JPH04154309A - Phase tracking type fm demodulating circuit - Google Patents

Abstract

PURPOSE:To decrease the transmission loss by using an IC filter constituted of a low-pass filter or a high-pass filter which uses one inductor and one varactor. CONSTITUTION:An input FM signal from an input terminal In is supplied to an amplitude limiting amplifier 13 through a buffer amplifier 12, and also, supplied to a voltage control variable capacity diode CD through a capacitor C1 for constituting a phase comparator 18 and a tracking filter 21. The FM signal which passes through the tracking filter 21 is supplied to a phase comparator 18 through a buffer amplifier 14 and an amplitude limiting amplifier 15 phase comparison with the FM signal outputted from the amplitude limiting amplifier 13 is executed, and an error signal being proportional to its phase difference is obtained, and outputted from an output terminal Out through an amplifier 16. In such a way, it is unnecessary to enlarge the gain of the amplitude limiting amplifier, and the FM demodulating circuit can be constituted more briefly.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an FM negative feedback type FM demodulation circuit, and in particular,
Great for improving the FM threshold level! The present invention relates to a phase tracking type FM demodulation circuit suitable for.

[Technical background]

By feedback-controlling the oscillation frequency of the voltage-controlled oscillator of the FM negative feedback type FMfi modulation circuit, the FM demodulator, and the voltage-controlled oscillator of the heterodyne circuit provided in the preceding stage with the FM\i modulation signal, the bandwidth of the intermediate frequency can be narrowed. An FM negative feedback system that performs FM demodulation and an FM demodulation circuit using a PLL (phase locked loop) are known. In addition, there are other examples, such as the demodulation method for angle-modulated wave signals disclosed and described in Japanese Patent Publication No. 56-47722 filed by the present applicant, and the "FM13j modulation circuit using a phase tracking loop" introduced in Japanese Patent Publication No. 62-8962. There are known FM negative feedback type FM fi tuning circuits that do not use a voltage controlled oscillator.

The basis of these is PTL (Phas
eTraCkrnQ toop) demodulation method, which is an FMffitlM method consisting of a phase tracking loop,
This is an original technology of Jinlinsha, the applicant. Until now, P.T.
We have developed a day modulator for discrete 4-channel (CD-4') record playback using the La tone method, and have commercialized several models of PTL-FM tuners. It has been well received internationally and has received high praise from related academic societies.

Specifically, a tracking filter with narrow bandpass characteristics is provided at the input section of the FM demodulation section, the input signal and output signal of this filter are compared in phase, and the error output (demodulation output) is sent to the tracking filter. The tuning frequency of this filter tracks the instantaneous frequency of the FM signal. As a result, the tracking filter is accompanied by a kind of inverse modulation effect, and the FM signal is controlled by a loop feedback control in which the input FM signal of the tracking filter and the filter output subjected to inverse modulation are phase-compared to perform FM modulation.
There is an M modulation method.

The originality of the PTLt method lies in its FM multiplexing technology using FM negative feedback without a transmitting section, and has various excellent features as follows.

■In addition to basic features, you can bring out features that are specific to your application.

■The FM threshold level can be improved in the same way as the conventional FMjj return method and PLL demodulation.

(2) Since the gain of DC feedback is greater than that of AC feedback, the temperature characteristics, aging characteristics, etc. of circuit components in the loop can be suppressed.

■Linearity (differential gain) in demodulation due to negative feedback effect
is improved, and demodulation with low distortion can be performed.

■Overvariant noise seen in PLLffL key during overmodulation (
aliasing noise) does not occur.

■Since it does not have an oscillation circuit, it is not possible to use the conventional FMM feedback method or P
In principle, the problem of the oscillation component returning to the input section (causing a decrease in sensitivity) and the problem of interference with other circuits, which are seen in LL demodulation, do not occur.

■Since the circuit configuration is relatively simple, the cost is low, and a wide range of applications such as consumer equipment and business equipment are possible.

[Conventional technology]

The basic principle of PTL restoration will be explained. Here, the operation will be explained by taking as an example a PTL circuit using a bypass type tracking filter suitable for application to a cordless telephone.

Figure 4 shows a typical example of a conventional PTL demodulation circuit, in which 5 is a tracking filter, 7 is a limiter, 9 is an LPF (low pass filter), 16 is a DC amplifier, 1
8 is a ±90 degree type phase comparator. FIG. 5 shows only the tracking filter 5 taken out.

Assuming that the capacitance of the variable capacitor Cx changes with time due to the demodulated output f(t), then Cχ=Co+ACHf(t)...
It can be expressed as (1) (where AC: sensitivity of variable capacitor cx).

Here, the stationary transfer function T (voice) when f(t)-〇 is T(jllJ) = C/ (C+Cx)X (voice)'
÷[(j(iJ)' + j(IJ/((C+CX
) R)+1/(L (C+Cx) month”Go(j”J)’/((,JLIJ)’tenωo(
P)/Q+(IL)G')-(2). Therefore, from the above formula, Q=ω. (C+
Cx) R = Rt L (C+CxN ”/ L...
・・・・・・・・・・・・・・・(3) G o = C/
(C+Cx) ・・・・・・・・・・・・・・・
・・・・・・・・・(4) ωo = <L, (C0Cx
)) → ・・・・・・・・・・・・・・・・・・・・・
・(5)H-(C+-Cx)R/CX ([C+Cx
)/L)'...(6) (However, R: dump resistance R
resistance value of 1).

FIG. 6 shows this frequency characteristic (curve (1') l and phase characteristic (curve width)).

Here, the behavior of the tracking filter 5 will be investigated by supplying the demodulated output to the tracking filter 5.

In other words, by substituting equation (1) into equation 0 and calculating the transfer function T(jw, t) of the desired tracking filter, we get T(jLIJ, t) = (P)'X [(Co
+ h CJ(t)+C)(jcu)'÷C+(jl
lI)/RC+1/LC]-' (7), where the resonant frequency of the tracking filter 5 when f(t)=O is ωC, which is equal to the carrier frequency of the FM signal. Therefore, ωc = (L (C+Co
))→ ・・・・・・・・・・・・・・・(8) Then, the number of transmission intervals T(jcu,
t) is as follows.

Ding (voice, t) = [A(J(t)/CJ (L
(C0Co)) ”/RC]−”−(9) When the operation is analyzed by supplying the unmodulated carrier Cnt+(t) to the tracking filter 5, Crrm(j) = exp(J
ωct) ・・・・・・・・・・・・・・・・・・
...(10), so the tracking filter output C□
'(t) is C positive'(t) = 13Xl) [J((+l) ct +
'1 K-1an-' (A CXRf(t)/f mouth T with 7)) J・・・・・・・・・(11),
That is, the unmodulated carrier crrr, (t) is tan-
'[4CRf(t)(L (C + Co) 111 times the amount of inverse modulation.

Next, we will analyze the operation of the basic configuration of PTL. FM
The signal cB) can be expressed as follows.

C(t) =exp[j(ωct+f”tt(λ)
dλl ·−·−(12) Furthermore, when f(t)=O, the FM signal C0(t) that has passed through the tracking filter 5 and limiter 7 is as follows.

Co(t) = eXp[J(ωct + f”tt (
λ)dλ−τμ(1)+π/2)]・・・・・・・・・
(13) However, the group delay μ(t) of the τnitracking filter: the modulation signal, that is, this is the signal itself that has passed through the phase shift circuit and limiter 7 in conventional quadrature demodulation. . In addition, the 0th
In Equation 3), the 0-group delay τ, which takes into account the condition that the modulation signal μ(1) gives a frequency shift that is sufficiently large compared to the carrier angular frequency ωC, becomes τ when given the condition Co>ac. →2Co R・・・・・・・・・・・・・・・・・・
...(14) From the formulas 0 or more Of), 03), and (b), F after passing through the tracking filter 5 and limiter 7
The M signal c'(t) is expressed as C'(t)=J eXt)[j(
ωct + /′″μ(λ)dλ−2Co Rμ(t)
acRj (t) ÷ E "Complete 7 te go)]...
・・・・・・・・・・・・・・・(15) However, tan-' [AcRf(t) (L (C+C
od'] is a sufficiently reasonable value, so jan-' (acRf(t) (L (C+Co))
']”q a CRf(t) (L (C+ Co)
It is set as l→. Therefore, by comparing the phase of the FM signal c(B) and the output signal c'(t) of the tracking filter 5, the error output signal Er(t) is obtained.

Er(t)=-sin[2cg Rμft) +acR
f(t)x (L (C0Co)l']...
・・・・・・・・・(16) In this formula 06),
C+C0LfCO, and the values in square brackets [ ] are sufficiently small, so the error output signal Er(t) is Er(t) "'? (2Cg R)t (t)+
a CRf(t) (L Co)' l--(17).

Based on the above considerations, demodulation 2j(S) is f(s)=
G fs)・Er(s)・A=-AG(S)(2Co
Rμ(S) +ACRf(S) (LCo)→) 2 Co RA G (S)・μ(S) ,,,
,,, (H3)1+aCRAG(s)(LCo)→ (where A: loop gain).

The loop filter G (s) generally uses a first-order LPF. That is, G(S)=(s+ωL)-1...
(19) Therefore, the demodulation equation f(S) becomes as follows, resulting in a first-order LPF characteristic as shown in FIG.

From the above analysis results, the basic properties of PTL demodulation can be summarized as follows.

■Demodulation gain is proportional to loop gain IA.

(2) Demodulation gain is proportional to group delay time (=1/band width).

(2) The demodulation bandwidth becomes wider as the loop gain A and/or the dump resistance R becomes larger.

■The demodulation bandwidth is the sensitivity of the voltage controlled variable capacitor (variable capacitor)! The larger the IC, the wider it becomes.

[Problems to be solved by the present invention]

The conventional phase tracking type FM demodulation circuit has been explained with reference to FIG. , it is difficult to achieve high performance unless the transmission loss of the transmitted FM signal is further reduced.

[Means to solve the problem]

The present invention provides a tracking filter that tracks the instantaneous frequency of an input FM signal while changing the tuning frequency by an FMffE tuning signal, and inputs an input signal and an output signal of this tracking filter to one and the other inputs of a phase comparator, respectively. The error signal (F
In the FM demodulation circuit using phase tracking, which is controlled by feeding back the Mfflrl signal) to the tracking filter, the tracking filter is basically a low-pass filter or high-frequency r-wave using one inductor and one varactor. The above problem was solved by using an LC filter consisting of a filter.

〔Example〕

A first embodiment of the phase tracking type FM demodulation circuit of the present invention will be described with reference to the circuit configuration diagram in FIG. 1 and the characteristic diagram in FIG. 8. In FIG. 1, 12.14 is a buffer amplifier, 13.15 is an amplitude limiting amplifier, 16 is a DC amplifier, and 18 is a ±90 degree phase comparator (multiplier).

Further, C1 is a capacitor, Ll is an inductor, CD is a voltage filJll variable capacitance diode (varactor, pressure controlled variable capacitor), and R1 is a resistor. By connecting these as shown, a tracking filter 21 is constructed. . FIG. 2 shows the isometric path of the tracking filter 21.

In the configuration shown in FIG. 1, an input FM signal C(t) from an input terminal bt is supplied to an amplitude limiting amplifier 13 via a buffer amplifier 12, and a capacitor C1 forming a phase comparator 18 and a tracking filter 21. is supplied to the voltage-controlled variable capacitance diode CD via the voltage-controlled variable capacitance diode CD. Here, the voltage controlled variable capacitance diode CD constituting the tracking filter 21
Let the capacitance be CX, and let the inductance of inductor L1 be,
Resistance R.

Let the resistance value of the amplifier 12 be approximately zero [Ω
], the input resistance of the amplifier 14 is almost infinite, the transfer function T(jw) of the tracking filter 21 at rest is T(j(2))=(j(2))'X(T voice)'+jL&
I/RCx+1/LCx)-' ・・・・・・・・・
It can be expressed as (21). Therefore, from the above formula, Go = 1. ωo = (L
Cx)→Q=R(Cχ/L), and the frequency characteristics and phase characteristics thereof are shown by curves G and T, respectively, in FIG. As can be seen from this figure, the resonance angular frequency ω0 is determined by the capacitance CX and the inductance L, and the phase at the resonance point is 90°.

The FM signal that has passed through the tracking filter 21 is sent to the buffer amplifier 1.
4 and the FM signal that is supplied to the phase comparator 18 via the amplitude limiting amplifier 15 and output from the amplitude limiting amplifier 13, and an error signal (
FMf3j tone signal) is obtained. The error signal is
It is output from the output terminal shaft via the amplifier 16, and
The voltage is supplied to the voltage controlled variable capacitance diode CD (tracking filter 21) through the loop filter 23 which also serves as a coupling circuit consisting of the resistor R2 and the capacitor C1, and -
FM demodulation is performed by a circular feedback control operation.

This tracking filter 21 has features such as a relatively simple circuit configuration and a large transmission gain at the resonance point, but on the other hand, if the output resistance of the amplifier 12 is not small enough, the filter Since the Q of
The low output resistance of the amplifier 12 means that the tracking filter 21
This is a condition for ensuring performance.

FIG. 3 shows the iso-exit path when the output resistance R, of the amplifier 12 is present. The transfer function T(jw) in the tracking filter in this figure is as shown in equation (22), and the tuning angular frequencies ω0 and Q are shown in equation (23) and (23), respectively.
24) The result is as shown in equation 24).

T(jLIJ) = R(P)'/ (R+Ro)x [
(j”)'+j(al (L 10 Cx RRo
)/ (L CX (RO+R)) 10R/ (LCX
(RO”R) Month-' −・−・−・・−・(22)
ωo=(LCx) (R/(Ro+R))"-"12
3) Q=R”Cx L (Ro+R) / [(Cx
LX(Ro+R))' (Cx RRo” L
)i”--(24) where RQ<<R, i.e.
(R/(Ro+R))”→1, ω0 and Q become as follows.

ωO= (L Cx)' ------100
,56905,(25)Q= (LCx)” (Ro+
R) ÷ (C, RRo 1 L)...
・・・・・・・・・・・・ (26) - As an example, C=2
101) If F, L = 560μH, R = IMΩ, to make Q≧150, the output resistance RO of the amplifier 12
must be 10Ω or less.

By the way, the tracking filter 21 does not need to be limited to the above-mentioned configuration. For example, as shown in FIG. It is possible to configure a tracking filter of 1 and use this to configure a phase tracking type FM demodulation circuit (2).

In this figure, the same components as those in FIG. ±
Although the 90 degree (+π) phase type is used, the present invention is not limited thereto.

〔effect〕

As described above, according to the phase tracking type FM demodulation circuit of the present invention, the configuration of the tracking filter and the loop filter can be simplified,
The configuration of the phase tracking type FMffl adjustment circuit can be simplified. Furthermore, since the transmission gain is determined by "R(Cx/L)", the gain is large and is proportional to the resistance value of the resistor R constituting the tracking filter, so there is no need to increase the gain of the amplitude limiting amplifier in the next stage. This also has the excellent feature that the FM demodulation circuit can be configured more simply.

[Brief explanation of the drawing]

1 and 9 show the first phase tracking type FM demodulation circuit of the present invention, respectively. A block diagram of the second embodiment, FIGS. 2 and 3 are schematic diagrams of a tracking filter constituting the circuit of the present invention, FIG. 4 is a block diagram of a conventional circuit, and FIG. 5 is a conventional PT.
A circuit diagram of a tracking filter that constitutes the L demodulation circuit,
6 is an operating characteristic diagram showing the frequency characteristics and phase characteristics of the tracking filter in FIG. FIG. 3 is an operation characteristic diagram showing the frequency characteristics and phase characteristics of the fecal and oral passages. 1.2... Phase tracking type FM demodulation circuit, 12 to 16...
・Amplifier, 21... Tracking filter, 23... Loop filter (coupling time f!@), c I, c 2
...Capacitor, CD...Varactor (voltage controlled variable capacitance diode),
Ll...inductor, RO~R2...resistance. Patent applicant Takube Bogami, Representative of Victor Japan Co., Ltd.

Claims (2)

[Claims] (1) A tracking filter that tracks the instantaneous frequency of the input FM signal while changing the tuning frequency using the FM demodulation signal is provided, and the input signal and output signal of the tracking filter are input to one and the other input terminals of the phase comparator, respectively. Error signal (FM demodulated signal) obtained by the phase comparator
In the FM demodulation circuit using phase tracking, which is controlled by feeding back to the tracking filter, the tracking filter basically consists of a low-pass filter or a high-pass filter using one inductor and one varactor. A phase tracking type FM demodulation circuit characterized in that it is configured using an LC filter. (2) The phase tracking according to claim (1), wherein the FM demodulated signal to be supplied to the tracking filter is supplied via a loop filter configured to also serve as a coupling circuit on the input side of the tracking filter. type FM demodulation circuit.