JPH0326693Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a CW narrowband filter in a wireless communication device.

[Prior Art] Conventionally, in wireless communication devices such as transceivers, the transmitting frequency and the receiving frequency often do not match.
Correction was performed using a BFO (Beat Frequency Oscillator), but in both cases the circuit was complicated. Therefore, when receiving CW, a narrowband filter is sometimes used to reduce interference and noise and increase selectivity, but due to the narrow band of the narrowband filter, the center frequency of the narrowband filter and the intermediate frequency signal do not match. There were times when I lowered the sensitivity over and over again.

[Problem to be solved by the invention] In order to match the signal with the center frequency of the narrowband filter as described above, the invention adjusts the oscillation frequency of the local oscillator and converts the signal from the intermediate frequency to the narrowband filter. Is it a way to center it, or conversely,
There are two ways to adjust the center frequency of the narrowband filter, and the purpose of the above two methods is to provide a narrowband filter passing circuit that can be used depending on whether the intermediate frequency is high or low.

[Means for Solving the Problems] The present invention will be explained with reference to the block diagram shown in FIG. In the figure, 1 is an intermediate frequency amplification circuit, 2 is an intermediate frequency mixer circuit, and 3 is a changeover switch, which switches between the through circuit 4 and the narrow band filter 5 by operation. 9 is a beat detector in which the above circuits are connected in cascade. The changeover switch 3 has contacts a, c,
The through circuit 4 that passes through the narrowband filter 5 and contacts a, b, d, and f is selected and switched. A variable resistance element 11 is provided in the narrowband filter circuit 5 to move the center frequency of the narrowband filter 5. Furthermore, the local oscillator 6 is provided with a variable capacitance element 13 in parallel with the crystal oscillator 12, and the oscillation output is connected to the intermediate frequency mixer circuit 2.
The BFO mixer circuit 7 inputs the output frequency of the BFO oscillator 8 and the output frequency of the local oscillator 6, converts it and sends it out, and demodulates the output of the narrowband filter 5 with the beat detector 9. The structure is such that the signal is transmitted as an audible sound.

[Example] An embodiment of the present invention will be described with reference to FIG.

The intermediate frequency of intermediate frequency amplifier circuit 1 is ₀ , the frequency of local oscillator 6 is ₀ ±10kHz, and ₀ +10kHz.
or ₀ to 10 kHz is frequency-converted by the intermediate frequency mixer circuit 2, the center frequency of the narrow band filter 5 is set to 10 kHz, and the signal is supplied to the beat detector 9 through this narrow band filter.

On the other hand, the output frequency from the BFO oscillator 8 and the local oscillation frequency of the local oscillator 6 are input to a BFO mixer circuit 7, frequency-converted, and then applied to a beat detector 9. This beat detector 9 performs beat detection using the output of the narrowband filter 5 and the input from the BFO mixer circuit 7 to obtain an audible frequency, which is then sent to a low frequency amplifier circuit (not shown) via a low frequency detector 10. Send.

In this example, the center frequency of the narrowband filter is
This will be explained as 10kHz. In fact, the bandwidth of CW narrowband filters is narrow, several hundred Hz, which increases selectivity and reduces interference. However, because of this narrow bandwidth, there is a risk that even a slight frequency shift may result in a decrease in sensitivity. Therefore, in order to pass the received signal at the center of the narrow bandwidth, there are two methods: varying the center frequency of the narrowband filter to match the frequency of the signal passing through, and fixing the center frequency of the narrowband filter and adjusting the frequency of the signal passing through the narrowband filter. There is a means for varying the frequency to match the center frequency of the narrow band filter. This is selected depending on the intermediate frequency of the intermediate frequency amplifier 1 in the previous stage. For example, in the case of an intermediate frequency of 455kHz, the variable capacitance element 1 is connected in parallel to the crystal oscillator 12 of the local oscillation circuit 6.
Even if you try to vary the oscillation frequency in step 3, it is easy to oscillate in the MHz band using the oscillation method of the crystal oscillator circuit, but it is difficult to oscillate stably in the 455kHz band. When the value becomes small, unstable operation such as stopping of oscillation often occurs. For this reason, with a kHz band crystal oscillator, the circuit constants are fixed and the oscillation frequency is oscillated at a constant value, or when variable frequency is required, the outputs of two MHz band crystal oscillators are used by changing the frequency using a mixer circuit. be. Therefore, in the present invention, instead of changing the oscillation frequency, a method is used in which the band of the band waver is varied. Therefore, 455kHz±
Using a fixed oscillator at 10kHz, the oscillation frequency and the intermediate frequency are adjusted without moving the variable capacitance element 13.
455kHz is frequency-converted by an intermediate frequency mixer circuit 2 and sent to a narrowband filter 5 with a center frequency of 10kHz. This narrowband filter 5 is a bandpass filter using a feedback method of an active element 5, and the center frequency of the narrowband filter is varied by a variable resistance element 11 provided between the input side feedback point and the contact point to match the pass frequency. On the other hand, the output of 455±1.5 kHz from the BFO oscillator 8 and the 455±10 kHz output from the local oscillator 6 are frequency-converted by the BFO mixer circuit 7 to 10±1.5 kHz, which is input to the beat detector 9, and then passed through the narrow band filter 5. It is mixed with the output of 10kHz and demodulated to become an audible sound and sent out through the low frequency wave generator 10.

Next, in the second case, the intermediate frequency amplifier circuit 1
For example, 9MHz for devices with a high intermediate frequency from
In this case, even if the variable capacitance element 13 connected in parallel to the crystal oscillator 12 of the local oscillation circuit 6 is varied, the frequency can be varied in a stable oscillation state. Therefore,
The frequency is varied by adjusting the variable capacitance element 13, and 9M
The Hz±10kHz oscillation frequency is input to the intermediate frequency mixer circuit 2, and the frequency is converted with the 9MHz intermediate frequency.
10kHz matched to the center frequency of narrowband filter 5
Send out a frequency. In this case, the center frequency of the narrowband filter 5 is not varied. Other signals branched from the output of the local oscillator 6 are converted to the output frequency of 9MHz±1.5kHz of the BFO oscillator 8 and the BFO mixer circuit 7.
10kHz±1.5kHz. Therefore, even if the frequency passing through the narrowband filter 5 changes somewhat, it is demodulated by the BFO signal adjusted by the BFO oscillator 8 and becomes a desired audible sound, which is transmitted through the low frequency wave generator 10.

The condition for selecting this narrow band filter is that when the received signal has a sufficient level and it is sufficient to simply adjust the BFO oscillator, the changeover switch 3 will match through the through circuit side, but if the received signal has a poor S/N ratio or interference occurs. In such cases, clear demodulated sound can be obtained by switching the changeover switch 3 to the narrowband filter 5 side.

[Effects of the invention] As described above, in the invention, either the variable resistance element of the narrowband filter or the variable capacitance element of the local oscillation frequency is used, whichever is appropriate, to change the band of the narrowband filter or the passthrough. By varying the signal frequency, the frequency of the CW narrow band filter and the passing signal can be matched to increase the selectivity, and a good audible sound of the CW signal can be obtained with extremely simple operation, which has a practical effect. is big.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an intermediate frequency tuning circuit showing one embodiment of the present invention, and FIG. 2 is a wiring diagram thereof. DESCRIPTION OF SYMBOLS 1... Intermediate frequency amplifier circuit, 2... Intermediate frequency mixer circuit, 3... Changeover switch, 4... Through circuit, 5... Narrowband filter, 6... Local oscillator,
7...Beat detector, 8...BFO oscillator, 9...
...BFO mixer circuit, 10...Low frequency device, 11
...Variable resistance element, 12...Crystal oscillator, 13...
...Variable capacitance element.

Claims (1)

[Scope of utility model registration request] In a wireless communication device in which an intermediate frequency amplification circuit of either a high frequency or a low frequency, an intermediate frequency mixer circuit, a beat detector, and a low-frequency wave detector are connected in cascade, between the intermediate frequency mixer circuit and the beat detector. A narrowband filter and a through circuit for improving interference or S/N are arranged in parallel and are provided with means for selecting and switching them with a changeover switch, and the narrowband filter is a bandpass filter having an active element and a feedback path, and is a bandpass filter having an active element and a feedback path. A means for varying the passband width of the narrowband filter, which has a variable resistance element provided between the feedback point on the element input side and the ground, and a local oscillator whose output is input to the intermediate frequency mixer circuit. ,
A variable capacitance element is provided in parallel with the crystal oscillator to vary the oscillation frequency when the intermediate frequency is high, and the BFO signal input to the beat detector is
The local oscillation output and the BFO oscillator output are connected to the BFO
An intermediate frequency narrow band filter tuning circuit configured to be connected to a beat detector via means for frequency conversion using a mixer circuit.