question about depth of modulation for a 40M transmitter.

[obrien135]

Hello,

As seen in video 10 in the website below, the transmited signal sounds unclear when received on the general purpose receiver. I can't tell if it is overmodulating or undermodulating. It has one stage of LM386 with gain = 200 for amplification prior to the mixer. The output is buffered by a common collector stage followed by a common base stage. Unfortunatly I sold all three of my oscilloscopes last year, so it's difficult to tell exactly what the problem is. Can anyone tell from theh video if it is under, or over modulating?

George

**broken link removed**

 

[chuckey]

Build your self a RF diode probe detector, use a cap of 100Pf which without any modulation will give a reference voltage for you carrier amplitude. Now increase the capacitor to 50 MF and gradually wind up the modulation. The level should go from the previous to double that voltage at 100% mod if it then flattens out as you increase your modulation level you are over modulating!
Frank

 

[D.A.(Tony)Stewart]

Looks like sound principles in the design, The configuration seems ok. I didn't check modulation level or have to time to check everything, but the layout needs to be tightened up into a smaller loop to prevent stray crosstalk and distortion. I'm not sure the supply was clean and amount of leakage with long wires.

I might suggest a $60 USB scope, but I cannot presume that is within your means. Try to condense the layout or add a foil ground plane.

 

[obrien135]

I noticed that when I tried to measure the audio in the lm386 circuit using a capacitor in series with the ac voltmeter on the 10VAC range, I would get a full scale deflection on both the input and output even with no signal applied. If I remember correctly I even got it on the power supply terminals. Is this because of the 7MHz osclillator signal radiating throughout the circuitry? Ir's an analog meter.

Actually, now that I remember more accuratly, there was full scale deflection on the out put on both sides of the coupling capacitor, but on the input there was full scale deflection on the input side of the coupling cap but on the output side of the input coupling cap there was about 1VRMS with the mic keyed and about 2VRMS when talking into the mic loudly.

That would seem to indicate over modulation right? But why the full scale on input of input coupling cap? Is it because of the radiated RF?

I am going to try th RF probe . I have everything but the 50MFD caap. But I will try it and post the results as soon as poosible.

George

- - - Updated - - -

Maybe I could put about a 2Kohm resister in series with the capacitor betwewen pins 8 and 1 on the LM386 to lower the gain in case it is overmodulating.

 

[D.A.(Tony)Stewart]

with unshielded wires scattered , they act as antenna, even the DMM leads , measurement errors are easy. 50MFD caps are not all the same these days. They use to put 10uF, 0.1 and 0.01uF for a reason in the old days because of self resonant freq. But in any case, the lack of a ground plane makes it hard to find for reference when there is RF.

Don't take any chances and start from your supply and get clean readings and clean interconnections.
I use twisting pair jumpers if I have anything >1MHz clocks for analog, or better still use coax.

Keep the Tx and Rx farther apart.