Method and apparatus for controlling a voltage controlled oscillator tuning range in a frequency synthesizer

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG.
1 of the accompanying drawings shows a block diagram of a frequency synthesizer 100 in accordance with a first embodiment of the present invention.
A reference frequency signal, f.
sub.
ref, generated from a reference oscillator 102 is received at phase detector 104.
Phase detector 104 phase compares the reference frequency signal, fref, to a divided output frequency, f.
sub.
v, received from a loop divider 106.
Phase detector 104 generates an error signal 108 which is transferred to loop filter 110.
The loop filter 110 generates a control voltage signal, Vctrl, in response to the error signal 108.
In accordance with the present invention, the control voltage signal, Vctrl, is used as an input to a plurality of voltage controlled oscillator (VCO) circuits 112 (VCO.
sub.
1 -VCO.
sub.
n).
In accordance with the present invention, a VCO control circuit 114 alternately enables each of the plurality of VCO circuits 112 through select ports (sel.
sub.
1 -sel.
sub.
n) in order to alternately generate output frequencies with which the synthesizer 100 can attempt to lack on frequency.
Thus, VCO control circuit 114 provides for the automatic selection of a VCO frequency from amongst a plurality of selectable VCO frequency ranges.
The plurality of VCO circuits 112 will also be referred as a VCO array 112.
The VCOs within VCO array 112 preferably have overlapping frequency ranges to provide a wideband tuning range for synthesizer 100.
The VCO control circuit 114 begins an iteration of alternately enabling and disabling individual VCOs within VCO array 112 in response to a trigger signal 115 generated from a trigger source (not shown), such as a controller.
The output frequency generated from a currently enabled VCO is fed through buffer 116 as output frequency, f.
sub.
out, and then fed back to loop divider 106.
Loop divider 106 receives inputs from a controller (not shown) which programs the loop divider to divide the output frequency, f.
sub.
out, by a predetermined amount (/n)