JPS5834981B2 - Automatic frequency control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is mainly used for automatic frequency control (hereinafter referred to as AFC), which is useful for receiving local oscillators of small-capacity microwave band multiplex radios.
(referred to as ) method.

In such radios, frequency stability of the transmitting and receiving local oscillators is a problem.

In other words, since the bandwidth of the reception intermediate frequency (hereinafter referred to as IF) filter is narrow, if the stability is poor, the reception signal will deviate significantly from the center of the reception IF filter, causing distortion and deteriorating the line quality.

For this reason, as shown in Fig. 1, conventional high-precision reception is achieved by using a crystal oscillator 3 for a mixer 1 that converts the received signal to a reception IF, and a bandpass filter 2 that removes unnecessary sidebands included in the converted IF. It required a local oscillator, and although it was a small-capacity microwave band multiplex radio, it had the drawback of not being inexpensive.

On the other hand, in a large-capacity microwave band multiplex radio, as shown in FIG. It is said that a receiver has been put into practical use very recently, consisting of a band frequency discriminator 5 and having an AFC loop (hereinafter referred to as the first AFC loop) that controls the receiving local oscillation frequency using the receiving local oscillation frequency. However, since the frequency accuracy is lower than that of a crystal oscillator, it is still common practice to use a crystal oscillator as a receiving local oscillator.

However, in the small capacity microwave band multiplexing radio of the present application, the reception IF filter has a narrow band compared to the reception IF filter of the large capacity microwave band multiplexing radio. method cannot be used.

On the other hand, in some simple radio devices, as shown in FIG.
, an IF band frequency discriminator 6, and a low-pass filter 7 for removing a baseband signal accompanying frequency modulation of the received wave, and an AF that controls the received local oscillation frequency using the IF.
A receiver with a C loop (hereinafter referred to as the second AFC loop) is theoretically conceivable, but it is difficult to achieve synchronization, and the crystal oscillator used in simple radios has a relatively low frequency. A crystal oscillator is generally used as a receiving local oscillator because it can be realized at low cost.

However, in the small-capacity microwave band multiplex radio device of the present application, it is difficult to obtain synchronization, so this second
The method of AFC loop G is unbearable to use.

As described above, in the fields of large capacity, small capacity, and simple radio equipment, crystal oscillators are currently used as receiving local oscillators.

Therefore, as mentioned above, a highly accurate receiving local oscillator using a crystal oscillator is required, and although it is a small-capacity microwave band multiplex radio, it has the disadvantage that it is not inexpensive.

The present invention eliminates the above conventional drawbacks and provides an AFC method that does not require a highly accurate receiving local oscillator using a crystal oscillator and can obtain a highly accurate receiving local oscillation frequency using an inexpensive receiving local oscillator. The purpose is to provide.

According to the present invention, in an automatic frequency control method for controlling the receiving local oscillation frequency of a small-capacity microwave band multiplex radio, this object is to provide a first automatic frequency control system for controlling the receiving local oscillation frequency using the receiving local oscillation frequency. This is achieved by providing an automatic frequency control method characterized by comprising a second automatic frequency control loop that controls the received local oscillation frequency using a frequency control loop and a received intermediate frequency.

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

FIG. 4 is a block diagram of one embodiment of the present invention.

In the same figure, 1 is a mixer, 2 is a reception IF filter, and 4 is a mixer.
is a voltage controlled oscillator, 5 is a microwave band frequency discriminator,
6 is an IP band frequency discriminator, 7 is a low-pass filter, 8
is a synthesizer.

The microwave band frequency discriminator 5, synthesizer 8, and voltage controlled oscillator 4 constitute a first AFC loop.

The purpose of this loop is to control the frequency of the voltage controlled oscillator 4 to maintain the received signal to the extent that it does not go out of the band of the receiving IF filter 2, and frequency stability as an AFC is not so strictly required.

Therefore, the microwave band frequency discriminator 5 required for this purpose can be realized relatively easily.

Next, mixer 1, reception IP filter 2, voltage controlled oscillator 4
, an IP band frequency discriminator 6, a low-pass filter 7, and a synthesizer 8 constitute a second AFC loop.

The purpose of this loop is to control the voltage controlled oscillator 4 so that the received signal passes through the center of the receiving IF filter 2, and the center of the IP band frequency discriminator 6 coincides with the center of the receiving IP filter 2. .

Here, the loop gain of the second AFC loop is
The voltage controlled oscillator 4 is controlled by combining both AFC outputs, which are set larger than the loop gain of the loop.

Figure 2 shows the characteristics of the frequency discriminator, Figure 5a shows the characteristics of the microwave band frequency discriminator 5, Figure 5 shows the characteristics of the IF band frequency discriminator 6, and Figure 5C shows the overall characteristics. It shows.

That is, even if the center of the frequency discriminator 5 in the microwave band changes by Δf, the overall characteristic will shift by Δf'.

In other words, the characteristics of the second loop are dominant within the IP band, and the received wave is kept at the center of the reception ↑F filter 2.

Furthermore, when the IP level decreases due to fading, etc., the output of the frequency discriminator 6 in the IP band decreases, and the output of the first AFC gradually decreases.
dominated by loops.

Furthermore, once the IF level is restored, it returns to its original state, and no discontinuous frequency movement occurs during this time.

As described above, according to the present invention, a narrowband reception I can be achieved at low cost without requiring a high-precision reception local oscillator using a crystal oscillator.
It is possible to realize a receiving local oscillator that can be used in small-capacity microwave band multiplex radio equipment that uses F filters, and its advantages are very significant.

[Brief explanation of the drawing]

Figures 1, 2, and 3 are block diagrams of conventional receivers,
FIG. 4 is a block diagram of an embodiment according to the present invention. FIG. 5a shows the characteristics of the frequency discriminator in the microwave band, FIG. 5 shows the characteristics of the frequency discriminator in the IF band, and FIG. 5C shows the overall characteristics. In the drawing, 1 is a mixer 2 is a reception IF filter, 3
4 is a crystal oscillator, 4 is a voltage controlled oscillator, 5 is a microwave band frequency discriminator, 6 is an IF band frequency discriminator, γ is a low-pass filter, and 8 is a synthesizer.

Claims (1)

[Claims] 1. In an automatic frequency control method for controlling the receiving local oscillation frequency of a small-capacity microwave band multiplex radio, the first method is to control the receiving local oscillation frequency using the receiving local oscillation frequency.
An automatic frequency control method comprising: a second automatic frequency control loop that controls the reception local oscillation frequency using the automatic frequency control loop and the reception intermediate frequency.