I am attempting to do some TOF experiments with TI's TDC7200EVM utilizing two RF signals - A Master Unit generates a 433 MHz pulse (ping) which is sent to the target (TOF1) whereby analog circuitry at the target generates the 315 MHz return pulse (pong) as quickly as possible (TOF2).
The TDC7200 Start signal is initiated with the 433 MHz Ping and the Stop signal is generated as quickly as possible upon reception of the 315 MHz pong from the target. Obviously, once the 433 MHz Ping is sent there can not be any clocked processing of the Ping/Ponged signal until the TDC7200 Stop input fires.
I believe I just need to generate an unmodulated 433MHz carrier Pong pulse and have analog circuitry on the target end detect the 433 MHz carrier, and then with as little delay as possible, use the 433MHz received signal (possibly amplified) to drive the 315 MHz transmitter on the target end, thus generating the 315 MHz Pong signal. Once the Master Unit detects the Pong signal it processes it as necessary and routes it to the TDC7200 Stop input to stop the counter and determine TOF.
I thought utilizing the 433/315 MHz Tx/Rx wireless com boards would be a good start to generate the Ping and Pong signal but the output pins of both the Rx modules output a digital square wave at approx. 4 KHz (max data rate) when the Tx module Data inputs are 0 waiting for my Start pulse on the front end going into the 433 MHz Tx. Once the Master unit initiates the Ping signal the returned Pong signal generates a digital 1. However, it appears to be clocked at a 4 KHz rate as you might expect and the Pong rising edge when it is detected fluctuates over 100usec. That will correspond to over 100,000 feet of error. Obviously, I should have expected this but after a 40 year hiatus from utilizing my EE degree I'm not too ashamed of my obvious oversight.
My spectrum analyzer arrives this weekend so I'll be able to verify if the RF output of the Tx modules is off when a digital 0 is applied to the Tx Data pins. If so, I can at least generate a pure 433 MHz pulse and the first half of my problem is solved. I was hoping some bright minds out there might recommend a highly efficient "turn around" circuit that just detects 433 MHz signal and can generate an input to drive the 315 MHz Pong transmitter. What ever this solution, is I know I can modify it to work similarly on the Master Unit side to detect the 315 MHz Pong and feed it to the Stop input of the TDC7200EVM.
For my TOF measurements to work with any consistency and accuracy, the components for the RF detection circuitry and "turn around" circuitry to generate the Pong signal are going to need a minimal number of components and pretty repeatable rise times from measurement pulse to measurement pulse as the signal is detected and processed. In a perfect world where the components delays were repeatable the added delays introduced can just be calibrated out. I realize all delays introduced are not going to be calibrated out and I have to accept that every nanosecond of uncertainty is going to generate about 1 foot of error in the measurement.
I would really appreciate any thoughts on how to potentially accomplish my goal of getting better than 1 inch resolution. The TDC7200 has a counting resolution of 56 psec (0.6 inches in terms of Speed of Light). Since laser measurement devices can resolve better than 1/16 of an inch this should be theoretically possible. However, the amount of circuitry required to generate a Start/Stop signal when firing a laser at a target and using optics to detect reflected light is going to be considerably less (= fewer induced timing errors) than what will be required to implement my concept. Thank you in advance to anyone who is willing to spend their time mulling over this challenge.