JPS645397Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a DX/L _p switching circuit that is capable of obtaining sufficient attenuation without worsening distortion.

A DX/ _Lp switching circuit installed in a car radio or the like switches the antenna input circuit according to the strength of the received electric field to maintain a good reception condition at all times.

Figures 1 to 3 are circuit diagrams showing conventional examples of such DX/L _p switching circuits, where ANT is an antenna,
CIR is the antenna tuning circuit, Q is the N-channel FET that constitutes the high frequency amplifier (RF amplifier), and R ₁
~ _R4 is a resistor, _C1 to _C5 are capacitors, _L1 and _L2 are coils, D is a diode, S is a DX/L _p changeover switch, and Vcc is a power supply.

Of these, in Figure 1, FET _Q
The gain is changed and attenuated when off, operating as the L _p side. Similarly, in FIG. 2, the DX/ _Lp changeover switch S sets _C3 , which acts as a bypass capacitor, to ground and opens it, so that when it is off, it is set to the _Lp side and the gain is attenuated. Also, in Figure 3, DX/
By turning L _p changeover switch S on and off.
D, which is made up of PIN diodes, is made conductive and non-conductive, respectively, and the gain of the RF amplifier is changed to cut off diode D when it is off, thereby attenuating the RF signal and operating it on the L _p side. .

However, these conventional ones, on the DX side,
There is no sensitivity difference between the L _{and P} sides, and from the DX side when there is a strong electric field.
L Switching to _{the p} side has almost no effect.

FIG. 4 shows the attenuation characteristics of the case shown in FIG. 3 as an example, where the horizontal axis shows the power supply Vcc, the left vertical axis shows the attenuation amount α, and the right vertical axis shows the third-order distortion Xm.
Now, assuming that the DX/L _p selector switch S is turned on and switched to the DX side, for example, when Vcc is applied at 5V, the RF signal attenuation α becomes -1.0 dB, and the third-order distortion Xm becomes -56 dB. . (However, it is assumed that the output level of the diode D is always kept at a constant value even if the attenuation degree α changes.) Next, turn off the DX/L _p selector switch S, and turn off the L _p
When switching to the side, the third-order distortion Xm becomes extremely worse and approaches 0 dB, and the attenuation degree α at this time only shows a value of about -25 dB.

As described above, in the conventional circuit, since a sufficient amount of attenuation cannot be obtained, even if the circuit is switched to the L _p side in the case of a strong input electric field, the effect of countermeasure against interference cannot be obtained, and furthermore, there is a drawback that distortion worsens. Furthermore, when a PIN diode was used as the DX/L _p changeover switch S, there was a drawback of increased costs.

The present invention has been developed in response to the above problems, and is designed so that the AM band noise prevention coil provided in the antenna input circuit is shared with the FM band DX/L _p switching circuit. It is an object of the present invention to provide a DX/L _p switching circuit which eliminates the drawbacks. Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a circuit diagram showing a DX/L _p switching circuit according to an embodiment of the present invention. The same parts as in _{FIG .}
Dual gate FET that composes FM RF amplifier,
_R5 to _R12 are resistors, _C6 to _C13 are capacitors, C _T1 ,
C _T2 is a trimmer capacitor, L ₃ and L ₄ are AM band noise prevention coils, D ₁ to _{D 3} are variable capacitance diodes,
T ₁ and T ₂ are coupling transformers, and Vc ₁ and Vc ₂ are control voltages for variable capacitance diodes. When the DX/L _p changeover switch S is switched to the contact a side, it is on the DX side,
When the contact is switched to the b side, it becomes the L _p side. In addition, the AM noise prevention coils L ₃ and L ₄ provided in the antenna input circuit are each 100 μH to 200 μH, and several
10μH, and the connection point between both coils L ₃ and L ₄ .
DX/L _p changeover switch S is connected. Coil L ₃
The combined inductance L _p of L 4 and L ₄ is expressed as L _a ,
When L _b is assumed, L _p =L _a +L _b +M (where M is mutual conductance).

In particular, the coil _L4 functions to separate the AM antenna input circuit and the FM antenna input circuit in terms of impedance when the DX/L _p switching circuit S is switched to the L _p side of terminal b.

In the above configuration, the AM signal from the antenna is routed to FETQ ₁ (AM RF amplifier) via coil L ₃ → coil L ₄ → coupling capacitor C ₆ , regardless of the switching position of DX/L _p changeover switch S. Added. During this AM reception, the presence of coil _L3 will cause shortwave broadcast interference signals or noise from electrical equipment (approximately
Band noise (band noise above 2MHz) is significantly reduced.

Next, when receiving FM, if the DX/L _p selector switch S is located on the DX side of terminal a, the antenna
_The FM signal from ANT does not pass through coil L ₃ , but is routed from terminal a to FETQ ₂ (FM
RF amplifier). In this state, when there is a strong input interference wave, when the DX/L _p changeover switch S is switched to the L _p side of terminal b, the FM signal from the antenna ANT is transferred to the FETQ via the coil L ₃ → terminal b → coupling capacitor C ₉ . Added to ₂ .

6 and 7 show the attenuation characteristics obtained with the above configuration, where the horizontal axis shows the frequency and the vertical axis shows the amount of attenuation. In Figure 6, the dotted line shows the resonance curve between the antenna capacitance and inductance when the antenna is connected, which occurs at about 2.3MHz, but in other frequency bands there are almost no irregular changes in the attenuation characteristics, and interference noise This shows that a sufficient suppressing effect can be obtained. for example
It can be seen that it has an attenuation effect of approximately 50 dB against noise around 6.4 MHz.

In addition, Figure 7 shows the attenuation characteristics when the frequency band is expanded to a wide range including the FM broadcast band, and in particular, it shows about 45 dB in the range of 76 to 110 MHz.
A high amount of attenuation can be obtained. When receiving FM like this
Switch to the L _p side and install the AM noise prevention coil L ₃ .
By sharing it with the FM antenna input circuit, a high degree of attenuation can be obtained, and distortion can be almost completely eliminated. Therefore, a PIN diode or the like is not required in the DX/ _Lp switching circuit, resulting in cost reduction.

Note that when receiving AM, no matter which side the DX/L _p selector switch is located, the existence of coil L ₄ allows AM
Since the antenna input circuit and the FM antenna input circuit are separated in terms of impedance, there are no operational problems.

As is clear from the above description, according to the present invention, the AM band noise prevention coil provided in the antenna input circuit is configured to be shared with the FM band DX/L _p switching circuit. Sufficient attenuation can be obtained without worsening distortion.

[Brief explanation of the drawing]

Figures 1 to 3 and 4 are circuit diagrams and characteristic diagrams showing conventional examples, Figures 5 and 6, and Figure 7.
The figures are a circuit diagram and a characteristic diagram showing an embodiment of the present invention. Q ₁ ... RF amplifier for AM, Q ₂ ... RF amplifier for FM, S ... DX/L _p changeover switch, L ₃ , L ₄ ... Coil.

Claims (1)

[Scope of Claim for Utility Model Registration] (1) In an FM/AM radio receiver, an AM band noise prevention coil provided in the antenna input circuit and connected in series to an AM RF amplifier, and a noise prevention coil for the AM band not via the above coil. DX/L, which has a first contact connected to the antenna through the coil, and a second contact connected to the antenna via the coil, and connects the first or second contact to an RF amplifier for the FM band. A DX/L _p switching circuit characterized by being equipped with _{a p} switching switch. (2) In order to separate the AM antenna input circuit and the FM antenna input circuit in terms of impedance, a second coil is provided that is connected in series between the coil and the AM RF amplifier. DX/ as stated in Paragraph 1 of Claims for Utility Model Registration
L _p switching circuit.