Comparing signals from two receivers
I have two receivers so thought it would be interesting to put one in a good reception position, approx 2 metres away from any electrical equipment, and another on my lab desk and compare the signals from each. As has been observed previously receivers near my computer monitor shows rapid flickering and loss of lock whereas the ones near the lab window shows a contented pulsing as it receives the time signal.
Control configuration – Two receivers, both away from noise
By way of a control I first put both receivers away from PC equipment and compare output from each receiver.
Here we see two nicely correlated traces. The amplitude differs because each has an independent supply, one being powered from the 5v USB supply of the PC and the other by 2 AA-cells but otherwise these signals are clean of noise and illustrate that when well sited the receivers can produce a clean, easily decoded output.
Moving one receiver closer to the computer – Feel the noise!
Here we see the clean signal at the bottom whilst at the top the signal is blown away by the computer induced noise. No chance of decoding the top receiver!
A closer inspection shows that the two peaks in the lower trace, this is an A=0,B=1 second are not easily visible in the top trace. There is some information in the top trace but recovery looks challenging.
Looking at some other examples and differencing the traces, see middle line, shows that in the upper trace some features have been completely erased by the noise. No amount of signal processing will recover those!
Again, here we see the top trace showing no sign of the 100mS pulse in the lower, the trace has effectively flat-lined for the first half of the measurement missing the 100mS entirely, we can assume that both A and B bits would be unrecoverable in this scenario.
Moving the roving receiver to a position that is mid-way between the computer and the lab window, i.e. middle noise levels, shows a different situation. Here we see noise on the top trace but the information is clearly present and some form of integration/low pass filter may be able to recover it. However the width of the pulses is not consistent, the bottom trace might be A=1, B=0, whereas the top trace looks more like A=0,B=0. Looks like these pulses really aren’t able to cope with much real-world interference before they become unrecoverable.
When a clear signal is present the receiver does a good job of filtering the input and presenting a precise representation, near sources of significant interference, (computer power supply, computer monitors, computer keyboards?) the noise can be significant enough to make recovery of the signal impossible, implementation of clever digital filtering techniques seems pointless. However, when the receivers are adjacent but not too close to noise sources creates a mode where the MSF signal may be recoverable with a degree of signal conditioning.
To start with I think I will go for the simple case where noise has been effectively discriminated by the receiver, the aerial having been located in a favourable position, later I may look at averaging to increase reliability in the presence of moderate noise. Tricky stuff!