Electronic Circuits and Projects Forum

Hi audioguru,

By AFC do you mean Automatic frequency control? I am aware that the frequency might change and have tried to tune the radio to nearby frequency when the audio seems weak but that does not help. I also notice the frequency drift, but only as much as +/- 0.2MHz from the main frequency.

What are the symptoms when I do not tune to the main frequency? I guess it would be reception loss within a few meters' range. My transmitter goes pretty far so I hope I am tuning to the correct frequency. I have a few radios here, both analog (manual tuning) and digital. In 2 of my analog radios, the transmitter appear at multiple places. Other stations are also affected (distorted transmitted audio can be heard in the background). On the other radios, including the PLL radio, the transmitter appears at 1 place only (88.6MHz).

I will mount the transmitter on a different location and try the parallel antenna method tomorrow, just to test if it works better

Also what is the purpose of the 2nd (rightmost) LC oscillator of the circuit? Must it be tuned to the same frequency as the first oscillator? I though it acts purely as an RF amplifier. So far, my range and frequency do not seem to be effected by tuning C13.

Thanks audioguru. So effectively, both oscillators need to be on same frequency for maximum range. Is there a way to tune this easily? Because within the premises of my apartment, the signal is strong even without the RF amplifier stage. I assume I can only place the radio much further away (at least 100m) from the transmitter and tune C13 until the strongest signal is received, which is difficult with only 1 person working on the circuit.

Originally Posted by audioguru

The transmitter has only one oscillator. The second LC tuned circuit is in the RF amplifier at the output.
I made a field strength meter with a 1N34A geranium diode, a couple of capacitors and a resistor to feed DC into a voltmeter.
Some field strength meters (look at Talking Electronics or in Google) add a transistor, battery and LED. When the LED is the brightest then the tuning is peaked.

Thanks audio guru. Will try to make my own field strengh meter.

I retuned C13 a bit today and mount my transmitter on another place, looking out to an open field. The results are great. Distance is approximate 1km line of sight. Could probably be higher if there are no trees in between.

One thing I notice is that the transmitter consumes lots of power. A new 9V battery (measures 9.7V) reduces to 8V within 1hour, and to 7.5V within the next 15 mins. Perhaps I need to run it from a wall-wart or from 6xAA batteries instead.