MOD 3-2: Local Oscillator Modification for 430MHz IF
Replace the crystal and adjust local oscillator frequency by turing the trimmer.
IF and crystal frequency
The original local oscillator frequency is about 2280MHz. Its IF frequency is 120MHz for receiving 2400MHz signal.
The relation between IF and crystal frequency is following.
(Output Frequency) = ((Crystal Frequency) x 256) + (IF)
Replace the Crystal with a 7.6953125 - 7.6640625MHz crystal.
- Crystal Frequency for 430MHz IF is
- (2400MHz - 430MHz) / 256 = 7.6953125MHz
- (2400MHz - 438MHz) / 256 = 7.6640625MHz
* I found a 7.68MHz crystal at Akihabara. That's why my IF is 433.92MHz.
photo 7: replace the X'tal
Only change the crystal, a converter does not work on 430MHz IF. PLL circuit does not lock. An another modification is needed.
- Voltage Check
- Turn on the power and check the voltage of blue arrow point showing on photo 8.
- When the PLL circuit is locked, the voltage shows 4 - 7V. Only change the crystal, the voltage will not be in that range.
- Change the capacitor pointed by red arrow on photo 8 to 10pF or near. An important point is to mount the capacitor on the same place.
- Check the blue arrow voltage. It will be 4 - 7V. Then the PLL has locked.
- << Special thanks to JH0TOG and N4CNW for this mod.>>
photo 8: 430MHz IF modification (near crystal)
Local Oscillator Turning
After replace the X'tal, adjust local oscillator frequency by turning the trimmer(photo 7, blue arrow). Frequency counter helps this adjustment.
- Receiving known frequency signal is also good way. I will show you a simple SG method. Fig 1 shows a simple signal generater(SG). This SG creates 48 times input frequency. Input 50MHz signal goes to 2400MHz.
- Setup the converter and SG.
- Put a short wire to the converter's N-type connecter as an antenna. Close to the SG(diode and resister) to the antenna.
- Set the 50MHz TX frequency and expected RX frequency.
- TX = 50.01MHz then SG creates 2400.48MHz. Set RX = (2400.48MHz - (Crystal Frequency) * 256)
- Transmit 50MHz TX. Don't input much power. Less than 1w is better.
- Search RX signal and adjust to expected frequency by turning trimmer.
fig 1: simple Signal Generater
(by JA7BLS: CQ ham radio Magazine; Nov, 1990)
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