Posted on Feb 7, 2014

It is a programable frequency multiplier, usually using digital logic integrated circuits. The synthesizer is aranged to multiply a reference frequency by a programable amount to achieve just about any frequency you want. If, for example you had a reference frequency of, say, 1KHz and a `programable multiplier` then you could program the multiplier to give you

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1KHz (X1), 3KHz (X3), 1. 025MHz (X1025), 98. 325MHz (X98325) or any other frequency you want. Sounds easy Actually, it is so easy, once you understand some of the basics. So let us start off by building a simple synthesizer that cover 3KHz to 4000KHz (4MHz). A Phase Locked Loop (PLL) consists of a Voltage Controlled Oscillator (VCO), the output frequency of which is monitored and controlled. An error voltage steers the VCO and brings it back onto the correct frequency. The error voltage is generated by a Phase Sensitive Detector (PSD) which compares the VCO frequency with a reference frequency. Consider the following block diagram: Here we have a 1KHz reference oscillator feeding the PSD Input-B, and a VCO feeding the PSD Input-A. The output of the PSD can be a square-wave and the difference between the positive pulse and the negative is averaged by the low-pass filter (LPF). This filtering results in a DC voltage that is fed back to the VCO to increase or decrease the frequency, as required. It is interesting to note that you can put a DC voltmeter on the DC control line and watch the DC vary. The voltage will vary if either the VCO centre-frequency changes or the reference frequency changes. This circuit alone can be built using a single CD4046, but without the reference oscillator. The reference osc input can then be an external connection to make a selection of usefull instruments, such as: The PSD signal input of the...

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