Re: MW Transmitter with 100% modulation.

by dare4444 Wed Sep 22, 2021 7:24 pm

Ivan wrote:Hi,
1. What is the value of RFC2?
2. Is it critical for a given frequency?
3. How the magnetic saturation of RFC2 by the DC component of current is prevented?
4. The DC coupling between E-T1 and B-T2 is correct?

VBR from Ivan

It was 20T on bn43-2402 ferrite core. Tested with 6.4 and 7.2 mhz. Doesn't seem to be Freq dependent. Max peak current is 45ma, no saturation, runs cold. T1-T2 coupling is correct.

by **Ivan** Wed Sep 22, 2021 7:06 pm

Hi,
1. What is the value of RFC2?
2. Is it critical for a given frequency?
3. How the magnetic saturation of RFC2 by the DC component of current is prevented?
4. The DC coupling between E-T1 and B-T2 is correct?

VBR from Ivan

by dare4444 Tue Sep 21, 2021 9:44 pm

I've found that by placing an inductor in the emitter of a colpitts oscillator, the AM modulation depth can be increased from 50% to 100%. The current must be pretty high to supply power during modulation peaks, prevent flattening, and distortion. It's achieved by using only 50ohm emitter resistance or two 100ohm resistors in parallel. C2 and C3 are feedback capacitors and their values can vary anywhere from 470pF to 1000pF.

RFC 1 is for RF blocking purpose and not let it interfere with LM386 modulator chip. Typical value is 100uH readymade axial inductor. It's better if RFC2 is wound on a ferrite toroid or binocular core. It helps trap and store the RF energy and allows modulation level to reach 100% just by modulating the oscillator. The audio quality remains surprisingly good. Buffer T2 amplifies the AM signal from T1. It could be a 2n3053, 2n2219A, 2n2222A, 2n3866, etc. Resistor R5 should be 390ohm. T2's emitter is at 4.6V so 12V power supply - 4.6V = 7.4V * 11.8ma = 88mW of DC input power to the final stage. Connect directly a 6 foot wire antenna or develop an impedance matching network to convert the 50ohm output impedance present at C4 to several thousand ohms for the wire antenna to get a better range. The two crystals can be of any value from 600KHz to 1.6MHz. The two bit switches selects one frequency for transmission. 1MHz crystals are readily available and can be used here. Pin 3 of LM386 can be disconnected from R7 and Mic1 and audio input directly fed to C8. Increase C8 to 820n when feeding audio from an external source. Use 12V regulated power supply. T2 does not need a heatsink.

MW Transmitter with 100% modulation. Img_2143

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