一款优秀的分立元件发射机电路(英文)

文章摘要： As you can see, the worst case is the 3rd harmonic at -60dBc. The carrier level was 23dBm (200mW) at 10v supply. This falls off a little to about 160mW at the ends of the bands. Brief specifications are given below. I have not been all that meticulous with the figures. When I got a reading of 73mA

一款优秀的分立元件发射机电路(英文),标签：电路设计,http://www.88dzw.com

As you can see, the worst case is the 3rd harmonic at -60dBc. The carrier level was 23dBm (200mW) at 10v supply. This falls off a little to about 160mW at the ends of the bands. Brief specifications are given below. I have not been all that meticulous with the figures. When I got a reading of 73mA I rounded it up to 75mA to keep the figures simple. The figures are only a guide anyway.

Parameter Supply=9v Supply=12.5v Supply=13.8v Freq range 76 - 116MHz 77 - 119MHz 78 - 121MHz Supply Current (98MHz) 75mA 85mA 95mA Output power (88MHz) 160mW 310mW 370mW Output power (98MHz) 180mW 360mW 420mW Output power (108MHz) 165mW 320mW 380mW Spurious Outputs (DC - 1GHz) -60dBc -60dBc -60dBc RMS AF for /-75KHz deviation 210mV 200mV 195mV AF response 0/-3dB 3Hz - 70KHz 3Hz - 70KHz 3Hz - 70KHz

Alignment

Adjust the variable capacitor to get the transmitter on the frequency you want. Is that simple enough?

Applications

There are several different applications since the unit will modulate from DC to several thousand kilohetrz, the most obvious being music. It may be, however, that you wish to change the frequency (to somewhere legal?) and use FSK data for moving information between computers. Perhaps you even want to convey DC changes or just stabilise the TX frequency. Let us now cover these items, beginning with a recap of the circuit diagram:

Frequency Control

(Not to be used with synthesiser)

As you can see from the original circuit, the varicap voltage is kept high by R1. Without this resistor the DC voltage on the diode will be zero, causing the oscillator to stop. R1 shall be removed if using external synthesiser control. We can, however, use the CTRL terminal to have a preset "frequency" potentiometer on the outside of the box. All we need is a 500K Linear potentiometer. Nothing else! no capacitors, nothing. This will give typically 10MHz tuning range

Frequency Stabiliser

(Not to be used with synthesiser)

Given that R1 is connected directly to the battery supply voltage, if the battery voltage were to vary then so would the TX frequency. You should really be using a stabilised power supply, or a high-current battery that has a fairly constant supply voltage. If this is NOT the case then you can use the CTRL terminal to bypass R1, without making any modifications to the TX. All we need is an external zener diode and a 6K8 resistor. The Zener diode should be as high as possible. If you have a 12.5v supply, for example, then a 10v diode would be great. With a 9v battery then a 6v8 diode is about the maximum practical. 8v2 would be Ok until the battery voltage went down a bit. We will asume that VE is a 9v battery.

Frequency Modulation

If you are using DC modulation then the AF input will allow this. This can be used to give low frequency Frequency Shift Keying. This should only be done in conjunction with the voltage regulator above. If you are NOT wanting to have a DC shift, and your input source has a "DC Continuity" (resistance) then there is problem. If you were to connect a magnetic microphone or CD player, for example, to the AF input then the TX frequency would jump. You therefore need to add a capacitor to block the DC shift at the input. 10uF will do nicely. A 4K7 resistor should also be added to give a load to the audio source and to help prevent "hum" or "pickup" from 50Hz (60Hz) wiring. The value of the resistor should be selected to match the audio source impedance of the device. 4K7 is normal for computer and CD LINE-OUT signals.

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