examples: add TDR toy example

examples/master/repository/tdr.py

@@ -0,0 +1,75 @@
+from artiq import *
+
+
+class PulseNotReceivedError(Exception):
+    pass
+
+
+class TDR(EnvExperiment):
+    """Time domain reflectometer.
+
+    From ttl2 an impedance matched pulse is send onto a coax
+    cable with an open end. pmt0 (very short stub, high impedance) also
+    listens on the transmission line near ttl2.
+
+    When the forward propagating pulse passes pmt0, the voltage is half of the
+    logic voltage and does not register as a rising edge. Once the
+    rising edge is reflected at an open end (same sign) and passes by pmt0 on
+    its way back to ttl2, it is detected. Analogously, hysteresis leads to
+    detection of the falling edge once the reflection reaches pmt0 after
+    one round trip time.
+
+    This works marginally and is just a proof of principle: it relies on
+    hysteresis at FPGA inputs around half voltage and good impedance steps,
+    as well as reasonably low loss cable. It does not work well for longer
+    cables (>100 ns RTT). The default drive strength of 12 mA and 3.3 V would
+    be ~300 Ω but it seems 40 Ω series impedance at the output matches
+    the hysteresis of the input.
+
+    This is also equivalent to a loopback tester or a delay measurement.
+    """
+    def build(self):
+        self.attr_device("core")
+        self.attr_device("pmt0")
+        self.attr_device("ttl2")
+
+    def run(self):
+        n = 1000  # repetitions
+        latency = 50e-9  # calibrated latency without a transmission line
+        pulse = 1e-6  # pulse length, larger than rtt
+        try:
+            self.many(n, seconds_to_mu(pulse, self.core))
+        except PulseNotReceivedError:
+            print("to few edges: cable too long or wiring bad")
+        else:
+            print(self.t)
+            t_rise = mu_to_seconds(self.t[0], self.core)/n - latency
+            t_fall = mu_to_seconds(self.t[1], self.core)/n - latency - pulse
+            print("round trip times:")
+            print("rising: {:5g} ns, falling {:5g} ns".format(
+                t_rise/1e-9, t_fall/1e-9))
+
+    def rep(self, t):
+        self.t = t
+
+    @kernel
+    def many(self, n, p):
+        t = [0 for i in range(2)]
+        self.core.break_realtime()
+        for i in range(n):
+            self.one(t, p)
+        self.rep(t)
+
+    @kernel
+    def one(self, t, p):
+        with parallel:
+            self.pmt0.gate_both_mu(2*p)
+            with sequential:
+                t0 = now_mu()
+                self.ttl2.pulse_mu(p)
+        for i in range(len(t)):
+            ti = self.pmt0.timestamp_mu()
+            if ti <= 0:
+                raise PulseNotReceivedError
+            t[i] += ti - t0
+        self.pmt0.count()