Phase indication by vectorscope

Description: An oscilloscope capable of displaying in X/Y mode and a reference "LO" signal source are required. In-phase and quadrature reference square wave signals are generated using a divide-by-4, 2-bit ring counter with two D-type flip-flops. To utilize an RF generator as the reference source, this division by 4 is preceded by a division by 100 counter, resulting in a reference signal input of 320 kHz for an 800 Hz audio input. Dividing the generator output allows for fine adjustments of the reference frequency in small fractions of a Hz, which is essential for stabilizing the display unless the reference and signal are phase-locked. The squarer circuit functions with input levels of a few tens of millivolts or more. A Direct Digital Synthesizer (DDS) or similar synthesizer is ideal; a stable Voltage-Controlled Oscillator (VCO) would also suffice. If a signal source provides audio frequency quadrature outputs, the dividers are unnecessary. The I and Q reference signals control two mixers, implemented as two analog switches in a 4053 integrated circuit. An input phase splitter circuit generates a push-pull signal for the mixers. The mixer outputs are processed through single resistor-capacitor (R/C) low-pass filters, which are sufficient for casual experiments and allow for experimentation with various bandwidths. A bandwidth of 1 Hz or less is optimal for G4JNT's 10-second dot periods; however, for examining "normal" modulated signals like Frequency Shift Keying (FSK) or Minimum Shift Keying (MSK), a wider bandwidth is necessary to ensure the display can accurately track the signal. The diagram indicates that a 1 MΩ/2.2 nF (approximately 70 Hz) filter is about the maximum before the display becomes excessively blurred by the 2f signal at the mixer output. For improved noise rejection, higher-order low-pass filters may be employed. The outputs are buffered by two operational amplifier followers, which are not strictly necessary if only an oscilloscope is connected to the output. The circuit operates on a single 5V supply, requiring low-voltage operational amplifiers; the CMOS types shown are suitable. Standard 741 op-amps will not function unless a split-rail power supply is used for the analog components. Implementing a split-rail supply would be advantageous as it eliminates the +2.5V DC offset present at the outputs. In such a case, a +/- 5V supply should be utilized, with the -5V rail connected to the negative supply pins of all op-amps and the Vee pin of the 4053. Additionally, the non-inverting input of the lower op-amp in the phase splitter should be connected to 0V. The maximum total supply voltage for the 4053 is 15V. The input audio signal should be a few volts peak-to-peak, with the output matching the input amplitude, necessitating the oscilloscope to be set to 1 or 2V/div. Without an input signal, the oscilloscope controls should be adjusted to center the spot on the screen. When a Continuous Wave (CW) signal is applied, the distance the dot moves from the center indicates the amplitude, while the phase difference is represented by the angle between a vertical line and a line drawn from the center of the screen to the spot. If a CW signal with a slight frequency deviation is introduced, the spot will orbit around the center of the screen, illustrating the phase trajectory of an angle-modulated signal, which can be complex with MSK.

The circuit described is an implementation of a phase detection system using a dual-channel approach, leveraging both in-phase (I) and quadrature (Q) signals to analyze modulated signals effectively. The use of a 4053 analog switch allows for efficient switching between different signal paths, facilitating the mixing process essential for phase detection. The phase splitter's push-pull configuration ensures that the mixers receive balanced signals, optimizing performance and reducing distortion. The single R/C low-pass filters serve to smooth the output, allowing for easier visualization of the modulated signals on the oscilloscope.

For applications that require higher fidelity or precision, consideration should be given to the implementation of higher-order filters, which can significantly improve the system's noise rejection capabilities. The choice of operational amplifiers is critical; low-voltage CMOS op-amps are recommended due to their compatibility with the 5V supply and their ability to operate effectively in low-power applications. The design's flexibility in terms of bandwidth adjustment allows users to tailor the system to specific signal types, enhancing its versatility for various modulation schemes.

In summary, this circuit is well-suited for educational and experimental purposes in signal processing, particularly for analyzing phase-modulated signals. Its design accommodates a range of input conditions and provides a clear visual representation of signal characteristics, making it an invaluable tool for understanding complex modulation techniques.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.

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 CMOS ones shown are fine. 741s and the like definitely won`t work - unless you use a split rail power supply for the analogue parts.

This would actually be better, since this would remove the +2. 5V DC offset on the outputs that exists at the moment. If you do this use +/- 5V, and connect the -5V rail to the negative supply pins of all the op-amps, and to the Vee pin of the 4053. also, connect the non-inverting input of the lower op-amp in the phase splitter to 0V. The maximum total supply you can apply to the 4053 is 15V. The input audio signal should be a few V pk-pk. The output will be the same amplitude as the input, so the scope should be set to 1 or 2V/div. With no input signal, set the scope controls so that the spot is in the centre of the screen. If a CW signal is applied, the distance the dot moves from the centre is the amplitude, whilst the phase difference is the angle between a vertical line and a line drawn between the centre of the screen and the spot.

If a CW signal is applied with a slight frequency difference, the spot orbits around the centre of the screen in a circle. The display shows the phase trajectory of an angle modulated signal, which can be quite intricate with MSK.