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My FM transmitter can deviate just as loud as a radio station and still sound good.mr.gone said:I was wondering about your transmitter. Do you ever have a problem with over deviation? You know that sounds like crap in the receiver or do you just have a real low audio level?
Very few audio power amplifiers are class-A. Most operate in class-AB for much less heating. Class-AB is class-B but with a little amount of idle current added to eliminate crossover distortion.
Nope, you have it backwards. If you don't use pre-emphasis then a voice will sound dull and muffled, not tinny.mr.gone said:Now getting back to over deviation. Even without pre-emphasis the high frequencies will drive the frequency swing harder at high levels. That is why if you don't use pre-emphasis a typical voice signal will sound tinny. So once again you have it backwards. It's a low pass filter isn't it? So it is designed to reduce the highs in a transmitter
All FM broadcast band radios have de-emphasis (treble cut) to cancel the pre-emphasis (treble boost) done during transmit and to cut hiss.mr.gone said:That's what de-emphasis is for
I used digital multimeter to measure the voltage at the mic, it kept changing from 1.x V to 2.x V. I put the probe at the collector of the 1st transistor, it shows only sinewave, and doesn't affected by any voice. I've also put the probe at the mic, there is also sinewave. When there is voice, the sinewave become faint.audioguru said:Horray it works! (almost)
Connect a separate antenna to the oscilloscope to tune C13 for its peak. If you directly connect the 'scope to the antenna then the capacitance of the 'scope's cable will be in parallel with C13 and will change the tuned frequency.
I don't think you will see modulation on the carrier with a 'scope because it is very small.
Measure the voltage at the microphone and at the collector of the 1st transistor Q1. It should be about +2.5V. Connect the 'scope there to see audio.
Oh I see.. I'll try this soon.audioguru said:We need to know the DC voltage at the collector of Q1 to see if it is biased correctly. Use a 'scope because a multimeter might be affected by the RF.
2N3904, I follow your circuit.What is the part number of your transistors?
Yes, that's what I wired. The manufacturer always get wrong with the pins? I've seen this for only once, remember?audioguru said:I hope that your 2N3904 transistors are wired like this:
I remember that you used a FET where different pins layputs were on different datasheets. This transistor is much more common than the FET so the pins layout is correct.bananasiong said:Yes, that's what I wired. The manufacturer always get wrong with the pins? I've seen this for only once, remember?
The antenna of the transmitter is around 10cm, vertical. I thought the distance of transmission is reduced if the length of the antenna is reduced?audioguru said:Try reducing the length of the transmitter's and radio's antennas and space them apart to avoid overload, then only a single correct frequency will be on the radio's dial.
I have to speak very near to the mic, less than 5cm from the mic. Or else I couldn't hear anything at the receiver side.The mic preamp is very sensitive and will cause acoustical feedback howling if the radio is in the same room as the transmitter, and severe distortion if you speak too loud or too close to the mic. It picks up voices at a distance of 2m to 3m.