JPS5846588Y2 - FM-AM radio receiver band switching circuit - Google Patents

Description

[Detailed description of the invention] The present invention provides an FM-AM radio receiver, in particular, one transistor is used for both FM intermediate frequency amplification and AM converter, and a ceramic filter is used as the FM side load of the transistor. An FM oscillator coil is connected in parallel to the ceramic filter as a side load, and a load resistor is provided on the input/output side of the ceramic filter.
The present invention relates to a band switching circuit for an AM radio receiver.

However, this type of radio receiver has the following problems.

(a) The optimum gain point of the transistor differs depending on whether the transistor is used for FM intermediate frequency amplification or for AM converter.

(b) Since the FM intermediate frequency is 10.7M, and the AM high frequency frequency is 505 to 1650 KHz, the handling frequency is different depending on whether the transistor is used for FM intermediate frequency amplification or as an AM converter.

Furthermore, when a transistor is used for an AM converter, since the handling frequency is not a single frequency, it is necessary to make the gain uniform over all frequencies, and at the same time, performance such as large input characteristics is required.

(C) An FM ceramic filter and an AM oscillation coil are connected in parallel as the transistor load, and the FM
During reception, the load resistances on the input and output sides of the ceramic filter are selected to the optimum state and maintained at high impedance, so there is no influence from the AM side circuit.
Since a wide range of frequencies are handled during reception, there is a concern that abnormal oscillations may occur at certain frequencies due to the effects of the ceramic filter.

This is because the input and output ends of the ceramic filter are set to high impedance, so depending on the component arrangement and the input side impedance of the next AM intermediate frequency stage, the output side of the AM converter and the input side of the next AM intermediate frequency stage may be affected. This is because the two are coupled through the ceramic filter, resulting in positive feedback.

Conventionally, a circuit shown in FIG. 1 has been used as a circuit to solve the problems (a) to (C) above.

In the circuit shown in Fig. 1, 1 is an FM mixing circuit, Ql is an I transistor that constitutes the mixing circuit, 2 is an AM antenna circuit, and Q2 is an emitter that is used for both FMM frequency amplification and AM converter. A grounded transistor, C-F, is connected to the transistor Q as the FM side load of the transistor.
2 is a ceramic filter connected between the collector and as (-B), R1 and R2 are load resistors provided on the input/output side of the ceramic filter, 3 is an AM oscillation circuit, and an oscillation coil that constitutes the oscillation circuit. The secondary winding l of L is resistor R7
Together with the secondary winding l' of the next-stage AM intermediate frequency transformer 5, it serves as a negative load on the AM side of the transistor Q2 and is connected in parallel with the ceramic filter CF.

4 is an intermediate frequency amplifier circuit for FM-AMM, 5 and 6 are AMM
There is one intermediate frequency.

In the circuit shown in FIG. 1, the following measures are taken to solve the problems (a) to (C) above.

(a) Solution to problem.

By using the band selection switch S3, only R3 is used as the emitter resistor of transistors 1 to Q2 during AM reception, and resistor R3 and resistor R4 connected in parallel to the resistor are used during FM reception. Problem (a) is solved by changing the bias point of the transistor between the AM signal and AM reception.

(b) Solution to the problem.

At this point, in the circuit of Fig. 1, "I" is the first transistor of transistor Q2.
This problem is solved by connecting the sensitivity adjustment resistor R5 in series with the bypass capacitor C1.

That is, by providing the resistor R5, the sensitivity at low frequencies during AM reception is suppressed, the gain is made uniform over the entire frequency range, and large inputs are suppressed.

However, if the resistor R5 is included in the bypass circuit of the emitter of the l transistor Q2, the bypass effect during FM reception will be reduced, making it impossible to obtain the optimum gain during FM reception.

Therefore, in the circuit shown in Fig. 1, the signal can be sufficiently bypassed even during FM reception by switching between the series circuit of bypass capacitor C1 and resistor R5 and the second bypass capacitor C2 using band selection switch S3. That's what I do.

(C) Solution to the problem The basic solution to this problem is the ceramic filter C-F
At the time of AM reception, the input side load resistance R1 is short-circuited with switch S2, and the switch S is connected as shown by the broken line in FIG.
4, or a ceramic filter C-F (not shown)
It may be possible to solve the problem by providing a switch on the input side of the ceramic filter that connects the input terminal of the ceramic filter and the collector of the transistor Q2 only when receiving FM and disconnects it when receiving AM, but this increases the number of switch circuits and increases the cost. There are drawbacks to it.

In view of the above, the present invention aims to provide a band switching circuit for an FMAMM geo receiver that solves all of the problems (a) to (C) above with a small number of switches. One embodiment will be described with reference to FIG.

Note that the same parts in Figure 2 as in Figure 1 are shown in Figure 1.
The same figure numbers as in the figure will be used.

In FIG. 2, Sl' is a first circuit which selectively switches the high frequency stage of each band, that is, the FMM mixing circuit 1, and the AM antenna circuit 2 to the base side of the transistor Q2, which is provided on the base side of the transistor Q2. A bias point adjusting resistor R6 is connected between the AM side fixed contact (AM) of the first band changeover switch Sl' and the ground-B.

Similar to the circuit shown in FIG. 1, the second band changeover switch S2' connects the movable contact CO to one end of the input side load resistance R1 of the ceramic filters C and F, and connects the AM side fixed contact AM to the load resistance R1. Connect to the other end, FM side fixed contact FM
is connected to the intermediate tap 8 of the tank circuit 7 of the FMM mixing circuit 1.

Then, the switch S2' short-circuits the load resistor R1 when receiving AM, and the switch S2' when receiving FM.
2' to the FM front end side, FM in Figure 2
A power source B is supplied to the mixing transistor Q1.

The third band changeover switch S3' connects the movable contact CO to one end of the output side load resistor R2 of the ceramic filter C-F and the AC ground point 9 (i.e., the capacitor C3 whose one end is connected to the ground -B). the other end), and the AM side fixed contact AM is connected to the other end of the output side load resistor R2 of the ceramic filter C-F, and the FM side, which was an idle terminal in the band changeover switch S4 in Fig. 1, is connected to the other end. Fixed contact FM
The second terminal is connected to the emitter of resistor Q2 at one end.
Connect to the other end of the bypass capacitor C2.

Further, an emitter resistor R3, a first bypass capacitor C1 connected in series, and a sensitivity adjustment resistor R5 are connected in parallel between the emitter of the transistor Q2 and the ground -B.

Note that the first to third band changeover switches Sl' to S
3' are used that interlock with each other.

Furthermore, the constants of the main circuit components used in the circuits shown in FIGS. 1 and 2 are as follows.

R1=560Ω, R2=330Q, R3=1.5KQ
, R5=22 f! , C1=0.01μF, C2=0.
022μF, C3=0.001μF, C4=0.02
2μF0 In a circuit configured in this way, the above (a)
How the problems in (C) are solved will be explained next.

Regarding the solution to problem (a).

By connecting the bias point adjustment resistor R6 between the base of transistor Q2 and ground-B during AM reception using the first band selection switch SL', the bias point of transistor Q2 can be switched between FM reception and AM reception. hand,(
Problem a) is solved.

Regarding the solution to problem (b).

During AM reception, the high-frequency circuit of the emitter of the transistor Q2 is constituted by the series circuit 10 of the first bypass capacitor C1 and the resistor R5, so that a sufficient bypass effect is achieved.

Also, during FM reception, the series circuit 10 is connected in parallel with a second bypass capacitor C2.

That is, since the second bypass capacitor C2 is connected to the ground-B via the third band changeover switch S3' and the capacitors C3 and C4, there is a bypass effect due to the resistor R5 being included in the series circuit 10. can be prevented from decreasing, and an optimal gain can be obtained.

About solving problem (C).

As a solution to the problem (C), that is, the problem caused by the influence of the ceramic filter during AM reception, the impedance on the input and output side of the ceramic filter C-F is lowered to eliminate the influence of the ceramic filter C-F. Therefore, the load resistances R1 and R2 on the input and output sides of the ceramic filter C-F are short-circuited by the second and third band changeover switches S2' and S3'.

In particular, in the present invention, a third band changeover switch S used for switching between the first and second bypass capacitors C1 and C2 provided between the emitter of the transistor Q2 and the ground-B is provided.
3', when receiving AM, ceramic filter C-
Since the load resistor R2 on the output side of F is short-circuited, the number of band changeover switches can be reduced by one compared to the circuit shown in FIG.

As described above, one transistor is used for both FM intermediate frequency amplification and AM converter, a ceramic filter is used as the FM side load of the transistor, and an AM oscillation coil is connected in parallel to the ceramic filter as the AM side load. There are various problems in the FM-AM radio receiver in which a load resistor is provided on the input/output side of the ceramic filter, but the present invention solves these problems with a band changeover switch with a small number of circuits. Since it is possible to solve this problem, it is possible to prevent cost increases, and it is also possible to obtain high quality because there is no need to sacrifice performance.

[Brief explanation of drawings]

Fig. 1 is a circuit connection diagram showing a band switching circuit of a conventional FM-AM radio receiver, and Fig. 2 is a circuit diagram showing a band switching circuit of a conventional FM-AM radio receiver.
FIG. 3 is a circuit connection diagram showing a band switching circuit of the M radio receiver. Q2...Transistor for FM intermediate frequency amplification and AM converter, C-F...Ceramic filter, R1, R2...Load resistance, R5...
... Resistor for sensitivity adjustment, R6 ... Resistor for bias point adjustment, S 1', S 2', S3'...
- First, second and third band changeover switch, CI. C2...First and second bypass capacitor.

Claims (1)

[Scope of utility model registration request] One grounded emitter transistor is used for both FM intermediate frequency amplification and AM converter, a ceramic filter is used as the FM side load of the transistor, an AM oscillation coil is connected in parallel to the ceramic filter as the AM side load, and the In an FM-AM radio receiver in which a load resistor is provided on the input/output side of a ceramic filter, the high frequency stage of each FM-AM band is selectively switched to the base of the transistor, and the bias point of the transistor is a first band changeover switch that switches for each band; a second band changeover switch that short-circuits the load resistance at the input end of the ceramic filter during AM reception and supplies power to the FM front end during FM reception; Switching operation between the series circuit of the first bypass capacitor and the sensitivity adjustment resistor provided between the emitter and the ground, and the second bypass capacitor, and A
A band switching circuit for an FM-AM radio receiver, comprising a third band switching switch that simultaneously short-circuits the output side load resistance of the ceramic filter during M reception.