JPH0366231A - Intermediate frequency circuit for fm tuner - Google Patents

Abstract

PURPOSE:To make a signal pass band narrow remarkably by varying the center frequency of a signal pass band of an intermediate frequency in interlocking with a change of the input signal frequency to an intermediate frequency stage. CONSTITUTION:An intermediate frequency 10.7MHz is outputted from an FM front end unit, amplified by an amplifier, a DC voltage is outputted by an F/V converter and an output VT is fed to varactor diodes D1, D2 through resistors R1, R2. Thus, the center frequency is varied in coincidence with a change in the IF input frequency. Since the window is decreased extremely and the position of the window is moved quickly and accurately in matching with the position of the signal coming at random to pass the signal therethrough, no noise is passed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an intermediate frequency stage (IP stage) circuit of an FM tuner.

[Prior Art] The performance of super-hetero gain tuners has been significantly improved. This super-hetero gain method introduces the idea of an intermediate frequency (IP) stage. In FM (frequency modulation), where performance is particularly important, the IP stage plays a large role. However, the current parts that make up the IP stage have reached their limits in performance and quality, and it is extremely difficult to improve their performance any further. In other words, in order to further improve the performance, an IP stage whose principle is fundamentally different and which is nearly perfect is required.

The intermediate frequency stage (IP stage) of an FM tuner is an important circuit that greatly influences the various performances of the FM tuner.
In order to improve the signal-to-noise ratio (S/N ratio), it is necessary to set the signal passing band width as narrow as possible. However, with the current system, it is not possible to design this signal passing bandwidth narrowly. Therefore, in addition to signals, noise also passes through, resulting in a poor S/N ratio. In order to prevent this S/N ratio from deteriorating, various IFTs and ceramic filters as shown in FIGS. 4 to 7 are used as intermediate frequency filters.

The single-tuned IFT'' shown in FIG. 4 is low cost or has a wide band. Therefore, the S/N ratio is poor.

The double-tuned IP'f'' in Figure 5 is obtained by using two single-tuned IFTs, each with a slightly smoothed resonance frequency curve as shown by the dotted line, and a composite curve as shown in the solid line in Figure 4. The band is narrower than that.

The ceramic filter shown in Figure 6 has a narrow bandwidth, but the second-order
The tertiary resonance point is noticeable.

The composite filter shown in Figure 7 is a combination of those shown in Figures 5 and 6.The IF transformer cuts off the secondary and tertiary resonance points, resulting in a narrow band and close to ideal trapezoidal characteristics. Become.

The one shown in Fig. 4 is used for low-cost tuners, the ones shown in Figs. 5 and 6 are used for middle-class tuners, and the one shown in Fig. 7 is used for high-performance tuners.

Generally, the IF of an FM tuner is 10.7 MH2, with a deviation of 75 KHz at 100% modulation. Taking this as an example, 10°7MHz±75K
Hz, i.e. 150 K I as shown in Figure 7.
-The bandwidth cannot be lower than Iz. Dare to be 150KHz
If the bandwidth is set below, distortion will occur in the detection output. Also, if dynamic range is important, 2
Requires a bandwidth of 50 KHz.

[Problem to be solved by the invention] By the way, the narrower the signal passing band width of the 11-stage FM tuner is designed, the more the selectivity is improved, and the S/
N and other performance can be improved, but in principle it is not possible to make the bandwidth smaller than the frequency displacement. This is because if you do this, the main signal itself will be truncated.

In the conventional FM stereo tuner IF, the signal passing band width is approximately ±250 to 1 (z (intermediate frequency: 10.7
MI-Iz), approximately ±25KHz for data communication
(Intermediate frequency: 455 KHz) is common. The frequency of the signal entering the 11th stage changes within the above range from time to time along the modulation signal.

Now, if the center value of the signal passing bandwidth of the IF stage is moved in accordance with the changing input signal, even if the bandwidth is made narrow, it is possible to pass all the input signals. The present invention was made with attention to this point, and by using 11 stages that change the center frequency in conjunction with input frequency displacement, it is possible to reduce the bandwidth to below the conventional theoretical limit IP bandwidth. An object of the present invention is to provide an intermediate frequency stage circuit with high S/N ratio.

[Means for Solving the Problems] In order to achieve the above object, the intermediate frequency stage circuit of the FM tuner of the present invention changes the signal passing bandwidth of the intermediate frequency stage in conjunction with the displacement of the input frequency of the intermediate frequency stage. It has a configuration in which the center frequency changes, and the bandwidth is narrow.

[Function] FM tuner's IF stage FSK (FREQUE
NCYSHIFT KEYING) band frequency is generally designed to be 200KHz to 300KHz,
Set this to as extreme as possible, such as 10KHz or 5KHz.
Make the F stage narrowband (let's say 5KHz now). This makes it difficult for noise to pass through and allows only the main signal to pass. On the other hand, the intermediate frequency is 10.7MHz ± 75 in the case of 100% modulation. KHz changes, so 5K
In a Hz bandpass filter, most of ±75 KHz cannot be passed by the conventional method. To deal with this, the center frequency of the IF with a bandwidth of 5 KHz is varied in conjunction with a change of ±75 KHz. As a result, the main signal passes through without causing any signal loss or distortion, and moreover, it is possible to obtain a significantly higher S/N ratio than in the case of a bandwidth of 200 to 300 KHz.

By the way, the 11th stage can be said to be a window through which signals do not pass. Since the signal is randomly waved by the voice, in the past, this window was left wide open to receive it, which may have caused noise to come in. In the present invention, the window is made extremely small to match the position of the randomly arriving signal. The window is moved quickly and precisely to allow the signal to pass through, so no noise gets through. Conventional tuners do not use this technique of moving the window position.

[Example] Hereinafter, it will be explained with reference to the drawings. In the IF stage circuit of the present invention,
As shown in Figure 1, the band is much narrower than the conventional band (
In this example, an IF filter (5 KHz) is used, and its center frequency (for example, 10.7 MHz) is changed in conjunction with the IF input frequency. Note that the output level is constant.

FIG. 2 shows the circuit configuration of the FM-IF stage according to the first embodiment of the present invention. An intermediate frequency of 10.7 MHz is output from the FM front end unit.

10.7M) (If z is unmodulated, after amplifying it with an amplifier (AMP), a DC voltage is output with an F/V converter (frequency/voltage conversion circuit: a type that outputs a higher DC voltage as the frequency increases). Output ■1 divided by resistors R3 and R4
is applied to the varactor diode D1 through the resistor R1, and the capacitance at both ends of the varactor diode DI (which may be a varicap diode or the like) becomes Cxl. Cxl is adjusted to have a resonance frequency of 10.7 MHz with the secondary coil L1 of the intermediate frequency transformer T1 through the capacitor C1. (Adjust the intermediate frequency transformer T1) Similarly, the intermediate frequency transformer T 2111J is also 10.7MH
Let it resonate at z.

Next, when the intermediate frequency of 10.7 MHz is modulated by ±75 KH2, at the moment of +75 KHz, the DC output voltage of the F/■ converter becomes correspondingly higher and is applied to the varactor diode D1. As the voltage across the varactor diode D1 increases, the capacitance across it decreases, and as a result, the varactor diode D1, the capacitor C1,
Intermediate frequency transformer T1 and varactor diode D2
, capacitor C2, and intermediate frequency transformer']2 become higher in resonance frequency. The frequency when it gets high is 107 M
Hz +75 KHz Resistor R3, resistor R
Decide on the resistance division ratio of 4.

Next, at the moment of -75KHz, the output of the F/V converter is low, and the varactor diode D1, capacitor C1
, the intermediate frequency transformer 1'1, the varactor diode D2, the capacitor C2, and the intermediate frequency transformer T2 have a resonant frequency of 10.7 MHz to 75 KHz.

Here, the resistors R3 and R4 will be explained.

When the input frequency to the F/V circuit changes by +Δf1, F
Assume that the output voltage change of the /V converter is +Δ■. +
When Δ■ is added to varactor diode D1 and D2, intermediate frequency transformer T1 and varactor diode D
The resonant frequencies of the resonant circuit of 1, intermediate frequency transformer 1゛2, and varactor diode D2 are Δf2, Δ
If f3 is designed to have the relationship Δfl<Δf2-Δf3, by appropriately selecting the division ratio of resistors R3R4, the center frequency of IF will change in accordance with the change in IP input frequency. The desired operation and effect as shown in FIG. 1 can be obtained. Note that the resistance R3 may be adjusted by using a semi-fixed volume.

FIG. 3 shows a circuit configuration of an FM-IP stage according to a second embodiment of the present invention. The F/V converter in the above embodiment is a variety of ICs.
is on the market, but there is also an F/V circuit using a face-locked loop circuit (PLL), which is surrounded by the dotted line in Figure 3.
A circuit that performs the same plowing as the converter will be explained.

Assuming that the 10.7MHz output of the FM front end unit is unmodulated, the 10.7MHz output is output by an amplifier (AM
P) into the mixer (M I XER). If the reference oscillator is 4 MHz, the voltage controlled oscillator (VCO) is locked to 6.7 MHz. At this time,
MI XER output is 10.7MHz +6.7MHz
z = 17.4M1lz10, 7HIIz -6,7M
Two types of 1lz = 4.0Hllz are output. The 17.4MHz frequency is cut off by the low-pass filter (LPF), and the 4MHz frequency (z passes through the LPF,
It passes through a 7° amplifier and the reference 4 MH
Since they match, the I-'LL loop 6 becomes locked.

Next, suppose that io, 7MHz is deviacin 100K
If Hz modulation is applied, 10.7MHz
At +100 KHz moment, the VCO is 68 M
Hz and then at the 10.7 MHz-100 KHz moment, the VCO is turned off to 6.6 MHz. The control voltage V-r of the VCO at this time is higher than that at 6.7 MHz, and the voltage at 6.8 MHz is higher;
6M1-1 Since the voltage ■ when z is low, the second
Voltage (1), which operates in the same way as shown in the figure, is obtained.

0 Note that the present invention is not limited to the configuration of the above embodiment, and can be applied to both digital FM tuners and analog FM tuners.

[Effects of the Invention] As described above, according to the present invention, the center frequency of the signal passing band width of the intermediate frequency stage is changed in conjunction with the displacement of the input frequency of the intermediate frequency stage. The bandwidth can be made significantly narrower, and as a result, the passage of noise can be suppressed, and the S/N ratio of the tuner can be greatly improved. In addition, the interlocking width of this center frequency position corresponds to the bandwidth of the conventional intermediate frequency stage,
Since this movement width can be set to 250 KHz or 500 KHz, the dynamic range can be made larger than before if necessary.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the concept of the circuit configuration of the present invention, FIG. 2 is a diagram of the circuit configuration of the first embodiment of the present invention, FIG. 3 is a diagram of the circuit configuration of the second embodiment, and FIG. a)(b) to Figure 7(a)(
b) is a diagram showing the configuration and characteristics of a filter of a conventional F1 M tuner. T1...Intermediate frequency transformer, Dl...Varactor diode, F/V...Frequency/voltage converter, P L
L...Face locked loop circuit.

Claims (1)

[Claims] (1) An FM tuner characterized in that the center frequency of the signal passing band width of the intermediate frequency stage changes in conjunction with the displacement of the input frequency of the intermediate frequency stage, and the bandwidth is made narrow. intermediate frequency stage circuit.