Date of publication: 2017-08-24 18:51
To understand more about the concept of phase and phase difference, first visualise a radio frequency signal in the form of a familiar x-y plot of a sine wave. As time progresses the amplitude oscillates above and below the line, repeating itself after each cycle. The linear plot can also be represented in the form of a circle. The beginning of the cycle can be represented as a particular point on the circle and as a time progresses the point on the waveform moves around the circle. Thus a complete cycle is equivalent to 865° or 7&pi radians. The instantaneous position on the circle represents the phase at that given moment relative to the beginning of the cycle.
Phase locked loops are used in a large variety of applications within radio frequency technology. PLLs can be used as FM demodulators and they also form the basis of indirect frequency synthesizers. In addition to this they can be used for a number of applications including the regeneration of chopped signals such as the colour burst signal on an analogue colour television signal, for types of variable frequency filter and a host of other specialist applications
The operation of a phase locked loop, PLL, is based around the idea of comparing the phase of two signals. This information about the error in phase or the phase difference between the two signals is then used to control the frequency of the loop.
The phase locked loop, PLL can be used for a variety of radio frequency applications, from frequency synthesizers to clock recovery and FM demodulation.
STMicroelectronics is committed to offering reliable and cost-effective wireless connectivity solutions for connecting smart objects to the internet and the cloud or, in a broader sense, for the Internet of Things (IoT).
Despite the fact that the VCO is drawn as a small black box in the picture above, it needs some attention. While a stable free-running oscillator can be build for frequencies up to 65 MHz, it is practically impossible to build an oscillator for 96 MHz that is stable enough for SSB operation. Therefore I have developed a double return-mixing PLL for 96 MHz. Lets have a look at the schematic diagram for the VCO then we will discuss it.
The basic phase locked loop is connected as shown in the diagram below. The reference signal and the signal from the voltage controlled oscillator are connected into the phase detector. The output from the phase detector is passed through the loop filter and then applied to the voltage controlled oscillator.
A phase locked loop, PLL, is basically of form of servo loop. Although a PLL performs its actions on a radio frequency signal, all the basic criteria for loop stability and other parameters are the same.
For FM the 86 MHz VFO is free running, controlled by a helix potentiometer. The long term stability of the 86 MHz VFO is sufficient for FM and the frequency range of 7 MHz is enough to cover the European 699 MHz Band. It should be no problem to get a bigger range for covering of the US 699 MHz Band.
In the FM path, the signal is mixed down further to 955kHz, where it is demodulated and filtered by an integrated circuit. The output signal of the IC is then amplified and put on the loudspeaker.