JPH023332B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a frequency conversion method in an FM receiver.

In general, FM receivers have a configuration in which the received wave (high frequency signal) is first converted into an intermediate frequency signal and then detected, and has the advantage of an excellent S/N ratio. When electric power is present and a weak electric field is below a certain value, the S/N ratio deteriorates rapidly and reception becomes impossible. Since this receivable limit power is proportional to the receiver bandwidth if other conditions are constant, it is well known that high-sensitivity reception becomes possible by narrowing the bandwidth of the intermediate frequency amplification section. . However, in the conventional system, the frequency shift of the intermediate frequency signal is the same as that of the received wave, and distortion increases as the band becomes narrower, so it was not possible to narrow the band unnecessarily.

Further, as shown in FIG. 1, the received wave is converted into an intermediate frequency signal by a frequency converter 1, amplified by a narrow band intermediate frequency amplifier 2, and then detected by an FM detector 3. In a configuration configured to obtain an FM detection output, frequency modulation is applied to the local oscillator 4 using the FM detection output, and the local oscillation frequency is changed to follow the frequency deviation of the received wave, thereby eliminating the frequency deviation in the intermediate frequency amplification section. So-called FM
A negative feedback method is known. However, in this method, a high-gain narrowband intermediate frequency amplifier enters the FM negative feedback loop, and the phase delay of the signal becomes significantly large due to narrowing the band, so the feedback amount is large from the viewpoint of stability. Therefore, the frequency shift in the intermediate frequency amplification section cannot be significantly reduced, so there is a drawback that no improvement beyond a certain level can be obtained.

The present invention improves these conventional drawbacks, and will be explained below with reference to the block diagram of FIG.

[] Received wave from antenna (high frequency signal)
(frequency F=±d) and the output signal (first local oscillation frequency _L1 ) of the first local oscillator 6 are mixed by the first mixer 5, and a first signal having a frequency that is the difference between the frequencies of both signals is mixed. Obtain an intermediate frequency signal.

The frequency F ₁ of this first intermediate frequency signal is F ₁ = (±d) − _L1 = (− _L1 ) ±d = ₁ ±d where: carrier frequency of high frequency signal d: frequency deviation _L1 of high frequency signal : First local oscillation frequency ₁ : Intermediate frequency (= _-L1 ).

[] This first intermediate frequency signal F ₁ is divided into 1/n frequency divider 7
The frequency is divided by 1/n to obtain a frequency-divided signal having the frequency Fn Fn=1/nF ₁ 1/n ( ₁ ±n).

[] This frequency-divided signal and a second local oscillation signal outputted from the second local oscillator 9 and having a second local oscillation frequency _{L2 L2} = n+1/n ₁ are mixed by the second mixer 8, A second intermediate frequency signal having a frequency that is the difference between the frequencies of both signals is obtained.

The frequency F ₂ of this second intermediate frequency signal is F ₂ = _L2 −Fn = n+1/n ₁ −1/n ( ₁ ±d) = ₁ ±1/nd, where the first term is the intermediate frequency and the second term is Represents frequency deviation. In other words, the frequency deviation of the second intermediate frequency signal is 1/n of the frequency deviation d of the high frequency signal.
becomes.

[] After this second intermediate frequency signal is amplified by the narrow band intermediate frequency amplifier 10, the FM detector 1
1 to obtain the FM detection output.

As a specific example, in the FM band in Japan, that is, when each frequency is set as follows: =76~90MHz d=±75KHz ₁ =10.7MHz, if the first local oscillation frequency _L1 is set to 65.3~79.3MHz. , the respective frequencies of the first intermediate frequency signal, the divided signal, and the second intermediate frequency signal are F ₁ = 10.7 MHz ± 75 KHz Fn = 1/n (10.7 MHz ± 75 KHz) F ₂ = 10.7 MHz ± 75/nKHz .

Here, if the frequency division ratio n=10, then F ₂ =10.7MHz±7.5KHz.

In this way, the frequency deviation in the intermediate frequency amplification section is 1/n of that in the conventional system, so for example, in the above specific example, the frequency deviation in the intermediate frequency amplification section is ±75KHz. In contrast, in the present invention, the bandwidth of the intermediate frequency amplifier was required to be 230 to 250 KHz.
A narrow band of 23 to 25KHz is sufficient, and reception sensitivity is improved by approximately 6dB.

Note that the frequency division ratio n of the frequency divider 7 and the intermediate frequency amplifier 1
By appropriately setting the zero bandwidth according to design specifications and other conditions, an FM receiver with high sensitivity and low distortion can be realized.

As described above, the present invention converts a high frequency signal from an antenna into a first intermediate frequency signal, and then converts the first intermediate frequency signal into a first intermediate frequency signal.
This frequency conversion method divides the frequency of an intermediate frequency signal to obtain a frequency-divided signal, and then converts this frequency-divided signal into a second intermediate frequency signal with a predetermined relationship. This allows the intermediate frequency amplification section to be narrow-banded, resulting in low distortion and high sensitivity.
It has the advantage of being able to provide an FM receiver.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an FM receiver using the FM negative feedback method, and FIG. 2 is a diagram showing the configuration of an FM receiver employing the frequency conversion method of the present invention.

Claims (1)

[Scope of Claims] 1. A frequency conversion method in an FM receiver comprising the following constituent elements (a) to (c). (b) Means for mixing the FM high frequency signal (frequency ±d) and the first local oscillation output signal (frequency _L1 ) to obtain a first intermediate frequency signal having a frequency ₁ ±d, which is the difference between the two frequencies. However, ₁ = - _L1 : Carrier frequency of FM high frequency signal d: Frequency deviation of FM high frequency signal _L1 : First local oscillation frequency, ₁ : Intermediate frequency (b) Divide the first intermediate frequency signal by the frequency division ratio n means to obtain a frequency-divided signal having a frequency of 1/n ( ₁ ±d). (c) Means for mixing the frequency-divided signal and a second local oscillation output signal having a frequency of n+1/ _n1 to obtain a second intermediate frequency signal having a frequency of the difference between the two frequencies, ₁ ±1/nd. .