The recent proliferation of wireless communications applications such as in the cellular telephone industry has led to demand for more and more channels over which to carry information to accommodate the growing number of users. Meeting this demand has been difficult, notably due to the limited portion of electromagnetic spectrum allocated to the commercial communications industry.
One solution to the problem of increasing the amount of channels in the limited commercial spectrum has been to dramatically narrow the bandwidth allocated for each channel. For example, a 10 MHz channel bandwidth allowing only 10 channels to fit within a 100 MHz range can be reduced to a channel bandwidth of 1 MHz thereby allowing 100 channels to fit within the same 100 MHz range. This "squeezing" of channel bandwidth has increased the amount of available channels but has led to other problems such as electromagnetic interference (EMI) and radio frequency interference (RFI) between adjacent channels separated by the smallest of the bandwidths.
To compensate for the increase in channels with such narrow bandwidths, the circuitry designed for handling wireless communications must be extremely precise. With increased precision, however, comes increased costs that are ultimately passed on to the consumer leading to higher prices. Because of the high precision circuitry needed, the elimination of unwanted noise and interference in such circuitry is now more critical than ever.
One of the most critical components in communications circuitry is the phase-locked loop (PLL). The PLL enables communications equipment to quickly "lock" onto a specifically selected frequency, typically the carrier frequency over which communications are sent. This fast locking ability is particularly important for devices such as cellular telephones, where the telephone must almost instantly switch carrier frequencies when traveling through different cellular zones or "cells." An essential component of a PLL is a voltage controlled oscillator (VCO), whose output voltage is controllable by the application of an input control voltage. The VCO, however, is very sensitive to fluctuations in the control voltage. Sensitivity of a VCO is typically expressed as MHz per volt. Assuming a linear frequency change versus tuning voltage characteristic for a 1,000-2,000 MHz VCO, tunable over a voltage range of 0.5 V to 10.5 V, the sensitivity will be 100 MHz/volt. This can be expressed as either 100 KHz/mV or 100 Hz/microvolt. Pick ups in the order of a few microvolts on the control line of a poorly designed VCO are not uncommon. However, any interference of the order of 1 microvolt is unacceptable in a PLL of a precision communication circuit.
Moreover, when a VCO is used in conjunction with other circuits which are capable of radiating interference signals, the control input line of the VCO should be guarded from such interference. Pushing and pulling phenomena in VCOs are well known in the industry, and improper loading of the VCO can modulate the output signal and shift the frequency. Accordingly, there is a great need to buffer the VCO output.
Because of the high degree of spectral purity required in today's communications circuits, there is a need to provide extremely precise PLL circuits. Therefore, to meet the demand of modern communications, there is a great and longfelt need to provide improved PLL and VCO circuits to solve the problems associated with the generation of stable and highly pure output frequencies.