I was about to comment on this. As you interpolate among the 8 phases,
time errors in the routing might need to compensated for in order to
represent a "flat" stepping of time-compensation. It will not be perfect
naturally.
Cheers,
Magnus
On 11/23/2014 03:57 PM, Anders Wallin wrote:
Anders,
The counter runs on a Pipistrello. I looked at the information on the web
about time taggers before starting. I decided to try an oversampling
scheme described by a group of Italian? physicists for a multichannel time
tagging instrument. They used 4x oversampling. My version is crude; it
uses the 50 MHz on-board clock but of course could use an external clock
source. The clock is multiplied to 1 GHz and then divided into four 125 Hz
clocks phased 45 degrees apart. There is a fifth 125 MHz clock at 0 phase
for the main counter and external interface.
There are four channels, each with 3 bits for value and a forth bit
indicating an event. The sixteen bits are followed by a 48 bit counter
value.
what, if any, signal conditioning do you have between the DMTD output and
the FPGA? I was thinking about copying the CERN DIO design which looks like
this:
http://ibin.co/1iEwLuAUQYJ4
it has a fuse, a resistor to set the input impedance, protection diodes,
and an ADCMP604 that outputs an LVDS pair to the FPGA.
The CERN design is for a 125 MHz clock. What would be the preferred way to
generate this for the Pipistrello, with an optional 10MHz reference input?
OCXO at 10MHz and a ADF4351 PLL+VCO up to 125MHz? Does someone have a
tested circuit that autodetects the external 10MHz and can switch between
the OCXO and ext-ref?
This yields 1 ns resolution (bin size) but the bins sizes are certainly
not all equal. I have few means to check the accuracy but for my purposes
(logging 100 Hz to 1 Hz zero crossings of a DMTD) it is certainly more
accurate than I need. I have experimented with .5 ns bin sizes, also using
the 8x oversampling with a 250 MHz clock. To keep the backend 125 MHz
structure I used a two phase multiplexer to combine two successive samples.
This runs but is not reliable and needs further work before it's useful.
Did you post the schematic for your DMTD?
Many of the time-to-digital papers calibrate the bin-width by collecting
time-stamps from an asynchronous pulse-source. If the bins are equal you
should get a flat histogram. Some use a ring-oscillator on the fpga for
generating the asynchronous hits.
Anders
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