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High-speed counter for frequency counter project

Discussion in 'Electronic Design' started by Ben Jackson, Jul 5, 2006.

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  1. Ben Jackson

    Ben Jackson Guest

    A while ago someone asked for a way to count at 100+MHz. I mentioned
    that I had made a counter with serial output using a MAX3000A series
    CPLD.

    I have been playing with a prototype of it now for a few weeks, and
    it seems to work, so I wrote it up so other people can use it in their
    frequency counter projects:

    http://ad7gd.net/counter/

    Now if someone would like to contribute a 200+MHz analog frontend, I
    may finish off the project with a controller + LCD display. :)
     
  2. jure

    jure Guest

    Ben,

    for the front end, you could use the minicircuits 50 Ohm gain
    blocks, I think that the ARRL's Radio amateur's handbook shows a
    circuit for a counter with such a front end.
    Don't recall which edition (year) though ...
     
  3. Luhan

    Luhan Guest

    Hi,

    I've never worked with CPLD's. How would I get started?

    By the way, I get 50Mhz out of a PIC with no external logic.

    http://members.cox.net/berniekm/super.html

    Luhan
     
  4. Ben Jackson

    Ben Jackson Guest

    I tell you where to get the software and what hardware you'll need on
    the page.
    My first draft went into more detail on the limits of counting with a
    PIC, but you can reach 50MHz with a suitable combination of CPU clk
    and scale factor. I took a quick look at your source and I see you're
    using a 256x prescale, which makes sense.

    The advantage of the external counter is higher resolution (assuming
    you have a good enough timebase, of course).
     
  5. budgie

    budgie Guest

    MC10216, cascaded stages. Good to >500MHz. JimT's probably got a heap left ;-)
     
  6. Guest

    I've got two from a 1989 project - one designed as a 50R termination,
    and one that looked like a classical 1M//10pF scope probe termination.

    How much sensitivity do you want, and how much over-voltage protection?
     
  7. pi

    pi Guest

    Nowadays there are many prescalers in the range of 500Mhz-3Ghz and
    upper.
    Some of them inside a PLL chips, others as individual devices.
    A possible goal is to be able to measure correct any RF signal at any
    levels between 0dBm to -120dBm with an overload of +10dbm.

    greetings,
    Vasile
     
  8. vic

    vic Guest

    You can have a resolution down to 1Hz (assuming a 1s gate time) with a
    pic too, even with a prescaler.
    There is a little trick to it : wire one if the IO pins to the counter
    input. When you've reached the end of the time window, disable the
    normal input and start feeding pulses until the prescaler overflows. You
    can then deduce the start value of the prescaler.

    It seems that's what Luhan has done in his schematics.

    The same trick would probably be applicable to high speed prescalers if
    you need really high resolution.

    vic
     
  9. Luhan

    Luhan Guest


    That is exactly right, and all done with one 470 ohm resistor. Timer-0
    is fed thru input RA4 for exactly 1 second. Then RA5 overpowers the
    input and starts driving extra pulses in until the overflow flag is
    set. This allows 50Mhz counting down to 1Hz.

    I did not originate this trick, but most of the other stuff is original
    with me.

    Luhan
     
  10. Jim Thompson

    Jim Thompson Guest

    Nope, don't have any of those.

    ...Jim Thompson
     
  11. Joerg

    Joerg Guest

    Hello Luhan,

    It requires a design suite pretty much like uC do. But the design
    methods and thinking processes are totally different. I designed most
    programmable logic via schematic entry but nowadays VHDL is the game.

    Running the LED with the port devices as a current limiter, wow, that's
    pretty brazen. Like relying on a governor to limit the engine rpm. Ouch...

    Just one word about that LDO in case you hear about problems: I believe
    it was the 2931 that I observed mis-behaving (oscillating) when the
    internal resistance of the power source became too high.
     
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