JPH0479491B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) This invention relates to a tone transponder circuit in a car telephone.

(Prior Art) In a cellular mobile telephone system, tones are sent from a base station for various monitoring purposes during a call.

When the mobile device receives this tone, it converts it into a clean waveform, synchronizes it with the received signal at 0° phase, and sends it back.

The conventional circuit has a circuit configuration as shown in Figure 3,
10 is a wireless receiver, 12 is an LC filter, 14 is a phase lock loop (PLL), 15 is a wireless transmitter, 16 is a received voice processing circuit, 17 is a speaker,
18 is a microphone, and 19 is a transmission audio processing circuit.
The received tone is extracted by the LC filter 12, the PLL 14 is synchronized with the output, and the tone is transmitted so that the phase from the receiving antenna to the transmitting antenna becomes 0°.

(Problems to be Solved by the Invention) However, the conventional circuit has the following drawbacks. There are three types of tones transmitted from a base station (5970, 6000, and 6030 Hz), and adjacent base stations that use the same wireless channel use different types of tones.

When a mobile device changes radio zones, a tone change is also performed, but this occurs in the marginal area of the radio zone, so the switching occurs under conditions where the C/N ratio (carrier signal-to-noise ratio) is very poor. becomes.

To reproduce a tone with poor C/N, it must be passed through a high Q filter.

A filter with a high Q tends to have a frequency characteristic shift due to temperature change, and a phase shift with respect to an input signal (in particular, a wide temperature range is required for an in-vehicle phone).

In order to minimize the effects of temperature changes,
The filter must be constructed from high-quality elements with good temperature characteristics, which makes the filter expensive.

The present invention aims to improve this drawback.

(Means for Improving the Problems) A feature of the present invention for achieving the above object is that in a control system of a mobile radio telephone, one side of the radio line sends out a tone and the other side sends back a tone synchronized with the tone. , the receiving side has a switched capacitor filter having a bandpass characteristic that follows the frequency of the tone, and a phase lock loop circuit connected to the output thereof and reproducing a signal synchronized with the tone, and the switched capacitor filter clock is provided by a phase lock loop circuit and is in a tone transponder circuit whose center frequency follows the received tone frequency.

(Operation) Unlike conventional LC filters, switched capacitance filters (SCFs) differ from conventional LC filters in that even if the tone frequency shifts, the center frequency changes accordingly, so the phase does not change. Therefore, it is possible to drive the subsequent phase lock loop (PLL) with a signal whose S/N has been improved by using the SCF with a sufficiently high Q value, and the object of the present invention can be achieved.

(Embodiment) FIG. 1 is a circuit showing an embodiment of the present invention. Switched capacity filter for filter
An SCF 30 is provided, and a clock (referred to as f _SCF ) for determining the center frequency of the SCF 30 is provided.

This filter has a phase characteristic of 0° at the center frequency.

Since the output of the SCF 30 is a staircase wave, it is passed through a simple low-pass filter LPF 32 to convert it into a sine wave.

Next, the process goes to the PLL 42, where the output of the voltage controlled oscillator VCO is set to be N times the input frequency (N is a natural number), and this is fed back to the SCF circuit 30 as f _SCF .

The VCO38 output is multiplied by 1/N, and the PLL outputs the same frequency as the input frequency.

For example, when a signal around 6 kHz is input from the input terminal, the signal reaches the PLL 42, and a signal f _SCF (= f _IN ×N) which is locked to the input signal f _IN is created. f _SCF is given as a clock signal of SCF 30, and the center frequency f _C of SCF 30 is f _IN .

Even when the frequency of the tone signal changes according to instructions from _the base station, the _SCF3
The center frequency of 0 will be the same as the input frequency.

When there is no input signal, the PLL 42 outputs a frequency around 6kHz. Therefore SCF30
The center frequency will be around 6kHz, and the circuit will be in the state of waiting for an input signal.

If the frequency of the tone to be used is known in advance, the center frequency of the SCF 30 can be tuned to the input tone frequency by controlling the VCO 44 using the VCO control terminal 44 so that the free run frequency of the PLL circuit 42 becomes the tone frequency. This makes it possible to design the PLL bandwidth narrowly, making it easy to improve noise resistance and stability.

In this way, the SCF 30 always follows changes in the input frequency, so one SCF can support several types of tones.

For convenience of explanation, the internal representation of the PLL is assumed to be analog, but it does not matter whether it is an analog PLL or a digital PLL.
It is clear that a similar effect can be obtained with PLL.

Figure 2 shows an example of characteristics showing the gain and phase of a switched capacitor filter (SCF), and it can be seen that the phase is always 0 for three center frequencies (5970Hz, 6000Hz, 6030Hz).

(Effect of the invention) As explained above, this invention provides
Since an SCF using the PLL output as a clock is provided, there is an advantage that the phase is stable against temperature changes even with a high Q filter configuration. It is the clock frequency that determines the center frequency and phase of the SCF, and changes in C and C due to temperature changes.
Even if elements such as R change, the center frequency and phase are hardly affected.

Therefore, even if the Q of the SCF filter is high, a circuit with stable phase can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a tone transponder circuit according to the present invention, FIG. 2 is a characteristic diagram showing the operation of a switched capacitor filter, and FIG. 3 is a configuration diagram of a conventional car telephone device. 30; Switched capacitor filter (SCF),
32; low pass filter; 42; phase lock loop circuit (PLL).

Claims (1)

[Claims] 1. In a control system for a mobile radio telephone where one side of a radio line sends out a tone and the other side sends back a tone synchronized with the tone, the receiving side uses a switched channel having bandpass characteristics to follow the frequency of the tone. A phase lock loop circuit is connected to the output of the passive filter and reproduces a signal synchronized with the tone, and the clock of the switched capacitor filter is supplied from the phase lock loop circuit, and the center frequency of the filter is A tone transponder circuit characterized in that the signal follows the received tone frequency. 2. The tone transponder circuit of claim 1, wherein the free run frequency of the phase lock loop is adjustable to match the input tone frequency.