out... and those is temperature sensitive. Similarly, 
usually you want both inputs to see about the same source resistance, in 
order to reduce offsets.

> 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) ?

You attempt to gain yourself out of having noise be the dominant source. 
A too high bandwidth will open up unnecessary noise, as it increases 
with bandwidth, at the same time you want high enough bandwidth for the 
output to support the slew-rate you want. For clock signals, a single 
amplifier stage usually suffice, but for lower frequency signals as 
coming from the beat frequency in a DMTD setup you want several stages 
of increased bandwidth and slew-rate.

The difficulty of the gain approach is that no amplifier will be optimum 
for a large range of frequencies. Look at the CNT-90/PM6690 for 
instance, it even had a dual bandwidth buffer-amplifier in parallel in 
order to achieve good performance even below 10 kHz.

Cheers,
Magnus