JPH0141242Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an FM receiver in which the amplitude limit of an intermediate frequency signal has a logarithmic characteristic.

[Conventional technology]

FM receivers that receive and demodulate frequency modulated (FM) signals generally use an amplitude limiter for intermediate frequency signals to remove unnecessary amplitude modulated (AM) components. Figure 5 shows the input/output characteristics of a conventional amplitude limiter, where the horizontal axis is the input electric field,
The vertical axis is the intermediate frequency (IF) output.

[Problem that the invention attempts to solve]

An amplitude limiter with this type of characteristic has no problem if the input electric field is higher than point a, but if it is lower than point a, it operates only as a linear amplifier and cannot provide sufficient amplitude limitation. For this reason, there is a drawback that AM components remain in the demodulated output when the electric field is weak.

[Means for solving problems]

The present invention attempts to remove the above AM component over a wide electric field range by utilizing logarithmic conversion characteristics. For this reason, the present invention is an FM receiver that receives a high frequency signal, converts it to an intermediate frequency signal, passes it through an amplitude limiter, and then demodulates it to a low frequency signal.The intermediate frequency signal is input to a logarithmic amplifier and its output is input to the intermediate frequency band pass filter,
Furthermore, the output of the filter is input to the amplitude limiter.

[Effect]

Since the frequency of the FM wave input to the amplitude limiter (usually an intermediate frequency of 10.7MHz) and the frequency of the AM component (low frequency) are far apart, the present invention utilizes the AM detection effect of a logarithmic amplifier to separate both. Separate. In other words, the pure FM wave V _FM is the AM component
IF output of the frequency converter V when accompanied by V _AM
is expressed as the product of both.

V=V _FM ×V _AM ...(1) This uses V _FM as the carrier wave and V _AM as the modulating wave.
Since it is an AM wave, when it is input to a logarithmic converter, the output is LogV = LogV _FM + LogV _AM ... (2). The first term on the right side of the above equation is a high frequency (in this case,
The intermediate frequency (intermediate frequency) and the second term have low frequencies as their main components, so they can be separated using a filter. In a normal FM receiver, the IF is installed before the amplitude limiter.
Since a band-pass filter is used, only the frequency components of V _FM included in LogV _FM can be extracted.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic block diagram showing an embodiment of the present invention, in which 1 is an antenna, 2 is a received high-frequency signal
High frequency amplifier that amplifies RF, 3 is intermediate frequency signal
Frequency converter to IF, 4 is a local oscillator that generates the necessary local signal LO, 5 is the above-mentioned
A logarithmic amplifier that produces the output LogV of equation (2) for the input V of equation (1), and 6 passes only the V _FM component.
An IF band pass filter, 7 an amplitude limiter which cuts amplitudes above a certain value, and 8 an FM demodulator which obtains a low frequency output AF from its output.

FIG. 2 is a circuit diagram showing a specific example of the logarithmic amplifier 5, where 51 is a non-inverting amplifier with a gain of +A using two-stage amplification transistors Q ₁ and Q ₂ , and 52 is a non-inverting amplifier with a gain of +A using two-stage amplification transistors Q ₁ and Q ₂ . an exponential amplifier 53 which operates in a complementary manner and converts the output Eo of the amplifier 51 into an index -e ^E0 by utilizing the exponential relationship between their collector current Ic and the base-emitter voltage _VBE ;
is an adder which adds the input Ei and the output -e ^E0 of the exponential amplifier 52, and as a whole constitutes a negative feedback amplifier as shown in FIG. The exponential conversion characteristic of the exponential amplifier 52 utilizes the nonlinearity between the bases and emitters of the transistors Q ₃ and Q ₄ . Therefore, zero bias is applied to both transistors by diodes D ₁ and D ₂ .
Transistor _Q3 then operates when output Eo is negative, producing a positive exponential conversion output, and transistor
_Q4 operates when the output Eo is positive and produces a negative exponential conversion output. −e ^E0 is the composite waveform of these.
C ₁ and C ₂ are DC blocking capacitors.

In this negative feedback amplifier, the input of the non-inverting amplifier 21 is
Since Ei-e ^E0 , Eo=(Ei-e ^E0 )・A, and since Eo≪Ae ^E0 , approximately A・e ^E0 =A・Ei. Therefore, if we take the logarithm of both sides, Eo=logEi, so it can be seen that there is a desired logarithmic relationship between input Ei and output Eo. FIG. 4 shows the input/output characteristics up to the amplitude limiter 7, which shows logarithmic characteristics when the electric field is weak below point a.

[Effect of idea]

As described above, according to the present invention, the degree of AM suppression of the FM receiver can be maintained satisfactorily over a wide input electric field range. In particular, there is an advantage that the frequency discrimination ability in a weak electric field is improved and the S/N ratio is improved.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.
Figures 2 and 3 are circuit diagrams and block diagrams showing details of the logarithmic amplifier, Figure 4 is an input/output characteristic diagram of the amplitude limiter according to the present invention, and Figure 5 is the input/output characteristic of a conventional amplitude limiter. It is a diagram. In the figure, 3 is a frequency converter, 5 is a logarithmic amplifier, and 6
is a bandpass filter, and 7 is an amplitude limiter.

Claims (1)

[Scope of utility model registration request] Receives high frequency signals and converts them to intermediate frequency signals,
Furthermore, in an FM receiver that demodulates a low frequency signal after passing through an amplitude limiter, the intermediate frequency signal is inputted to a logarithmic amplifier, its output is inputted to a bandpass filter for the intermediate frequency band, and the output of the filter is further inputted to the amplitude limiter. An FM receiver characterized in that the input is input to a limiter.