[time-nuts] schematics of frequency counter
Magnus Danielson
magnus at rubidium.dyndns.org
Fri Dec 26 14:21:15 UTC 2014
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
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