JPS58218237A - Intermediate frequency signal processor of fm receiver - Google Patents

Abstract

PURPOSE:To reduce distortion generated by a filter without degrading selectivity by permitting a control signal to cancel distortion in a signal of the 2nd intermediate frequency when converting an FM received signal from the 1st intermediate frequency to the 2nd intermediate frequency. CONSTITUTION:The FM received signal is converted by a front end 21 into a signal of frequenct f1, i.e. the 1st intermediate frequency IF, which is applied to a BPF22 and a delay device 25, so that transmission distortion appears at the output of the BPF22. A mixer 23 performs conversion to frequencies f1+ or -f2 by a signal of frequency f2 from a VCO31 and an LPF24 extracts the 2nd IF signal of frequency f1-f2. The 2nd IF signal and the 1st IF signal from the delay device 25 are mixed by a mixer 26 and a BPF27 extracts a signal of frequency f2. The 1st and the 2nd IF signals have the same frequency shift direction, so a modulated signal S is canceled to obtain only the distortion D. The distortion D is made into fine distortion (d) through the negative feedback loop of the system with an input to a detector 8. The distortion (d) detected by the detector 28 is passed through an LPF29 and phase-inverted and amplified by an inverting amplifier 30 to control the output of the VCO31 so that the distortion D in the 2nd IF signal is canceled.

Description

DETAILED DESCRIPTION OF THE INVENTION Jin's invention relates to an intermediate frequency signal processing device that improves distortion generated by an IF filter in an IF (intermediate frequency) amplifying means of an FM receiver.

A general block diagram of the IF filter signal processing means is shown in Figure 1, in which the IF multiplied signal I by the front end r1 is
The signal is input to the IF amplifier 3 via the P filter 2. This amplified output is detected by the FM detector 4.

The IF filter 2 has characteristics that satisfy both selection characteristics and low distortion characteristics in order to eliminate interfering signals, and therefore a filter with emphasis on amplitude characteristics and phase characteristics, such as a Bessel type LC filter, is used. However, since there is a limit to the improvement of the characteristics of the IF filter 2, it is not possible to fully satisfy the above characteristics, and it is difficult to achieve a low distortion rate.

Chain Diagram 2 is an IF signal processing circuit using an FM negative feedback method that has been put into practical use to improve the characteristics of the device shown in Chain Diagram 1. A radio frequency (RF) amplifier 5 mixes the RF multiplied signal with the output of a voltage controlled oscillator (VCO) 6 and converts it into an IF multiplied signal. This IF multiplied signal, IF filter 2. A low pass filter (LPF) is passed through the IP amplifier 3 and FM detector 4.
)8. The output of the LPF 8 is fed back as a control signal for the VCO 6 to control the oscillation frequency of the VCO 6, and the FM detection output is frequency-modulated by the VCO 6 and used as a mixing input of the mixer 7. With this configuration, the RF multiplied signal frequency deviation (±75 KH) is compressed by mixing with the output of the VCO 6 (for example, ±7°5 KHz)9, and the apparent passage of the IF filter 2 Since the bandwidth is wider than that shown in FIG. 1, distortion can be improved.

However, in this FM negative feedback method, the frequency shift is compressed, so the detection efficiency is reduced accordingly, and the interference signal is also compressed at the same time, resulting in poor selectivity. Furthermore, because of the negative feedback system, there is a risk of oscillation due to circuit force jamming. ′:□Separately, the present invention provides an IF amplifier with i? An object of the present invention is to provide an engineered F signal processing device which removes distortion generated by a filter without deteriorating selectivity.

The present invention will be explained below with reference to the drawings.

Figure 3 shows a block diagram of the present invention*21
ilt front end, and a delay device 25 and a first bandpass filter 22 are connected to its output. The output of the first band pass filter 22 is connected to one side input of the mixer 230 of a\1,
The output of VCO 31 is connected to the other input of 3. The output of the first mixer 23 is the low-pass filter 1-
24, and its output is connected to the first FM detection B32 and to one side input of the second mixer 26. The output of the FM detector 32 of the ship 1 is normally connected to an MPX demodulator afterwards as an FM detection output. On the other hand, slow Yuyu 2. A'ka 1,2 degrees oh. 6C)' to 5-
;1): connected to one input, and its output is input to the band-to-pass filter 427 of chain 2. #2 band pass filter 4-27 is outstanding. ,20FM'
Detection 528, low pass filter 41-29. and inversion A
The C amplifiers 30 are connected in sequence, and the output of the inverting AC amplifier 30 controls the VCO 30. And VCO31
The output of is connected to the other input of the mixer 23 of @1 as described above. Here, ml mixer 23. Low pass filter 24. Second mixer 26. The second bandpass filter 4-2710, the one-pass filter 291, the inverting AC amplifier 30, and the VCO 31 form a negative feedback loop.

As shown in FIG.
2 and the delay device 25, the first band-pass filter 22 removes the visiting signal, and the delay device 25 corrects the delay time of the first band-pass filter 22 and the low-pass filter 24. Ru. Furthermore, the first IF signal causes transmission distortion due to the ml bandpass filter 225. The free run frequency of the VCO 31 is set to 1, and the output of the mixer 23 of the wLl is set to the first IF frequency of 10.7 MHz and vc.

The sum and difference frequencies of the oscillation frequency 9.44MHz are obtained h1
At the output of the 0-pass filter 24 the difference frequency is 1.26
MHz is obtained. This 1.26 MHz second IF signal is subjected to FM detection by a 10th F'M detector and converted into an audio signal. Also, the l12IF signal passed through the delay device 25! A second mixer 26 is applied together with the IIF signal, and a second bandpass filter 27 extracts a difference frequency of 9° 44 MHz from its output. Here, the signal of 9.44 MHz, which is the frequency of this difference, is III
When the direction of frequency shift of the F signal and the second IF signal is the same, the modulated signal S#i included in the spectrum is canceled out. The remaining spectrum is only the distortion caused by the first bandpass filter, but this distortion does not reach the input of the second FM detector 28 because the system forms a negative feedback loop. It takes a return to zero. Therefore, this will be expressed as minute strain d. The bandpass filter 27□ of the output signal 11tlI2 of the mixer 26 of the flute 2 removes the interference signal, and the FM detector 28 of the wrinkle 2 detects this signal. The detected minute distortion d is band-limited by a low-pass filter (or loop filter) 29, and its phase is inverted and amplified by an inverting AC amplifier 30. This output controls VCO 31, and the coupling to VCO 31 is AC coupling. DC is applied to vCO in advance to determine the free run frequency. Here, it will be explained that distortion is canceled by negative feedback. 1.Free run frequency of VCO319.
Assuming that 44 MHz is constant, the spectrum of the second IF signal, which is the output of the first mixer 23, will be (S0D) (ordinarily it is not one-sided because feedback is required).

Here, S is a modulation signal, and D is transmission distortion generated in the first bandpass filter 22.

Now, if the direction of the frequency, number, and 1 deviation of this distortion in the first IF signal is upward 48 (higher frequency), then the second
The IP signal is also on the upper side. This is shown in Figure 3 (D↑
), and in the output of the g2 mixer 26, the direction of this distortion is oppositely downward. If we assume that the frequency-voltage conversion characteristic of the second FM detector 28, which is indicated by (D↓), increases as the frequency increases, the voltage also increases! Even in the second FM detector 28, the direction of this distortion is downward (in this case, the direction of lower voltage). The output of the inverting AC amplifier 30 is reversed in direction and is applied to the VCO 31 with the output being on the upper side. Here, if the voltage-frequency conversion characteristic of the VCO 31 is such that the frequency increases as the voltage increases, the direction of frequency shift of the distortion spectrum in the output of vcoai will be upward. This direction is to cancel the distortion spectrum at the output of the fjKl mixer 23.

Since a negative feedback loop is constructed in this way, the distortion 11
j minute strain d. Here, the second band □ pass filter 2.7! 20FM detector 28・1 Non-linearity is minute sparrow 1. ．． Since the frequency deviation of d is extremely small, it often becomes a problem, and the interference signal is
It is removed by an EI bandpass filter and a second bandpass filter. In this manner, according to the present invention, it is possible to improve distortion caused by the filter without deteriorating the selectivity.

In the above embodiment, the delay device 25 is used to correct the delay time caused by the band pass filter 22 and the low pass filter 24, but when these delay times are small compared to the modulation signal, this is not necessary. may be omitted.
0Also, although the explanation is given by giving a specific frequency relationship, other frequencies may be used.

However, the output frequency of the VCO 31 must be selected to be lower (than the first IF frequency).Furthermore, the inverting amplifier 30 becomes a positive phase amplifier when the voltage-frequency characteristics of the FM detector 28 of the VCO 31 or aI2 are opposite.

In addition, AC in the inverting AC amplifier 30 means that the coupling with the VCO is AC, and if it is DC (direct current), the free run frequency of the VCO is determined.
Capture = Impossible, but if you solve this capture problem, you can use DC amplification.

As described above, according to the present invention, the distortion generated by the engineered F filter can be removed without deteriorating the selectivity, and the FM detector (81110F
Since a negative feedback loop is applied from the front stage of the M detector 32), a pulse count detector with a large number of time steps can be used as this FM detector. As shown in the figure, negative feedback is applied from the rear stage of the FM detector, and oscillation can be prevented since it is 3IA.

[Brief explanation of drawings]

Fig. m1 and Fig. 2 are block diagrams showing an IF signal processing circuit of a conventional FM receiver, and Fig. m3 is a block diagram showing an embodiment of the IF signal processing circuit according to the present invention. 1.21...Front end 2...IP filter 3...IF amplifier 4.28.32...FM detector 6.31...
...vc. 8.24.29・・・Low pass filter 7.2
3, 26... Mixer 22. 27... Pantone filter 25.
...Delay unit 30 ...Inverting AC amplifier patent applicant Pioneer Corporation

Claims (1)

[Claims] Frequency location! a first conversion means for converting the il received signal into a first intermediate frequency signal; a conversion means @2 for converting the intermediate frequency signal of jlE1 into a second intermediate frequency signal; control signal generating means for generating a control signal for the second converting means from the intermediate frequency signal of the converter 2; and means for obtaining an FM detection output from the intermediate frequency signal of the converter 2; An intermediate frequency signal processing device for an FM receiver, characterized in that it operates in a direction to cancel a distortion component included in an intermediate frequency signal of ta2 with respect to a conversion means.