AM: AM band transmiter by Techlib

@12/7  2012

the 2 weeks i rebuilt a permanent version of my AM band superhet receiver.  i do need a transmitter first to test and adjust the receiver. looks like eggs and the chicken problem.

Tech lib provide a simple version AM transmitter, which does  not use audio power amplifier as the modulator.  i prefer it for testing, we don't need powerful transmitter here.

changes and tips

1) i built it with secondary version's oscillator and first version's modulator.

2) all 2n4401 change to 2n2222

3) 10k resistor near by the xtal change to 37k for clean sine wave, output level  200mVpeak.

4) An additional amplifier stage is built to provide 2Vpeak modulation signal. voltage Gain around 10X.

the limitation of the modulator

the differential modulator seems lack of method to control the amplitude(on MAX power output), let it can be 100% modulated, and keep there no peak limitation occur.

debugging 

1. the 0.1uF capacitor before the detector 1N914 seems buggy in my version( this might my fault i don't know why now), only when i shorted this capacitor,  the 100uA meter show value otherwise there is no current. this must be a PCB trace failure i think.

2. my version is 1Mhz transmitter,  output tuning tank use 100uH normal axial inductor, and around 300pF capacitor.

3. to use a 1.5meter antenna, i try some values loading coil but failed to get a perfect result. finally, a 800uH inductor, sent signal  3 meters away: test by IC-91A full band receiver.

4. output level around 10Vpp without audio input.  100% modulation < 3Vpp. the modulator has a voltage gain around (10Vpp/0.4Vpp)~= 25.