JPH0221692B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention has a voltage controlled local oscillator (VCO),
In a heterodyne receiving system that has a variable capacitance diode as a tuning element, the received wave is searched in an open loop operation until it is tuned to the target received wave, and after reaching a certain tuning condition, it forms a closed loop and enters the receiving state. This invention relates to a so-called search tuning system tuning control circuit. Conventional configuration and its problems This type of channel selection control circuit does not require any special memory and can be constructed at low cost, so it has been used in low-inch television receivers and the like. but,
In conventional devices, there is only one process for transitioning from open loop to closed loop: changing from a state of not receiving a broadcast signal to receiving a broadcast signal, and transitioning to closed loop when a certain tuning condition is satisfied. For this reason, if you are currently receiving a broadcast, it is necessary to switch to a state in which no broadcast signals are received. Once the switch is pressed, this state is maintained by a capacitor, and after a predetermined period of time, detection of whether or not broadcasting is being received is started. but,
The length of the period during which the state is maintained by the capacitor is uncertain, and there are cases where broadcast reception is still in progress after a predetermined period of time has elapsed, so it is not sufficient.
It is also not suitable for IC implementation. Object of the invention The object of the invention is to enable a reliable transition from an open loop to a closed loop. Structure of the Invention The present invention provides a search system having a set input terminal connected to a search instruction input terminal and a reset input terminal for resetting the memory state in a heterodyne reception system having a voltage controlled local oscillator using a variable capacitance diode as a tuning element. a state memory circuit, a broadcast presence detection circuit, an intermediate frequency signal detection circuit, a detection output comparator connected to the output of the detection circuit, an output of the detection output comparator, an output of the broadcast presence detection circuit, and The station is connected to the station search instruction input terminal, and the station search is performed in an open loop until the desired radio wave is received by the search instruction signal inputted from the search instruction input terminal, and the detection output comparator and broadcast presence/absence detection circuit are connected to the station search instruction input terminal. Select a loop opening/closing control circuit that controls the reception state in a closed loop when the output signal is a signal indicating a predetermined tuning state, an input terminal for open loop control, an input terminal for closed loop control, and one of these input terminals. a voltage control circuit having an input terminal for loop opening/closing control, whose output is connected to a voltage-controlled local oscillator of the heterodyne receiving system, and which generates a control output signal to control the oscillation frequency of the voltage-controlled local oscillator; The output of the search state storage circuit is connected to an input terminal for open loop control of the voltage control circuit, and the output of the detection output comparator and the output of the broadcast presence detection circuit are connected to the input terminal for closed loop control of the voltage control circuit. The output of the loop opening/closing control circuit is connected to the input terminal for loop opening/closing control of the voltage control circuit and the reset input terminal of the search state storage circuit, and the output of the loop opening/closing control circuit is connected to the input terminal for loop opening/closing control of the voltage control circuit and the reset input terminal of the search state storage circuit. The present invention is characterized in that the transition from the open loop to the closed loop is performed by two different state transitions depending on the condition, and the transition to the open loop is automatically made depending on the reception state after the closed loop. DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 to 5. FIG. 1 shows the configuration of the search tuning system. 1 is a control circuit which receives a search up instruction signal 2, a search down instruction signal 3, a broadcast detection signal 5 from a broadcast signal presence detection circuit 4, a comparison result signal 7 from a detection output comparator 6, and an input voltage 9 to the VCO 8. The switch 10 is controlled by five inputs. The switch 10 has three states: connected to the charging source 11, connected to the discharge source 12, and high impedance. These states correspond to raising, lowering, and holding the potential of the capacitor 13, respectively. Based on this potential, it is transmitted through the conversion circuit 14 according to the desired reception band, etc.
Generates input voltage 9 for VCO 8. In addition, 15
is an element such as a FET with high input impedance for detecting and outputting the holding voltage while suppressing the outflow of charge from the capacitor 13 when the switch 10 is set to the middle point. 16 is a mixer;
The local oscillation signal of frequency _L output from VCO 8 and the received wave of frequency _R are mixed and converted into a signal of intermediate frequency _I. The detection circuit 17 is a detection circuit that detects the output signal of the mixer 16, and the detection output comparator 6 compares the reference voltage with the output voltage V _AFC of the detection circuit 17 and outputs a binary comparison result signal 7. . This will be explained using FIG. 2. intermediate frequency _I
Assume that the relationship between the detection output V AFC and the detection output V _AFC is as shown in characteristic A. Near the reference intermediate frequency _IO , the relationship between the intermediate frequency _I and the detection output V _AFC is linear _, and _I
When I is greater than IO, V _AFC is lower than V _AFCO when I is less than _IO , and when _I is less than _IO , V _AFC is less than V _AFCO
rises more than Set V _a and V _b around V _AFCO in an area where the relationship between _I and V _AFC is sufficiently linear. The binary values of the comparison result signal 7 are assumed to be x and y. x is
When V _AFC is larger than V _b , the logic level is inverted to "H", and when it is smaller, it becomes "L". Y is
When V _AFC is smaller than V _a , it is inverted to "H", and when it is larger, it is set to "L". Next, the channel selection operation will be explained using FIG. 3.
Inside the broken line is an internal block diagram of the control circuit 1. The loop opening/closing control circuit 18 is the central circuit of the present invention. The control circuit 1 receives the binary values x and y of the comparison result 7 obtained by the detection output comparator 6, and the broadcast detection signal 5 from the signal presence/absence detection circuit 4 [Here, when there is a broadcast signal, the logic level is “H” ”, when it is absent, it is “L”. ], input voltage 9 to VCO 8,
The search up instruction signal 2 and the search down instruction signal 3 are applied as described above, and the output is the charge/discharge control output signal 19 that controls the switch 10. First, the operation of each block will be explained. In the search state storage circuit 20, the output UP is set to the logic level "H" by the search up instruction signal 2, and the output DN is set to the logic level "H" by the search down instruction signal 3.
is set to And loop opening/closing control circuit 1
Both outputs UP and DN are reset to "L" by the output signal 21 of 8. In addition, to avoid instability when simultaneous inputs occur, prioritize the inputs. Since the search up instruction signal 2 and the search down instruction signal 3 are signals input manually by a human, they are given priority over the output 21. Further, the search up instruction signal is given priority over the search down signal 3. Further, at least one of the outputs UP and DN is configured to be "L". The loop opening/closing control circuit 18 is a circuit that controls the mutual state transition between the loop closed state (tuned state) and the loop open state (detuned state), and all state control is performed by the output signal 21. Let's do it. When the output signal 21 is at logic level "L", it is in a detuned state and the loop is open. When the signal is "H", the loop is closed in the synchronized state. The charge/discharge control circuit 22 receives the outputs UP and DN of the search state storage circuit 20, the output signal 21 of the loop opening/closing control circuit 18, the binary signals x and y indicating the state of the AFC S curve, and the broadcast signal. A charge/discharge control output signal 19 is synthesized from a broadcast detection signal 5 (hereinafter referred to as z) indicating whether or not the charge/discharge control is being performed. The auto return circuit 23 detects the input voltage 9 applied to the VCO 8 under the control of the charge/discharge control output signal 19, and outputs the input voltage to the search state storage circuit 20 within the control circuit 1 according to its magnitude.
Provides output to set UP or PN. This auto-return circuit 23 is a circuit that has meaning only when detuned. For example, when the search up instruction signal 2 is given, the charge/discharge control output signal 19 becomes the charge/discharge output, and the input voltage 9 rises and reaches the upper limit of the VCO 8, the auto return circuit 23 operates. A signal is input to the search state storage circuit 20 so that the charge/discharge control output signal 19 becomes a discharge output. Conversely, when the search down instruction signal 3 is given, the charge/discharge control output signal 19 becomes a discharge output, the input voltage 9 decreases, and reaches the lower limit of the VCO 8, the auto return circuit 23 operates. A signal is input to the search state storage circuit 20 so that the charging/discharging control output signal 19 becomes a charging output. The initial setting circuit 24 is used to set the initial conditions for the tuning control circuit 1 when the power is turned on.When the power is turned on, the input voltage 9 to the VCO 8 is zero volts and is in a detuned state. A signal is given to the search state storage circuit 20 so as to search up inside the control circuit 1, and the output UP is set. Next, the overall operation of the control circuit 1 will be explained in detail. First, assume that search up instruction signal 2 is input from the outside. At this time, the search state storage circuit 2
At the same time as the output UP of 0 is set to “H”,
By the OR gate 32, the loop opening/closing control circuit 18
Search up instruction signal 2 is added to. This search up instruction signal 2 sets the initial state of the loop opening/closing control circuit 18, and the output signal 21 becomes "L". Note that unless the pulse of the search up instruction signal 2 is "H" until the output signal 21 becomes "L", the output UP will not be set, but when the search up instruction signal 2 is input and the output signal 21 is Since the time until it becomes "L" is sufficiently short, when the search up instruction signal 2 is input, the output UP is set. Now, when the output signal 21 is "L", the charge/discharge control circuit 22 outputs the binary value x,
It becomes unrelated to y and z, and the output UP or
Accepts input only from DN. In response to an input from the output UP, the charging control output signal 19 becomes a charging output, and in response to an input from the output DN, it becomes a discharging output. That is, this state is an open loop state. The transition from open loop to closed loop is performed in the loop opening/closing control path 18 as follows. First, when a pulse input for initial condition setting is received through the gate 25, if z=“L”, that is, no broadcast is being received at that time, the next z=“H” and x=y= “L”, that is, when the broadcast signal is applied and the detection output V _AFC is between V _a and V _b , the output signal 21 is inverted to “H”, and the output of the search state storage circuit 20 is thereby After being reset, the charge/discharge control circuit 22 determines the charge control output signal 19 based on the binary values x, y, and z.
That is, looking at FIG. 1, between the detection output comparator 6 and the control circuit 1, the binary values x, y of the comparison result signal 7 are
This means that the system has entered a closed-loop state. Next, when a pulse input for initial condition setting is received through the OR gate 32, z=
“H” In other words, if the broadcast is being received at that time, the charging control output signal 19 remains the charging output,
An up-search is performed once to the state where z=“L”, that is, the state where there is no broadcast signal. During this time, the output signal 21
remains at "L". Thereafter, a transition to the closed loop state is made in the same process as when z=“L” when the initial condition setting pulse was input. Next, the operation in the closed loop state will be explained. At this time, the output signal 21 is "H", and the outputs UP and DN of the search state storage circuit 20 are both reset and do not affect the charging control output signal 19. That is, the charging control output signal 19 is determined by the states of the binary values x, y, and z. Table 1 below shows these relationships.

[Table] In 1 to 3, z = “H”, that is, broadcasting waves are being received, and in a closed loop state, z = “H” normally.
It is. When 1, x=y=“L”, that is,
When V _AFC is between V _a and V _b of the AFC S curve, it is assumed that the intermediate frequency _I of the output of the mixer 16 is within the permissible range, and charging and discharging are not performed. That is, the charge control output signal 19 is a high impedance output and holds the charge of the capacitor 13. 2, x = "H", y = "L", that is, when the detection output V _AFC is greater than V _b , the input voltage 9
The charging control output signal 19 is set as the charging output so that the detection output V _AFC becomes equal to or lower than V _b by increasing the detection output V AFC. 3
When x = "L", y = "H", that is, when the detected output V _AFC is less than V _a , the input voltage 9 is lowered to make the detected output V _AFC more than V _a .
The charging control output signal 19 is used as the discharge output. As mentioned above, when z = “H”, the detection output V _AFC
The charging control output signal 19 is controlled according to the states of x and y so that the voltage falls within the range of V _a to V _b .
4 to 6 are cases where z=“L”, that is, no (weak) electric field state. The field strength of the received signal often changes even during closed loops, such as when a small TV is in a car and goes into a tunnel, or when a train passes by in a house near railroad tracks. At this time, as can be seen from Figure 1, when the received wave _R disappears, the intermediate frequency _I disappears and the loop opens. This received wave
Although the trace of _R has disappeared, if it takes a few seconds, the received signal will return again.
It is hoped that when _R returns, it will be able to lock onto the same channel as before. This is state 4. When the received wave _R disappears, the detection output V _AFC is
Force the voltage to be between V _a and V _b . That is, x=y=“L”. Then, in order to hold the charge in the capacitor 13 and maintain the current input voltage 9, the charging control output signal 19 is set high.
Impedance output. Now, suppose that the electric field returns to its original state after the received wave _R disappears for several seconds. At this time, if the deviation of the input voltage 9 is within the range of z=“H” (signal present) of the original channel, the original channel is locked by operations 1 to 3. This relationship will be explained using FIG. 4. This shows the S curve of FIG. 2 in terms of the relationship between the input voltage 9 and the detected output V _AFC for two channels, the A channel and the B channel. 25 and 26 represent closed loop reception states of channels A and B, respectively. Note that the deviation in the input voltage 9 is caused by the leakage current of the capacitor 13, the switch 10,
A deviation in the direction in which the input voltage 9 decreases is considered, such as a leakage current in the connection to the mixer 16 in the next stage. now,
If, when receiving at point P, the radio wave disappears and the electric field returns to its original state, the state is at point Q, as described above, the operation in step 2 returns to point R. However, when the electric field returns to its original state, if the input voltage 9 has decreased to point S, then the operation in step 5 returns to point R. That is, since x = "H", y = "L", and z = "L", search up instruction signal 2 is input and the process shifts to open loop operation. The operation when the search up instruction signal 2 is input is as described above with the output signal 21.
is reset, the output UP is set, the charging control output signal 19 becomes the charging output, and when the input voltage 9 rises to point R, the system shifts to closed loop operation. Furthermore, there may be a case where the input voltage 9 decreases significantly. This decrease in the input voltage 9 is due to the fact that the charge in the capacitor 13 cannot be ideally maintained as described above, and the magnitude of the decrease is approximately proportional to the time during which the external radio wave disappears. Also, as shown in Figure 4, the state of the AFC curve between channels is
For example, V _b and V _a often intersect due to the influence of audio carrier waves of adjacent channels. The operations in these cases will be explained by dividing them into the following cases. - If the radio wave disappears while receiving at point P and then returns to point T In this case, the state is 4 in Table 1, so the charging control output signal 19 becomes high impedance,
The charge in the capacitor 13 is held. When the leakage current reaches point U, this is the state 6, so the loop opening/closing control circuit 18 is reset by the search up instruction signal 2, and the circuit shifts to open loop operation. This also sets the output UP,
When the charging control output signal 19 becomes a charging output and the input voltage 9 rises to point R, the output signal 21 of the loop opening/closing control circuit 18 becomes "H", and the system shifts to closed loop operation. - If the radio wave disappears while receiving at point P, and then returns to point V. In this case, since it is in state 6 from the beginning, it immediately shifts to open-loop operation, and in the same way, close-loop operation at point R. to move to. - If the radio wave disappears while receiving at point P, and then returns to point W. In this case, as with point T, it is initially in state 4, so the input voltage 9 drops to point X due to leakage current. After waiting for this to occur, the system shifts to open-loop operation according to state 5, and then enters closed-loop operation at point R. - When the radio wave disappears while receiving at point P and then returns to point Y. As in this case, z = “H” of F channel
If the input voltage 9 drops to the range , the original B channel can no longer be locked. While remaining in a closed loop, it enters a receiving state at point Z due to state 3.
Note that if the input voltage at point Y is 9 or less, it is possible to enter the receiving state in the same way as for points Q to X, but it will not be possible to return to the original B channel and will be locked on another channel. . Conventionally used search tuning systems have had difficulty in transitioning from an open-loop state to a closed-loop state. First, when the search up instruction signal 2 and the search down instruction signal 3 are input in the closed loop state, in the configuration of the present invention, the loop opening/closing control circuit 1
The state of z is reset and open loop operation is started depending on the situation of z at that time. However, in the conventional example, a capacitor is used as a means for escaping the range of z=“H” of the channel currently being received. In other words, when the search up or search down instruction switch is pressed, the capacitor is charged at the same time, and it is as if the search up or search down instruction switch continues to be pressed until the z is out of the "H" range. operate as if it were. When the charged capacitor spontaneously discharges to a certain point, it starts detecting z again and enters a closed loop at the next z=“H”. In this method, for example, the period of spontaneous discharge changes due to temperature characteristics, and there is a possibility of malfunction, but with the configuration shown in FIG. 3, such problems can be eliminated. Furthermore, this configuration exhibits stable operation even with changes in electric field strength. In the conventional method, once the closed loop is entered, x, y
A method has been adopted in which the capacitor 13 is charged and discharged only depending on the state. Therefore, when the electric field returns after it has disappeared,
If it reaches point W in Figure 4, it will lock in the range from V _b to V _a with W point as the center, and it will not be able to escape from this state unless a search up or search down instruction is input from the outside. I can't.
However, with this configuration, the range in which the original channel can be relocked is wider than in the conventional case. Also, even after a long period of no trace of radio waves, a stable state is achieved by receiving one of the channels. It is conceivable to use some kind of analog memory instead of charging and discharging the capacitor 13, but in that case, the greatest advantage of the search tuning method, which is low cost, would be lost. When compared with other tuning methods such as frequency synthesizer and voltage synthesizer methods, it is inferior. FIG. 5 shows the configuration of the loop opening/closing control circuit 18. As mentioned above, the loop opening/closing control circuit 18
Controls transitions from an open-loop state to a closed-loop state and from a closed-loop state to an open-loop state. The part that plays the former role is the part that plays the role of the latter. The inputs are a pulse-like up search instruction signal 2 and a down search instruction signal 3, which are input from the outside using a push switch or the like.
There are five values: binary values x and y indicating the state of the AFC S curve, and z, the result of determining the presence or absence of a broadcast signal. output is output
Its purpose is to set the UP and perform open/close loop control. First, the operation related to the transition from the open loop state to the closed loop state will be explained. 25 and 26 are R-S flip-flops, and the reset input R _A , set input S _A and outputs Q _A and _A have the relationships shown in Table 2 below.

[Table] There is no input state where R _A = S _A = “H”. R-S
The reset input R _B of the flip-flop 26 is always "L", and the R-S flip-flop 25
When z=“H”, the output of the inverter 27 is “L”, the output of the AND gate 28 is “L”,
Therefore, R _A =“L”. Furthermore, when z="L", the outputs of AND gates 29 and 30 are "L", so S _A = "L" via OR gate 31. Therefore, the R-S flip-flop 25
There is no state where R _A = S _A = “H”. In addition, R-
Terminals C of S flip-flops 25 and 26 are “L”
When , the input/output relationship shown in the table above is obtained, but C="H"
When , Q _A = Q _B = “L” and _A = _B = “H”. As mentioned above, when the search up instruction signal 2 or the search down instruction signal 3 is applied, the loop must necessarily become open immediately after that. When the search up instruction signal 2 or the search down instruction signal 3 is given, the OR gate 32 outputs R-S.
Since the terminals C of the flip-flops 25 and 26 become "H", Q _A and Q _B become "L", and the output signal 21 of the AND gate 33 becomes "L". In other words, it is an open loop state. Subsequent changes vary depending on the state of z. first,
If currently z=“H”, that is, broadcasting is being received,
Once through the state of z = "L", the next channel's z = "H" and x = y = "L" inverts the output signal 21 from "L" to "H", changing from the open loop state to the closed loop. transition to state. By search up instruction signal 2 or search down instruction signal 3, C=
When it becomes “H” and Q _A = Q _B = “L”, z=
Set it to “H”. With the output “L” of the inverter 27,
The output of the AND gate 28 is “L”, that is, R _A =
It is “L”. Q _B = “H” when z = “L”
At this time, the output of the inverter 27 is "H" and _B = "H", so the AND gate 28
Therefore, S _B = “H”, therefore Q _B = “H”, _B =
It becomes “L”. Due to this _B = “L”, S _B = “L” immediately through the AND gate 28, and Q _B =
The states of "H" and Q _A = "L" are maintained thereafter. That is, Q _B becomes "L" when reversed to C = "H", "L" while z = "H", becomes "H" when z returns from "H" to "L", Thereafter, whether S _B is "L" or "H", Q _B remains "H" and does not change until the next C="H". On the other hand, for Q _A , C=
During "H", Q _A = "L". Next, when C=“L”, R _A is “L” like S _B while z= “H”. Since the output of the AND gate 29 is "H", S _A is set to "H" by the OR gate 31. Because the input of the AND gate 29 is z=
This is because “H”, _A = “H”, and _B = “H”. S _A
= “H”, R _A = “L”, Q _A = “H”, _A =
It becomes “L”. Therefore, andgate29,
30, the input from _A becomes "L", so both outputs become "L", S _A becomes "L", and this Q _A =
The states of “H” and _A = “L” are maintained. Next, consider the case when z changes from "H" to "L". At this time, the input of S _A from z through AND gates 29 and 30 becomes "L", so S _A =
It remains “L”. On the other hand, since R _A was _B = "H" when z = "H", the moment z = "L", both inputs of the AND gate 28 were "H".
R _A (S _B )=“H”. Therefore, Q _A = “L”
becomes. As mentioned above, since S _B becomes "H", _B becomes "L". At the same time, the output of the AND gate 28 becomes "L", and S _A = R _A = "L"
Therefore, Q _A = "L" and _A = "H" are maintained over the period of z = "L". Next, consider the case when z=“L” becomes “H”. At this time, R _A is "L" because both inputs of the AND gate 28 are "L". Also, since the AND gate 29 also has _B ="L", its output is "L". AND gate 30 is, _A is z=
When it changes from “L” to “H”, it is “H”, so
In addition to z="H", x="L", y="L"
When the output becomes "H", the output is inverted to "H". At this time, via the OR gate 31, S _A = "H", Q _A = "H", and _A = "L". _A = “L”
Therefore, the outputs of AND gates 29 and 30 are “L”
Then, S _A = “L”, Q _A = “H”, _A =
The "L" state is maintained. To summarize the above results, by inputting a pulse to terminal C, Q _A =Q _B =“L” first. z=“H”
During this period, Q _A = “H”, Q _B = “L”, and then z = “L”
Then, Q _A = “L”, Q _B = “H”, and then z =
When “H” and x=y=“L”, Q _A =
Q _B becomes “H”. The output signal 21, that is, the AND gate 33 becomes "H" in this final state.
At this time, a transition from open loop to closed loop is instructed. Next, by inputting a pulse to terminal C, Q _A = Q _B
The case where z=“L” when z=“L” will be explained. This is because it is not receiving broadcast waves to begin with.
The following signal is present (z=“H”) and x=y=
A transition from an open loop to a closed loop may be performed by "L". Now, when Q _A = Q _B = "L" and z = "L", first S _B is _B = "H", the output of the inverter 27 is "H", and the output of the AND gate 28 is "H". . In other words, Q _B =“H”. At this time _B =
Returns to “L”, and S _B of the output of AND gate 28 =
Due to "L", this state of Q _B = "H" is maintained. On the other hand, due to z=“L”, the AND gate 29,
30 are both "L", and the output S _A of the OR gate 31 remains "L". Therefore, whether R _A is "H" or "L", the original state of Q _A = "L" remains. Next, consider the case when z changes from "L" to "H". At this time, since R _A is _B = “L”,
It is set to "L" by the AND gate 28. Also _B
Therefore, the output of the AND gate 29 is "L".
Since z=“H” and _A =“H” of the AND gate 30, when x=y=“L”, the output becomes “H”.
The OR gate 31 is also "H", and S _A = "H", so Q _A = "H". Furthermore, AND gates 29 and 30 become "L" due to _A = "L", the output of OR gate 31 is "L", and S _A = "L", therefore, the state of Q _A = "H" is maintained. be done. S _B = R _A =
Since it is “L”, this state does not depend on x, y, z, etc.
The signal is held until the input pulse of the search up instruction signal 2 or the search down instruction signal 3 is input to the terminal C. To summarize the above results, when a pulse is input to terminal C, Q _A =Q _B =“L” first. Since z=“L” at the time the terminal C changes from “H” to “L”, Q _A = “L” and Q _B = “H”. Then z=“H”
And when x = y = “L”, Q _A = Q _B = “H”
This remains unchanged until the next input of search up instruction signal 2 or search down instruction signal 3 to terminal C. The output of the AND gate 33 becomes "H" in this final state, and an instruction to shift from the open loop to the closed loop is issued. In this way 30~
When the search up instruction signal 2 or search down instruction signal 3 switch is pressed by 39 and 44, the switch shifts to open loop operation, and if it is receiving broadcast waves at the time of pressing, it exits the current channel. The closed loop transitions when the detection output V _AFC of the next channel falls within V _b ~ _{V a} , and if no broadcast is being received at the time of pressing the button, the detection output V _AFC of the next channel falls within V _b ~ V a. The loop transitions to a closed loop at the point within _a . Next, the operation of the part will be explained. When the output signal 21 is "L", that is, during open loop operation, the AND gate 34 output is "L", the diode 35 is cut off, and is not related to the circuit operation. The output signal 21 is "H", that is, there is no signal after entering the closed loop operation (z = "L"), and x =
When “H”, by the AND gate 36, and
When y=“H”, the output of the OR gate 36 becomes “H” by the AND gate 37, the output of the AND gate 34 is inverted to “H”, the diode 35 is turned on, and the search up instruction input to the search state storage circuit 20 is performed. Set to “H”. At the same time, the output of the OR gate 32 also becomes "H", and the terminals C of the R-S flip-flops 25 and 26 are "H", and the output signal 21
is “L”, that is, it shifts to an open loop. By the output signal 21 becoming "L",
AND gate 34 output becomes “L” and terminal C=
"L" and starts the transition operation from open loop to closed loop as described above. In this way the fourth
It becomes possible to take measures against the temporary fluctuations in the received electric field as explained in the figure. To summarize the operation of the loop opening/closing control above, the transition from a closed loop (output signal 21 is “H”) to an open loop (output signal 21 is “L”) is due to the input of search up instruction signal 2 or search down instruction signal 3. . By resetting the two R-S flip-flops 25 and 26, the output signal 21 becomes "L". Even though the loop is closed, no broadcast signal has been received, and the detection output V _AFC is greater than or equal to V _b or less than V _a . Search state storage circuit 20
At the same time when setting the output UP of
By resetting the flip-flops 25 and 26, the output signal 21 is set to "L". The transition from open loop to closed loop is divided into two processes depending on the presence or absence of the broadcast signal, but when the broadcast signal of the next channel is received, V _AFC changes from V _a
When the voltage falls within the range of _Vb , the loop is closed (the output signal 21 is set to "H"). In conventional circuits, once the closed loop state is reached,
Since z was an invisible method, the transition from closed loop to open loop was only as described above. When the radio wave disappears once and is input again, the input voltage is within the range where it can re-lock to the current channel.
If the signal is deviated, it would be impossible to locate the station without inputting a search up instruction signal or search down instruction signal from the outside. Depending on the situation, it is possible to shift to open loop operation. In addition, in the conventional circuit example, the transition from open loop to closed loop is from z=“L” to z=
There was only one process: transition to “H” and x=y=“L”. For this reason, when currently receiving a broadcast, it is necessary to once shift z from "H" to "L", but this means that once S _o and S _D are pressed,
A method has been used in which this state is maintained with a capacitor and the detection of z is started after the transition from z=“H” to “L” has passed. This is because the length of the period in which the state is maintained by the capacitor is uncertain, but in this circuit, it is divided into two processes depending on the state of z when the search up instruction signal 2 or the search down instruction signal 3 is input. , such uncertainty can be removed. Effects of the Invention As explained above, according to the present invention, the process of transition from open loop to closed loop is divided into two depending on the presence or absence of broadcast reception when a search instruction is detected from outside, so can transition to a closed loop. In addition, since it is divided into two processes in this way, there is no need for a capacitor to maintain the state as in the past, and this circuit configuration can be easily integrated into an IC, so it is possible to provide a tuning circuit at a lower cost. is possible.

[Brief explanation of drawings]

Fig. 1 is a block diagram of one embodiment of the channel selection control circuit according to the present invention, Fig. 2 is an explanatory diagram of intermediate frequency and AFC voltage, Fig. 3 is a detailed block diagram of the main part of Fig. 1, and Fig. 4 is an explanatory diagram of the AFC voltage with respect to the VCO applied voltage, and FIG. 5 is a specific configuration diagram of the loop opening/closing control circuit. 1...Control circuit, 2...Search up instruction signal, 3...Search down instruction signal, 4...Broadcast presence detection circuit, 6...Detection output comparator, 8...
VCO, 10...Switch, 13...Capacitor,
16...mixer, 17...detection circuit.

Claims (1)

[Scope of Claims] 1. A heterodyne receiving system having a voltage-controlled local oscillation section using a variable capacitance diode as a tuning element, which has a set input terminal connected to a search instruction input terminal and a reset input terminal for resetting the memory state. a search state storage circuit, a broadcast presence detection circuit, an intermediate frequency signal detection circuit, a detection output comparator connected to the output of the detection circuit, an output of the detection output comparator, and an output of the broadcast presence detection circuit. and the station search instruction input terminal, and searches for stations in an open loop until the desired radio wave is received by the search instruction signal inputted from the search instruction input terminal, and the detection output comparator and broadcast presence/absence detection circuit a loop opening/closing control circuit that controls the output signal to enter a reception state in a closed loop when the output signal is a signal indicating a predetermined tuning state, an input terminal for open loop control, an input terminal for closed loop control, and one of these input terminals. a voltage control circuit having a loop opening/closing control input terminal for selection, whose output is connected to the voltage controlled local oscillator of the heterodyne receiving system, and which generates a control output signal to control the oscillation frequency of the voltage controlled local oscillator. , the output of the search state storage circuit is connected to an input terminal for open loop control of the voltage control circuit, and the output of the detection output comparator and the broadcast presence/absence detection circuit are connected to the input terminal for closed loop control of the voltage control circuit. The output of the loop opening/closing control circuit is connected to the loop opening/closing control input terminal of the voltage control circuit and the reset input terminal of the search state storage circuit. A channel selection control characterized in that the transition from open loop to closed loop is performed by two different state transitions depending on the presence or absence of the channel, and the channel selection control is configured to automatically transition to open loop depending on the reception state after the closed loop. circuit. 2. The channel selection control circuit according to claim 1, wherein the output of the loop opening/closing control circuit is 1-bit digital data. 3. The channel selection control circuit according to claim 1, wherein the loop opening/closing control circuit is constituted by an asynchronous sequential circuit. 4 Loop opening/closing control circuit is R-S with clear terminal
Two flip-flops are provided, and the search instruction signal from the search instruction input terminal is applied to the clear terminal of these R-S flip-flops, and the detection output signal is applied to the reset input terminal and set input terminal of these R-S flip-flops. Claims characterized in that the output of the comparator and the output of the broadcast detection circuit are applied via a combinational circuit, and the outputs of these R-S flip-flops are combined to output 1-bit digital data. The channel selection control circuit according to item 1. 5. The loop opening/closing control circuit is equipped with a circuit that combines the 1-bit digital data output, the output of the broadcast presence/absence detection circuit, and the output of the detection output comparator using a combination circuit, and applies the synthesized signal to the station search instruction input terminal. The channel selection control circuit according to claim 4, characterized in that: 6. The channel selection control circuit according to claim 1, wherein the detection output comparator is constituted by two voltage comparators having the output of the detection circuit as a common input.