> I did see a reduction in desense when I replaced the multiplier type > exciters with the PLL exciters. > > 73 - Jim W5ZIT
Yup... The biggest advantage of the GE VHF PLL is the reduction in side-band noise over the standard phase-modulated multiplier type. On a typical 600 kHz two meter split, 22 dB of side-band noise reduction is afforded with the use of a PLL exciter. How? Any fundamental oscillator has inherent noise. When this oscillator is multiplied in frequency, so is any noise or frequency shift. While the crystal oscillator is elementary low in noise, in a GE VHF exciter the crystal gets multiplied 12 times, and so does the amount of fundamental noise. Along with the multiplication of fundamental noise, each multiplier stage in itself adds some noise as well, as any active stage has a noise figure. The PLL exciter uses a L-C tank operating AT the transmitted frequency. The noise figure of this tank is fundamentally lower, because it's not been multiplied up to the final frequency. The GE PLL exciter reference is a crystal held within a FM ICOM. The reference crystal is multiplied up to the final transmitter frequency and this frequency is compared with the Phase Locked Loop against the free running L-C tank. When the PLL locks, the free running L-C oscillator has the frequency stability of the crystal reference, but doesn't have its inherent multiplied noise. How? The PLL is designed so its ability in adjusting the frequency of the tank is slow enough that it cannot follow noise generated by the crystal reference or its subsequent multiplication. It is, however, designed so that it is fast enough to follow the audio modulation of crystal reference, in this case, the FM ICOM. The result is a transmitter that has considerably lower side-band noise, excellent FM audio characteristics, and the same stability of a crystal. General Electric produced documentation that shows the advantages where noise is concerned. Jeff DePolo assembled this information in an easy to follow page: http://www.repeater-builder.com/pdf/GE_Isolation_Curves.pdf Hope this helps... Kevin Custer
