A Frequency Synthesizer for the VHF FM Band

摘要

The first stage was the construction of a keyboard controlled frequency synthesizer covering the range 88 to 108MHz with a channel spacing of 100kHz. This synthesizer should in no way be confused with the VHF DSP receiver we described more recently (Slifkin, Mayan and Globinsky, VHF receiver in DSP Electronics World, October 1999) which relies on completely different principles. We have yet to complete the whole project but feel that the synthesizer is a worthy project in its own right. Originally, frequency synthesizers were derived from a low frequency crystal oscillator by using multipliers. These were resonant circuits tuned to a low harmonic of the input frequency. Radio amateurs found these particularly useful as the amateur radio frequencies in the past were 3.5, 7, 14, 21, and 28MHz. So, from that one crystal oscillator, they could access all the higher amateur radio bands. However, with the appearance of IC phase lock loops, these have more or less fallen into disuse. We have discussed the phase lock loop (pll) several times in our earlier articles. However for new readers we will give again a brief description. Figure 1 shows a block diagram of the pll.