Phase indication by vectorscope

Posted on Feb 5, 2014

You will need an oscilloscope capable of displaying in X/Y mode, and a reference `LO` signal source. In-phase and Quadrature reference square wave signals are generated by a divide by 4, 2 bit ring counter using 2 D type flip - flops. So that I could use an RF generator as the reference source, the /4 is preceded by a /100 counter, so the reference signal input is at 320kHz for 800Hz audio input.

Phase indication by vectorscope
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Dividing down the generator output also allows adjusting the reference frequency in small fractions of a Hz, which is necessary to stabilise the display, unless reference and signal are phase-locked. The squarer circuit shown works with a few 10s of mV or more input. A DDS or other synthesizer is ideal, a stable VFO would be OK too. If you have a signal source that generates audio frequency quadrature outputs, the dividers are not required obviously. The I and Q reference signals drive two mixers, which are 2 analogue switches in a 4053 (sound familiar, Johan!). The input phase splitter circuit generates a push-pull signal for the mixers. The mixer outputs are filtered by single R/C low pass sections, which are adequate for casual experiments, and makes it easy to experiment with different bandwidths. A 1Hz or less bandwidth should be best for G4JNT`s 10s dot periods; to look at "normal" modulated signals like FSK or MSK, wider bandwidth is needed for the display to follow the signal, as shown on the diagram - 1meg/2n2 (70Hz) is about the limit before the display becomes excessively blurred by the 2f signal at the mixer output. For better noise rejection, higher order low pass filters could be used. The outputs are buffered by 2 op-amp followers, which are not strictly necessary if only a `scope is to be connected to the output. The circuit as shown runs on a single 5V supply - it requires low voltage type op-amps, the...

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