JPH0237722B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in FM detection circuits.

As a conventional detection circuit of this type, for example, the one shown in FIG. 1 has been known.

That is, the intermediate frequency amplifier 2 is connected to the input terminal 1, the output stage of the amplifier 2 is connected to the base of the emitter follower transistor 3, and the emitter of the transistor 3 is connected to the phase change capacitor 5 and the transistors 10 and 11. A first differential circuit A consisting of transistors 6, 7, 8, and 9 is connected to the capacitor 5 via a connection point P, and a second differential circuit C consisting of transistors 6, 7, 8, and It is known that a tuned circuit 30 consisting of a capacitor 31 and a coil 32 is connected between the ground 23 and the ground 23.

The conventional circuit operates as follows.

In other words, FM modulated FM is input to the input terminal 1.
When a signal is added, this signal is amplified by the amplifier 2, transmitted from the base of the transistor 3 to the emitter of the transistor 3, and output.

Next, the emitter signal of the transistor 3 is applied to the base of the transistor 10 in the first differential circuit A, and the emitter signal of the transistor 3 is applied to the base of the transistor 10 in the first differential circuit A.
0 and 11 turn on and off.

On the other hand, the emitter signal of the transistor 3 is applied to the tuning circuit 30 via the phase change capacitor 5, and the carrier frequency of the FM signal (here,
10.7 MHz), and when the frequency deviates from this tuning frequency, the phase changes and transistors 6, 7, 8, and 9 in the second differential circuit c are turned on and off.

The collectors of the transistors 10, 11 are connected to the emitters of the transistors 6, 7 and 8, 9 of the second differential circuit B, respectively, and the transistors 10, 11, 6, 7, 8, 9 are turned on.・
When it is turned off, FM detection is performed and a detected signal is taken out to the output terminal 22.

The Q value of the tuning circuit 30 (here, Q is
When the quality factor (an abbreviation for the sharpness of resonance) is low, as shown by the characteristic curve a in Figure 2, the level increases at the top and bottom (left and right in Figure 2) of the tuning frequency (near the carrier frequency) _f0 . Since the characteristics of the decrease in the frequency are gradual and the phase changes are small, it is possible to
It is possible to extract a detection output without much distortion.

However, if the Q value of the tuning circuit 30 is extremely high, as shown by characteristic curve b in FIG. The problem was that the FM detection output became a distorted signal.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an FM detection circuit with low distortion of the FM detection output by using an element with a special circuit configuration in place of the tuning circuit. , that is its purpose.

The configuration of the present invention includes a phase change capacitor and a first
and a second differential circuit, and is configured to perform FM detection by turning on and off the transistors in each of these differential circuits, between the phase change capacitor and ground, Series resonant frequency lower than the carrier frequency of the FM signal
f ₁ and a resistor and a tuned circuit element having a parallel resonance frequency f ₂ higher than the carrier frequency f ₀ are connected in series, and both frequencies f ₁ and f ₂ are connected in series.
, the intermediate frequency between frequencies f ₁ and f ₂ is FM
By increasing the difference between frequencies f ₁ and f 2 so that it becomes the carrier frequency of the signal, the part where the phase change between the frequencies f ₁ and _{f 2 is} close to direct is increased. It is.

Hereinafter, one embodiment of the present invention will be described in detail based on the drawings.

FIG. 3 is a circuit diagram of an FM detection circuit according to the present invention. In FIG. 3, the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof is omitted.

That is, 35 is a tuned circuit element, and this element 3
Reference numeral 5 denotes a device interposed between the connection point P and the ground 23, more specifically, between one end of the phase change capacitor 5 and the ground 23, and the element 35 has a frequency lower than the carrier frequency f ₀ of the FM signal. The circuit is configured to have a series resonant frequency f ₁ having a low frequency and a parallel resonant frequency f ₂ having a higher frequency than the carrier frequency f ₀ .

In other words, the tuning circuit element 35 has a coil L ₁
and capacitors C ₁ and C ₂ , and the frequency f 1 is obtained by series resonance with the coil L ₁ and capacitor C ₁ , and the frequency f ₁ is obtained by making the coil L ₁ and capacitors C ₁ and C ₂ resonate in parallel. The circuit is configured to obtain f ₂ .

In general, crystal or ceramic resonant elements have an equivalent Q of
L ₁ , C ₁ , of the tuned circuit element 35 in FIG.
It can be expressed as an equivalent circuit of C ₂ .

These crystal elements and ceramic resonant elements have a high Q value, so if only parallel resonance is used, if there is a large frequency shift during FM detection, the detected output will be highly distorted, but as in this example, the series resonant frequency f ₁
and the parallel resonant frequency f ₂ , so that the intermediate frequency between these frequencies f ₁ and f ₂ becomes the carrier frequency f ₀ of the FM signal, thereby increasing the difference between the frequencies f ₁ and f ₂ . By doing so, the portion where the phase change between the frequencies f ₁ and f ₂ is close to a straight line can be increased, and therefore FM detection can be performed with reduced distortion of the FM detection output signal.

That is, even if the element 35 with a high Q value is used, with this configuration, a detection output with little distortion can be obtained.

However, when the capacitance of the phase change capacitor 5 in Fig. 3 is small, the level decreases at the series resonance frequency f ₁ as shown by the solid line in Fig. 4 a, making it difficult to perform FM detection where the input signal is small. be. Therefore, in this embodiment, a resistor 36 is provided in series with the tuning circuit element 35, and the effect of this resistor 36 causes the above-mentioned series resonance frequency to be increased.
The detection output level at f ₁ is as shown by the dotted line in Figure 4a, and even when the signal input level is low, a signal around frequency f ₁ can be applied as a large signal to the bases of transistors 6 and 9. It is something.

In addition, in FIG. 3, the series resonance frequencies of the phase change capacitor 5 and the element 35 are expressed as the frequencies f ₁ and f _{2 .}
When _the carrier frequency of the FM signal is set near _{f 0} between This is a signal at a frequency shift of , and can facilitate FM detection even with a small input.

Furthermore, if a crystal or ceramic element 35 having series resonance and parallel resonance is used, the value of this element 35 cannot be changed in the circuit, so the frequency width of frequencies f ₁ and f ₂ should be increased in advance. and,
Since a signal with less distortion can be obtained without adjustment, it is possible to create a circuit without adjustment, and the number of steps and man-hours required for creating a receiver can be reduced.

As described in detail above, the present invention includes the phase change capacitor 5 and the first and second differential circuits A and C, and the transistors 6 to 11 in each of the differential circuits A and C are turned on and off. In the FM detection circuit configured to perform FM detection by turning off, a series resonance frequency f ₁ lower than the carrier frequency f ₀ of the FM signal is connected between the phase change capacitor 5 and the ground 23.
and a tuned circuit element 35 and a resistor 3 having a parallel resonance frequency f ₂ higher than the carrier frequency f ₀ .
6 are connected in series, and both frequencies
Using f ₁ and f ₂ , the intermediate frequency between frequencies f ₁ and f ₂ is the carrier frequency f ₀ of the FM signal (for example, 10.7M
Hz), and by increasing the difference between the two frequencies f ₁ and f ₂ , the portion where the phase change between the frequencies f ₁ and f ₂ is close to a straight line is increased. Also, the series-connected resistor makes it easy to detect even when the signal level is low.
This has the effect of reducing distortion of the FM detection output.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional example, FIG. 2 is a characteristic diagram showing characteristics of the conventional example, FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIGS. 4A and 4B are characteristic diagrams. 5 is a phase change capacitor, 6 to 11 are transistors, 23 is ground, 35 is a tuning circuit element, 3
6 is a resistor, A is a first differential circuit, and C is a second differential circuit.

Claims (1)

[Claims] 1. In an FM detection circuit that has a phase change capacitor and first and second differential circuits and is configured to perform FM detection by turning on and off transistors in each of these differential circuits, the phase change FM detection characterized in that a resistor and a tuned circuit element having a series resonance frequency lower than the carrier frequency of the FM signal and a parallel resonance frequency higher than the carrier frequency are connected in series between the capacitor and the ground. circuit.