Hi, On 12/26/2014 02:38 PM, Li Ang wrote:
Hi Charles & BruceI'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 _______________________________________________ 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.
