JPH0481899B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an AM receiver.

[Conventional technology]

A conventional AM receiver, as shown in Figure 2, uses RF
Select the receiving frequency by filter 12 and PLL
A local oscillator consisting of a circuit 16, a VCO 17, a mixer 15, and an IF filter 18 mix down to a predetermined IF frequency, for example, 455 kHz.The IF amplifier or RF amplifier is used as an AGC amplifier 19 to set the IF amplifier output to a constant level, and then the rectifier 23 The method used was envelope detection.

[Problems to be Solved by the Invention] The conventional AM receiver described above requires an AGC amplifier with a wide dynamic range for the RF amplifier or IF amplifier, and also requires an RF filter and an IF filter. Also, because it is a superheterodyne system, there are drawbacks such as spurious characteristics with respect to image frequencies.
The RF filter is also required to have filter characteristics that sufficiently attenuate the image frequency.

[Means for solving problems]

The AM receiver of the present invention includes a logarithmic amplifier that outputs a signal including a component proportional to the logarithm of the amplitude of an input signal, and a PLL connected to the output terminal of the logarithmic amplifier.
means for obtaining a local frequency signal synchronized with the input signal, comprising a circuit, a first filter whose input end is connected to the output of the PLL circuit, and a VCO whose frequency is controlled by the output of the first filter. and a mixer circuit that demodulates the output of the logarithmic amplifier using the local frequency signal;
A second filter that passes only the audio frequency band of the output of the mixer circuit, and an antilogarithmic amplifier that outputs a signal containing a component proportional to the antilogarithm of the amplitude of the output of the second filter. Features.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. AM modulated signal received by antenna 1
The amplitude value of the RF signal is logarithmically compressed and amplified by the logarithmic amplifier 2, and then sent to the mixer circuit 3.
The signal is input to the PLL circuit 4.

Therefore, the RF signal output from the logarithmic amplifier 2 is only logarithmically compressed in amplitude value, and its frequency remains unchanged. For example, even if there are RF signals of two frequencies with a difference in level at the antenna input, it can be considered that they are logarithmically compressed at the output of the logarithmic amplifier and converted into RF signals of two frequencies with a small difference in level. The RF signal output of this logarithmic amplifier is the PLL circuit 4
As a result, the frequency of the output signal of the VCO 5, which has previously oscillated at a frequency close to the desired frequency, is phase-locked and synchronized so that it becomes the same frequency as the RF signal output.

Here, the loop filter 6 that constitutes the PLL loop is a low-pass filter, and its cutoff frequency is set to a value lower than the modulation frequency band of the AM modulated wave from 0.3kHz to 3kHz. The output signal is a constant level signal synchronized with the receiving frequency at antenna 1.

On the other hand, the RF signal output of the logarithmic amplifier 2 is also input to the mixer circuit 3 and mixed with the VCO output. The mixer circuit 3 maintains linearity between the input signal and the output signal as long as the local signal is at a constant level. Therefore, the output of bandpass filter 7 is
The AM modulated wave is synchronously detected and only the AM demodulated signal is output.

However, since it is passed through a logarithmic amplifier in the RF section,
The amplitude characteristics of this AM demodulated signal are subjected to logarithmic compression like the RF signal, but the frequency is the same as that of the AM modulated signal.

Therefore, by connecting an antilogarithmic amplifier 8 after the bandpass filter 7, antilogarithmic transformation is performed, and the amplitude characteristic that has been logarithmically compressed by the logarithmic amplifier in the RF section is antilogarithmically expanded, and thus the logarithmic characteristic cancels out. Being
The AM modulated wave will be linearly detected.

Therefore, the speaker is driven by the demodulated signal of the AM modulated wave received by the antenna.

Here, the dynamic range of logarithmic amplifier 2 is
This is the dynamic range of the AM receiver, and the dynamic range of the logarithmic amplifier 2 can normally be about 100 dB. Further, the dynamic range of the inverse logarithmic amplifier 8 is the dynamic range of the demodulated signal, and usually about 20 dB is sufficient for an AM receiver. Therefore, the antilogarithmic amplifier 8 can be simple.

As explained above, the output of the logarithmic amplifier is the same logarithmically amplified output of all the frequency components input to the logarithmic amplifier, so all the input frequency components are obtained as logarithmic values, and therefore the AM component is also logarithmically amplified. Obtained by characteristics. Therefore, by connecting a mixer circuit to the output of the logarithmic amplifier and inputting a wave synchronized with the received wave to the local signal, the AM detection signal of the received wave can be obtained at the mixer output. Obtained by logarithmically transforming the AM component of the wave. This will be explained using a formula.

The input of the logarithmic amplifier is Vin, the output is Vout,
If the slope of the logarithmic characteristic of the logarithmic amplifier is B and the DC output component is C, then Vout(t)=Bln|Vin(t)|+C... If the input is an AM modulated wave, Vin (t) = A _O {1 + Kcos (pt + θ)} cosω _c t ... However, ω _c is the angular velocity of the carrier, and p is the AM modulated wave.
is the angular velocity of the modulated signal, and θ is its phase shift. The modulation degree k is O<k<1... Substituting the formula into the formula, we find Vout(t)=BlnA _O +Bln{1+kcos(pt+θ)}+Bln|cosω _c t|+C... In other words, it can be seen that fluctuations in the amplitude value A _O of the input wave result in fluctuations in the DC component at the output of the logarithmic amplifier. Therefore, only the AM component of Bln{1+kcos(pT+θ)} is obtained through the bandpass filter.

〔Effect of the invention〕

As explained above, the present invention is applicable to the RF band or
AF without the need for a bandpass filter in the IF band
The necessary selectivity can be obtained with a band-pass filter, and there is no need for a wide dynamic range AGC amplifier, and it can be realized with a circuit configuration suitable for IC implementation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a block diagram showing a conventional example. 2... Logarithmic amplifier, 4... PLL circuit, 6...
Loop filter, 5... VCO, 3... Mixer circuit, 7... Band pass filter, 8... Anti-logarithmic amplifier.

Claims (1)

[Claims] 1. A logarithmic amplifier that outputs a signal including a component proportional to the logarithm of the amplitude of an input signal, a PLL circuit connected to the output terminal of the logarithmic amplifier, and a first filter whose input terminal is connected to the output of the PLL circuit. , and a VCO whose frequency is controlled by the output of the first filter, means for obtaining a local frequency signal synchronized with the input signal, and a mixer circuit for demodulating the output of the logarithmic amplifier using the local frequency signal. , a second filter that passes only the audio frequency band of the output of the mixer circuit, and an antilogarithmic amplifier that outputs a signal containing a component proportional to the antilogarithm of the amplitude of the output of the second filter. An AM receiver featuring