JPS6157741B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a transmission frequency control device for correcting frequency fluctuations occurring in satellite lines in narrowband radio frequency telephone and data transmission systems.

In the field of satellite communications, INTELSAT's
Narrowband radio frequency telephone or data transmission using PSK or FM modulation is widely used, such as the SPADE method or the Maritime Satellite MARISAT method. This narrowband wireless transmission system (Single Channel Per Carrier, hereinafter referred to as SCPC) requires that the received wave be placed within a certain designated frequency range at the input of the demodulator. For this reason, earth station transceivers usually use crystal oscillation sources with high frequency stability of 10 ^-8 or less to meet this requirement, but frequency fluctuations that occur between satellites pose a problem. This frequency variation is primarily due to the frequency stability of the frequency converter in the satellite transponder, plus the Doppler shift.

The frequency fluctuation that occurs in this satellite section is approximately
The frequency deviation is on the order of 10 ^-5 (40KHz), which is greater than the occupied bandwidth of the SCPC wave. Therefore, means for correcting this frequency deviation is required in the earth station's transmitter/receiver. For this purpose, the SCPC method uses a communication pilot radio frequency and applies automatic frequency control (hereinafter referred to as AFC) to correct frequency deviations based on this pilot frequency. There are two methods for this AFC. That is, the first method is to receive this pilot frequency at the receiving station, detect frequency fluctuations in the satellite interval, uniformly correct the frequency spectrum of the received waves by this frequency fluctuation, and determine each received wave. This is a method of placing the signal within the specified frequency range. This method is effective for systems in which target earth stations mutually configure a communication network, for example.
This method is used in INTELSAT's SPADE or SCPC system.

In the second method, the earth station's receiver receives the pilot frequency, detects the frequency fluctuation in the satellite interval, and corrects the frequency of the own station's transmission wave by this frequency fluctuation before transmitting. This is a method of configuring. This method is effective for a system in which there is one central earth station and a large number of slave stations communicate with it, and frequency correction is performed by the transmitting device of the central station.

The present invention relates to this second method, and the principle of its operation will be explained in detail with reference to the drawings.

Figure 1 shows the frequency allocation diagram for transmission and reception.
The horizontal axis is the frequency, f _PT is the pilot transmission frequency, f
_PR represents the pilot reception frequency, f _T represents the transmission signal frequency, and f _R represents the reception signal frequency. The transmission pilot signal _fPT has high frequency stability and is transmitted from the transmitting station at a specified frequency.

On the other hand, the received pilot signal f _PR passing through the satellite section
If there is no frequency variation in the satellite interval, it is received at f _PR ' shown by the dotted line, but normally it is received at f _PR with a frequency shift, for example, +Δf shift as shown. In the pilot receiver, this deviation +△
In addition to detecting f, a means is required to control the frequency so that the transmitted signal wave f _{T '} is shifted in advance by 1 Δf so that the received signal wave f R can be received at a predetermined frequency, and is transmitted at f _T .

The conventional transmission frequency control device shown in the block diagram of FIG. 2 has been used. In the figure, an input signal with a receiving pilot frequency f _PR is connected to an input terminal 1, and a bandpass filter 2 removes unnecessary waves.
It is connected to a first frequency converter (mixer) 3 and mixed with the output of a local oscillator 4 having a highly stable oscillation frequency f _L (where f _L <f _PR ). The output of this first mixer 3 passes through a bandpass filter 5, is amplified by an intermediate frequency amplifier 6, and is amplified by a phase detector 7.
is supplied as one input. This phase detection output is amplified by a DC amplifier 8 and passed through a loop filter 9.
through a voltage controlled oscillator 10 with an output frequency f _V
(hereinafter referred to as VCO).

The output of VCO 10 is connected to hybrid branch 12
This one output is mixed in a second mixer 13 with the output of the reference frequency oscillator 11 having a frequency f _R (where f _V <f _R ). The output of this second mixer passes through a bandpass filter 14,
This serves as another input signal to the phase detector 7 and forms a phase synchronization system.

In this case, the receiving pilot frequency f _PR is +△f
If there is a fluctuation in the first mixer output frequency (f _PR −
f _L ) also fluctuates by +Δf. In this phase synchronization system, if the input frequency of one side of the phase synchronization system fluctuates by +△f, the other input signal must also fluctuate accordingly.
In order for the output (f _R −f _V ) of VCO 10 to fluctuate by +Δf, it is necessary to configure the VCO 10 so that its output becomes −Δf.

On the other hand, the transmission intermediate frequency signal is transmitted to the transmission input terminal 21.
The signal enters the signal from the VCO 10, is amplified by the intermediate frequency amplifier 22, and is mixed with the output branched from the VCO 10 by the transmission mixer 23. The output of this mixer 23 passes through a bandpass filter 24, is mixed with the output of a transmission pilot signal generator 25 at a hybrid combiner 26, passes through a bandpass filter 27, and is sent out from a transmission terminal 30 as a transmission signal.

Since the output of the VCO 10 operates to fluctuate by -Δf, the transmission signal using the output of this VCO fluctuates by -Δf. Therefore, for a +△f fluctuation in the received signal, the transmitted signal changes by -△f
It will be controlled to fluctuate and have automatic frequency control (AFC) function.

However, in the conventional device, the phase detector 7
As mentioned above, the frequency of the input signal fluctuates by about 40 KHz, and it is necessary to widen the bandwidth of the filters 5 and 14 to cover this fluctuation, so the S/N of the received signal cannot be improved. Therefore, it is necessary to take systematic improvement measures such as increasing the transmission output of the other party. Furthermore, the operating bandwidth of the phase detector 7 must also be widened. Furthermore, since the conventional reference frequency f _R is compared with the output of the VCO 10, it is a high frequency and needs to be configured as a stable high frequency oscillation source.

An object of the present invention is to eliminate such conventional drawbacks, improve the S/N of a received signal, and provide an inexpensive transmission frequency control device.

The present invention mixes the input frequency and the output frequency of the VCO, compares the phase of this output with the reference frequency, and
A transmission frequency control device is configured to drive a VCO using this phase comparison output.

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 3 shows a block diagram of an embodiment of the invention. In this embodiment, the received input signal f _PR is directly mixed with the output of the VCO 10 in the mixer 15, so the input frequency fluctuation of +Δf is directly output as the frequency fluctuation (+Δf) of the VCO 10. Therefore, in this embodiment, a local oscillator 28 and a second transmission mixer 29 are provided in the transmission system. In this transmission system, the input first intermediate frequency signal f _IF is transmitted to the amplifier 2
After being amplified in step 2, it is mixed with the local oscillation signal of VCO 10 in first mixer 23 to obtain a second intermediate frequency output signal. The second intermediate frequency signal is sent to the hybrid synthesizer 2
6 and a bandpass filter 27, and is connected to a second transmitting mixer 29. The local oscillator 28 has high frequency stability, and its oscillation frequency f _L is f _L > f
_PT or arranged so that f _L > f _T . Therefore, the frequency spectrum of the output signal of the second mixer 29 has its polarity inverted with respect to the input signal, and is connected to the transmission output terminal 30 through the output bandpass filter 31. On the other hand, the transmission pilot oscillator 25 has high frequency stability, and the output of the first transmission mixer is combined with the transmission signal wave by a hybrid combiner 26. That is, as described above, the output frequency f _L of the second transmitting local oscillator will give a frequency deviation of -Δf at the output terminal 30 of the input transmitting signal at the output of the second mixer.

In the case of the present invention, the intermediate frequency with a frequency variation of +Δf is mixed with the output frequency of the VCO and frequency converted. Therefore, the bandwidth of the filter 16 that performs bandpass filtering on the output of the mixer 15 can be made sufficiently narrow regardless of the pilot frequency fluctuation. For example, the bandwidth of this bandpass filter 16 is set to 3.
Traditional bandwidth can be ~5KHz
Compared to 40KHz, it is possible to improve the S/N by about 10dB. This means that by narrowing the bandwidth, the level of the received pilot signal can be lowered accordingly, and the transmission output of the satellite can also be kept low, making the overall system configuration easier. Furthermore, the operating bandwidth of the phase detector of the present invention can be made narrower accordingly. Also, narrowing the bandwidth creates a receive pilot signal acquisition problem, which can be solved by sweeping the VCO at a lower frequency. Furthermore, the reference oscillation frequency f of the present invention
Since _R is compared with the output of the mixer 15, the frequency can be set sufficiently low, so that frequency stability can be increased and the configuration can be simplified.

Note that the transmitting pilot oscillator of this embodiment is
It can also be configured using signals transmitted from other earth stations.

[Brief explanation of the drawing]

FIG. 1 is a layout diagram of transmitting and receiving frequencies, FIG. 2 is a block diagram of a conventional device, and FIG. 3 is a block diagram of an embodiment of the present invention. In the figure, 1...input terminal, 2, 5, 14,
16, 24, 27, 31...Band filter, 3, 1
3, 15, 23, 29... mixer, 4, 28...
... Local oscillator, 6, 22 ... Intermediate frequency amplifier, 7
... Phase detector, 8 ... DC amplifier, 9 ... Loop filter, 10 ... VCO, 11 ... Reference frequency oscillator, 12 ... Hybrid branching circuit, 21 ...
... intermediate frequency input terminal, 25 ... pilot signal oscillator, 26 ... hybrid synthesizer, 30 ... output terminal.

Claims (1)

[Claims] 1: a first means for receiving a radio pilot signal, extracting and outputting a received pilot signal; a voltage controlled oscillator for changing the frequency of an oscillation signal in response to a control signal; a first frequency converter for mixing; a bandpass filter for bandpass filtering the output of the first frequency converter; and a bandpass filter for performing phase detection on the output of the bandpass filter using a signal having a predetermined reference frequency. a second means for supplying the detected output as the control signal to the voltage controlled oscillator; a second frequency converter for frequency mixing the oscillation signal and the transmission signal; a third means for extracting a signal having a sum frequency of the frequency of the oscillation signal and the frequency of the transmission signal; a local oscillator for outputting a local oscillation signal having a frequency greater than the sum frequency; a third frequency converter for frequency mixing a signal having a sum frequency; and a fourth frequency converter for extracting a signal having a difference frequency between the frequency of the local oscillation signal and the sum frequency from the output of the third frequency converter. a transmitting frequency control device comprising: means for controlling the transmission frequency;