JPS6110376Y2 - - Google Patents

Description

[Detailed explanation of the invention] The purpose of this invention is to improve the frequency of marks and spaces.
SSB/FSK modulates the SSB transmitter and sends the signal
The purpose of the transmitter is to reduce carrier leakage during FSK transmission and prevent spurious signals due to harmonics of modulated tones from being transmitted.

Generally, in FSK (Frequency Shift Keying) communication, the mark frequency is 2125Hz, and the space frequency is lower than the mark frequency by FSK mode.
Using a lower shift frequency of 175Hz, 425Hz or 850Hz. Among these, when shifting by 850 Hz, that is, when the space frequency is set to 1275 Hz, there is a risk that the waveform of the tone signal will be poor, or that the second harmonic of 2550 Hz generated by the distortion of the modulator will be sent as a spurious signal. The reason for this will be explained next.

Figure 1 is a block diagram of the signal generation section of the SSB/FSK transmitter, in which the output C of the carrier oscillator 1 is modulated by the low frequency signal A in the balanced modulator 2, and the DSB wave D
Then, it passes through the side band pass filter 3.
It becomes an SSB wave, and is then transmitted in that radio format through an amplifier or frequency converter.

The low frequency amplifier 4 works as a microphone amplifier during SSB and as a tone signal amplifier during FSK.

Filter 3 typically has a 6dB bandwidth for SSB.
A frequency of 2.1 kHz or higher is used, and the standard difference between the center frequency and carrier frequency is 1.5 kHz, and the DSB wave D
passes through filter 3 and removes unnecessary sidebands to become SSB wave S, the carrier component is also 15 to 25 dB.
It is configured to receive attenuation of .

Figure 2 illustrates the relationship between filter characteristics and carrier frequency.The 6dB bandwidth of the filter is ±1.1kHz, and the difference between the center frequency and carrier frequency is 1.5kHz, so the carrier was used as the standard.
SSB modulation band is 0.4~2.6kHz or 400~2600
Hz.

When transmitting FSK, carrier C is a tone signal.
Since the signal is balanced modulated at 1275Hz or 2125Hz, it is a single tone wave at the 1275Hz or 2125Hz position in Figure 2, which naturally passes through a filter and is transmitted, but at the same time the second harmonic of 1275Hz 2550
Hz is also within the filter band of 2600 Hz, so it can pass through, and since there is no band limit after the filter, there is a risk that it will be sent out as is and cause interference as a proximity spur.

Therefore, in this invention, the carrier frequency is first shifted by α toward the attenuation side of the filter. That is, the carrier position is set to be (1.5+α) kHz from the center of the filter as shown by C'. Of course, if this continues, the frequency position of the FSK tone-modulated signal will shift, so the second
Mark and space the frequency of the tone signal with α
Take only a higher price. That is, the space signal is (1.275
+α)kHz, and the mark signal is (2.125+α)kHz, the position of the single tone in the filter is exactly the same as before the carrier shift. Therefore, the side receiving the radio waves can normally receive them in exactly the same state. However, by setting the 1.275kHz tone to (1.275+α)kHz, its second harmonic becomes (2.55+2α)kHz, so in Figure 2, this position is outside the passband of the filter. It can be seen that approximately 2.55+2α>3 is sufficient for this to occur. Subtracting 2.55 from both sides of the previous equation and dividing both sides by 2 yields α > 0.225kHz, that is, the carrier position is 225Hz on the attenuation side of the filter.
If the space tone harmonics are shifted by more than 100 dB, the filter will attenuate the harmonics by more than 20 dB, which has the advantage of emitting clean radio waves without spurious signals.

Carrier oscillators generally use a crystal oscillation circuit in terms of frequency stability, but depending on the carrier frequency, a frequency difference of the above degree can be achieved by changing the external constant of the crystal circuit (usually a parallel or series capacitance). There is no need to add an expensive oscillation crystal, and it is necessary to modify the original circuit by combining a small capacitor and diode switch, or a variable capacitance diode and bias switch. The cost is minimal and SSB
Automatically changing the carrier frequency between FSK and FSK can be easily done by interlocking electric circuits, so it can be said that there are almost no problems in implementing it. Also, by shifting the carrier in the direction of filter attenuation, the carrier leak will be reduced by the amount of attenuation.
It is also advantageous from that point of view.

A dedicated FSK transmitter or a focus on FSK in particular
SSB dual-purpose transmitters use a narrowband filter that matches the tone signal, so there is no need to worry about spurious signals caused by tone harmonics. Therefore, the practical value of the present invention, which can reduce spurious signals without adding an expensive dedicated filter, is great.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the signal generating section of the SSB/FSK transmitter, and FIG. 2 is an explanatory diagram of the relationship between filter characteristics, carriers, and signal frequencies. 1...Carrier oscillator, 2...Balanced modulator, 3
...Filter, 4...Low frequency amplifier.

Claims (1)

[Scope of utility model registration request] SSB with given mark frequency and space frequency
In an SSB/FSK transmitter that modulates the transmitter and sends out a signal, by shifting the carrier frequency to the attenuation side of the sideband pass filter during FSK transmission, carrier leakage during FSK transmission is reduced and harmonics of the modulated tone are reduced. FSK where is the frequency outside the passband of the sideband pass filter
modulation circuit.