JPS5812761B2 - Isouhenchiyouki - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a phase modulator using a phase locked loop (PLL).

In general, a PLL consists of a loop consisting of a phase detector 1, a loop filter 2, and a voltage-controlled oscillator (VCO) 3, as shown in Figure 1. Make an action to match.

Therefore, if the frequency of the carrier wave is defined by the crystal oscillator 4 and a modulated signal component is added to the loop, a phase modulator can be constructed, and a phase modulated wave can be taken out from the VCO 3.

However, at this time, attention must be paid to suppressing noise generated from the VCO 3 and improving modulation linearity.

The present invention was made in view of the above circumstances, and when stabilizing the frequency of a carrier wave using a PLL, suppresses noise generated from a VCO to improve S/N and improve modulation linearity. The purpose of this invention is to provide a phase modulator with a wide band.

The present invention will be described below with reference to the drawings.

FIG. 2 is a block diagram showing one embodiment of the present invention.

In the figure, 11 is 1/1 of the carrier wave in the phase modulated wave.
A crystal oscillator that oscillates at a frequency of n, 12 a phase detector,
13 is an adder that adds a modulation signal to the output of the phase detector 12, 14 is a loop filter, 15 is a VCO, and 16 is a VCO.
This is a frequency divider that divides the frequency of the 15 output to 1/n.

Here, the phase detection sensitivity by the phase detector 12 is Kp, V
The sensitivity of CO 15 is Kv, the transfer function of loop filter 14 is F(s), the phase of crystal oscillator 11 is θi, frequency divider 16
When the system of FIG. 2 is expressed as a block diagram of the control system, it becomes as shown in FIG. 3, assuming that the output phase of is θ0, the phase modulation output phase is Θ0, the modulation signal is Si, and the PM noise of the VCCO 15 is Ni.

From this block diagram in Figure 3 holds, the output phase Θ0 is It can be expressed as

From equation (1), it can be seen that the output phase Θ0 is the sum of the noise phase ΘN, the carrier wave phase Θc, and the modulated signal phase ΘS, so it can be expressed as follows.

If the loop propagation functions of noise, carrier wave, and modulated signal are HN(S), Hc(S), and Hs(S), then the following equations are obtained.

By the way, if the loop filter 14 in FIG. 2 is constructed of passive elements as shown in FIG.
It will look like the figure.

The transfer function F(s) of this loop filter 14 is as follows. Substituting this equation (6) into the above equation (4) yields the following equation.

At this time, Kp·Kv=K. That is, it becomes.

The loop transfer function of this carrier wave is the output of the frequency divider 16 (θ0
), we can obtain from equation (7).

Since this equation (8) simply means that the loop gain is reduced to 1/n by the frequency divider 16, by setting K/n=k and substituting these into equation (8), we get Become.

This equation (9) shows the loop transfer function of the carrier wave,
From automatic control theory, ωn and ξ each indicate the nature of a loop.

Also, from equation (5), it becomes.

Since this Hs(S) is the transfer function of the output phase to the input signal Si from equations (1) and (5), the frequency response of the output phase to the input signal Si is |Hs(jω)
It can be expressed as |.

Figure 6 is a diagram showing this frequency response, where ξ=√2/2
and the vertical axis is 20log | (Kp
)/nHs(jω)|, and the horizontal axis represents ω/ωn, which is obtained by normalizing the angular frequency ω by ωn.

That is, from equations (1) and (5) above, FIG. 6 shows the output phase for the modulated signal, and therefore shows the modulation frequency characteristics of the phase modulator according to this embodiment.

Therefore, as shown in FIG. 6, the input signal Si can obtain an even phase modulation characteristic within the loop band.

Further, the noise improvement amount is (input noise)/(output noise), and from the above equations (3) and (6), this noise improvement amount is as follows.

FIG. 7 is a diagram showing the amount of improvement in this noise.
g|1/(HN(jω))|=20log|1+1/(
jω)・(Kp・Kv)/n・(1+jωT2)/(1
+jωT1)| is plotted on the vertical axis and the angular frequency is plotted on the horizontal axis.

In addition, as shown in FIG. 7, in the PLL, VCO1
Even if the loop filter 14 has a band narrower than the modulation signal transmission band, the phase shift of the noise in the modulation signal transmission band of No. 5 is as follows.
The phase shift of the noise of the VCO 3 is output to the PLL output without any change and is not improved.

Now, for example, modulation signal Si=A sin ωt, ωL<ω
If <ωH, ωN≧10ωH, then ω as shown in Figure 6
Since ω/ωn is 0.1 or less in the range L<ω<ωH, the modulation frequency characteristic is constant (the output phase shift is constant).

In addition, the above equation (10) indicating the advancement function of the modulation signal (the expression indicating the output phase shift angle frequency characteristic with respect to the modulation signal angular frequency) is expressed as Hs(S)=Hs(jω), [S=jω]. can be expressed.

Therefore, in this embodiment, IHs(jωL)l2IHs(
By setting ωn so that jcc>H)l, ωn≧10ωH can be obtained as in the above example.

Furthermore, if it is necessary to improve the noise of the VCO 15 by 40 dB or more in the range of ωL<ω<ωH, the amount of noise improvement is 12 dB from the point of ωn as shown in FIG. B/OC
Since ωn≧10ωH, an improvement amount of 40 dB in ωH can be obtained.

That is, in the phase modulator 12 of this embodiment, as shown in FIGS. 6 and 7, by making the loop band sufficiently wider than the band of the modulation signal, the S/N can be improved, and
The phase modulation characteristic has a flat frequency characteristic.

Furthermore, as shown in equation (10) above, since the output phase shift of the modulation signal is uniquely determined by n/(Kp), the frequency divider 1
If the frequency division ratio of 6 is increased, the output phase shift can also be increased.

Therefore, by increasing the modulation sensitivity by using the frequency divider 16, it is possible to prevent deterioration of modulation nonlinearity due to the nonlinearity of the phase detector 12.

As explained above, according to the present invention, by making the loop filter wideband and making the loop band wider than the modulation signal transmission band, VCO noise is compressed by the PLL negative feedback loop, and the output phase shift of the noise is reduced. It can be improved.

Furthermore, since the modulation signal is added before the loop filter, it is possible to obtain a phase modulation frequency characteristic in which the output phase shift is constant with respect to the modulation signal frequency.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic configuration of the PLL, FIG. 2 is a block diagram showing an embodiment of the present invention, FIG. 3 is a control system block diagram of the system in FIG. 5 is a frequency characteristic diagram of the loop filter in FIG. 4, and FIG. 6 is a diagram of the loop transfer function Hs(j
FIG. 7 is a characteristic diagram showing the amount of noise improvement. 11... Crystal oscillator, 12... Phase detector, 13... Adder, 14... Loop filter, 15... Voltage control Oscillator, 16.
...Frequency divider.

Claims (1)

[Claims] 1. A voltage controlled oscillator, a frequency divider that divides a part of the voltage controlled oscillator output, a phase detector that performs a phase comparison between the frequency divider output and the reference oscillator output, and a phase detector that performs modulation on the phase detector output. comprising an adder that adds a signal, and a loop filter that has a pass characteristic sufficiently wider than the band of the modulation signal and transmits the output of the adder to the voltage controlled oscillator,
A phase modulator characterized in that a phase modulation output is obtained from the voltage controlled oscillator.