Thursday, 20 September 2012

1milli-second counter with 1ns resolution

To start with I am not too sure if this will work if it does i am still not sure if this will help me reach my goal. For my DIY GPR I need a way to count really really small time units. So i can determine how long the reflected radio wave have taken to reach an object bounce off it and return to my antenna.

 So I have devised a possible solution to solve my problem. what i have is a 1Ghz oscillator ASG-P-X-A-1.000GHZ, this device is $28au from digikey it has LVDS output which is nice for my first UHF PCB. coupled to that i have a MCA100EP016AFAG 8 bit counter capable of 2.2Ghz.

I started to lay the board out then i realized that if i make this board i will have spares as its a prototype for my GPR so i may use it somewhere else in the future if all goes well, so i changed the PCB from 50x50 to fit a arduino MEGA footprint. this would allow me to Hack on either an arduino MEGA or a MAX32. once i found the footprint of the MEGA (see my previous post) i noticed that i have Stacks of free space on the board so i thought about what else this counter might need.. i started with space for 16Mbytes of FLASH serial SPI memory, then i my attention moved to how i was going to trigger the counter to start and stop so i can read it at my pleasure as this counter will be configured as a single shot to avoid having to incorporate highspeed RAM to store results in, i figured a simple counter on the enable line might do the trick, so i have added a decade counter 744017and a  four element SR latch 74279.

As the Counter and oscillator are 3.3v devices this board really shouldn't be used on a mega, i will only use it with a MAX32 as they are a 3.3 volt board.

With the SR latch and the decade counter being stock standard 74 series logic chip i could replace these with any logic chip from that family. i have only wired the ground and power pins the others go to pads so they can be rats nested into any configuration that i may need in the future..

I also have a alternate Voltage regulator for 3.3v a stack of decoupling caps, the voltage regulator is either supplied off the arduino 5v rail or its own input header.the regulator accepts any 78xx SOT223 regulator that has a IN-GND-OUT pin footprint.

So the plan seems sound enough for now, i will spend $40 and get 10 of these boards made once i have laid out all the tracks. hopefully i will be able to get the oscillator running, the counter counting, then i can see about starting and stopping the counter with a signal. my plan is to use the SR latch to latch the enable pin to high (its active low) then when a ping is sent i release the counter to ... count :) then when a echo comes along we can stop it. read the time and reset everything and start again, this time we use the decade counter to miss/ignore the first echo and capture the second echo, this may require me to pre-seed the decade counter before i send a ping, but this board is so generic i can work that out while i wait for the oscillator and counter to arrive..





thats what i have laid out so far. just a few tracks here and there and a carefully placed ground plane and i should be good to send it off to the factory in china to be made..

I have picked my pins i hope carefully, so all the important information can be read and written in single word instructions, i have the first 8 bits of the 1Ghz counter on one port the next 12 bits are on another port and the preset data are on two ports of the PIC, this means some are side by side and others are spread across the PCB. the pins for the SPI flash chips are broken out so i can solder thwm to where ever i like. this is not the main purpose of the board so they will need to be rats nested to the MAX32 pins. you may notice that there are places for two variable resistors, for my initial purpose these pins are pulled high, but other oscillators use these pins to set the frequency, in case i find 1ns is not fine enough resolution i can simply change parts without needing fresh boards to be made or horrible hacks near the RFHF's in that area..  i will enclose the oscillator and support components under a tin can to keep the signal from radiating everywhere and potentially causing problems.

2 comments:

  1. I posted earlier that I had built one and can give you some clues, perhaps you missed it? I may also be interested in some of your for sale parts.
    Steve.

    ReplyDelete
  2. Thanks for sharing these great quotes.
    This is a fantastic website, could you be interested in going through an interview concerning just how you made it? You can visit my site.
    changeparts

    ReplyDelete