JPH0225113A - Frequency synthesizer - Google Patents

Abstract

PURPOSE:To obtain an open loop gain characteristic stable over a broad band by controlling the open loop gain of a loop filter of a frequency synthesizer by an open loop gain control signal in response to a frequency division control signal. CONSTITUTION:A phase locked loop controls a divided frequency N by using a frequency division control signal 8 from a frequency control circuit 7 and acts like a frequency synthesizer. In this case, switches S1A-S1D switching resistors R1A-R1D in an active filter 6a by using an open loop gain control signal 9 of the active loop filter from an external frequency control circuit 7 are controlled according to a frequency division control signal. That is, when a parallel combined resistance R1(N) of the resistors R1A-R1D switched on according to the frequency division number N of a frequency divider 3 is set in multi-stage according to the frequency division number N, even if the frequency division number N is fluctuated into 2XN, 3XN-, the fluctuation of the open loop gain KS of the synthesizer is suppressed by controlling the resistance R1(N) to be 1/2,1/3-.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a frequency synthesizer used in the microwave band.

[Conventional technology]

Figure 2 shows a conventional microphone A-wave frequency synthesizer. In the figure, 1 is a synthesized frequency signal;
2 is a voltage controlled oscillator, 3 is a frequency divider, 4 is a phase comparator, 5
is the reference frequency signal, 6 is the resistor R,,R,, capacitor C
3, etc., 7 is an external frequency control circuit, and 8 is a frequency division number control signal.

Next, the operation will be explained. The signal oscillated by the voltage controlled oscillator 2 is frequency-divided by a frequency divider 3 whose frequency can be varied by a frequency division number control signal 8 from an external frequency control circuit 7. 4 for comparison.

Due to the difference between these two frequencies, the phase comparator 4 generates an AC voltage, which passes through the active loop filter 6 and becomes an error voltage, and the voltage controlled oscillator 2 oscillates at a frequency corresponding to the error voltage. , which pulls the synthesizer into a synchronized state and uses this synchronized signal as the synthesized frequency signal l. At this time, the modulation sensitivity of the voltage controlled oscillator 2, the detection sensitivity of the phase comparator 4, and the frequency division number of the frequency divider are set to K1. Assuming Kt and N, the open loop gain of the entire device is given by equation (1).

Ka=ni+ ・K2 ・(R2/R1)/N...
(11 where Rt/Rl is the open-loop gain of the active loop filter.

[Problem to be solved by the invention]

Since the conventional microwave band frequency synthesizer is configured as described above, the frequency division number N of the frequency divider 3 is set to 2XN.
, 3XN,..., the open loop gain K of this device is
There were problems such as A being Ka/2, KA/3, . . . , and that there was a limit to the variable range of the frequency division number N.

This invention was made to solve the problems of the conventional devices as described above, and even when the frequency division number N of the frequency divider is changed, fluctuations in the open loop gain KA of the entire device can be suppressed. The purpose is to obtain a microwave band frequency synthesizer that can perform the following steps.

[Means to solve the problem]

The microwave band frequency synthesizer according to the present invention is one of the factors that determines the open loop gain of the active loop filter.
The value of the resistor R1 is changed simultaneously with the frequency division number N by an external frequency control circuit.

That is, the resistance between the phase comparator and the operational amplifier, which is one of the factors that determines the loop gain between the active loop filter 6, is replaced with a plurality of resistances connected in parallel with each other, and those resistances are In this case, switches are connected in series that are opened and closed by an open-loop gain control signal that changes according to the frequency division control signal, and the switches are controlled together with the frequency control so that the overall resistance value can be varied. It is.

[Effect]

In this invention, by configuring as described above, it is possible to control the frequency division number of the frequency divider as well as the open loop gain of the active loop filter, so that fluctuations in the open loop gain KA of the device can be suppressed. It is possible to construct a microwave band frequency synthesizer that is stable over a wide band.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure shows a frequency synthesizer according to an embodiment of the present invention, in which 1 is a synthesized frequency signal, 2 is a voltage controlled oscillator, 3 is a frequency divider, 4 is a phase comparator, 5 is a reference frequency signal, and 7 is a frequency synthesizer. The external frequency control circuit 8 is a frequency division control signal, which is the same as the conventional one. 9 is a loop gain control signal between the active loop filters, 6a is a resistor Rla-RID+ connected in series with a switch that is opened and closed by the loop gain control signal 9 during this period, a feedback resistor R1+ of the active loop filter, a feedback capacitor 01, etc. This is an active loop filter.

Next, the operation will be explained. Similar to the conventional one, a voltage controlled oscillator2. A phase-locked loop constituted by a frequency divider 35 whose frequency division number can be varied by a frequency division number control signal 8 from an external frequency control circuit 7, a phase comparator 4, and an active loop filter 6a is connected to a frequency division number control signal 8 from an external frequency control circuit 7. The frequency division number N is controlled by the frequency division number control signal 8, and it operates as a frequency synthesizer. At this time, a switch sea"'sin that opens and closes the resistors R, , "-R, and D in the active loop filter 6a in accordance with the open loop gain control signal 9 of the active loop filter from the external frequency control circuit 7 is switched on and off. When controlled according to the frequency control signal, the open loop gain of the device is 8, which is K m = K I - Kz ・ (R2/R,.
)/N.

However, K: ii: the modulation sensitivity of the voltage controlled oscillator, the detection sensitivity R of the two-phase comparator 4,) the resistor R that is switched on according to the frequency division number N;
Parallel sum N of IA'-'-R10: 2 for the frequency division number of the frequency divider 3: Open loop gain Rt of this device: Feedback resistance of the active loop filter 6a.

Therefore, if R+on is set in multiple stages according to the frequency division number N, even if the value of N changes to 2XN, 3XN, etc., RIIN) can be set by 1/2, 173 times, etc. By controlling it so that it is possible to suppress fluctuations in the loop gain of this device, it is possible to obtain a frequency synthesizer in the microwave band that has stable open-loop gain characteristics over a wide band.

Note that in the above embodiment, only the case where a plurality of resistors connected in parallel are used as the resistor between the operational amplifier and the phase comparator at the preceding stage thereof is described, but the present invention is limited to this case. Needless to say, it is sufficient to have a configuration in which the fluctuations in open loop gain due to changes in the frequency division number are canceled out by changing the resistance value between the operational amplifier and the phase comparator in the preceding stage. stomach.

〔Effect of the invention〕

As described above, according to the present invention, the open-loop gain of the loop filter of the frequency synthesizer is controlled by the open-loop gain control signal corresponding to the frequency division number control signal, so that the open-loop gain is stable over a wide band. There is an effect that a microwave band frequency synthesizer having gain characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a microwave band frequency synthesizer according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a conventional microwave band frequency synthesizer. In the figure, 1 is a synthesizer frequency signal, 2 is a voltage controlled oscillator, 3 is a frequency divider, 4 is a phase comparator, 5 is a reference frequency signal, 7 is a frequency control circuit, 8 is a frequency division control signal, and 9 is an open loop Gain control signal, 6°6a is active loop filter, 61 is operational amplifier, R, ”-R, cram is resistor, 84
~5ltl is a switch, R. is a feedback resistor, and C1 is a feedback capacitor. Figure 1

Claims (1)

[Claims] (1) A voltage controlled oscillator that oscillates an output signal, a frequency divider that divides the output of the voltage controlled oscillator, a phase comparator that compares the phase of the output of the frequency divider and a reference frequency signal, and the phase An active loop filter that filters the output of the comparator to create a control signal for the voltage controlled oscillator, and a divider control signal that controls the frequency division number of the frequency divider to control the oscillation frequency of the output signal. a frequency control circuit that determines the open loop gain of the loop filter, the resistor being provided between the phase comparator and the operational amplifier in the loop filter, and an open loop responsive to the frequency divider control signal. A frequency synthesizer comprising variable resistance means for changing the resistance value of the resistor according to a gain control signal, and capable of simultaneously performing frequency control and open-loop gain control of the synthesizer.