Sunday, June 20, 2010
NE567 Tone encoder for repeater access
What's the point in having an FM VHF transmitter if you can't use the repeaters?
Since my homebrew VHF transmitter will have low power making impracticable for direct QSO's I have to find a way of improving the range.
Looking for a simple and cheap tone generator I found this litle NE567/LM567 chip that does this and also a lot more (I just need it for the tone...).
One of the problems I had was with the amount of background noise the little chip makes (harmonics) so had to reduce the level to a bare minimum but still activating the repeater...in this case my TM-D700 configured for tone actuation. Even at the lowest level you can still find a little of noise, probably will have to increase microphone gain do overcome this issue. But for now it works, I intend to put a switch for selecting several variable trimpot's to all the tones I will need. For 74.4 hz the trimpot value is around 11.5K.
The lousy schematic (you can see a better one here: http://www.rason.org/Projects/decoder/decoder.htm )
I didn't had an 1k trimpot (emiter of the 2n3904) so made one with a 200 Ohm one and 820 resistor in series since I knew that the signal had to be low, the 200 Ohm is closer to the ground on the output of the 2n3904 amplifier.
And the injection point, I also tried near the varicap but the result was a little worst:
This vfo is part of the one a few post bellow on the blog: http://speakyssb.blogspot.com/2010/06/vhf-vfo-for-pll-use.html
The video... sorry for the frequency drift (no pll at the moment, just the LC VFO) but you can see that the tone squelch gets in as soon as you leave the vfo frequency and you can ear starting some feedback from the mic. The antenna was just a little piece of wire (20cm) connected to the VFO buffer output some 2m far from the receiver antenna.