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 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: )
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:

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.


Friday, June 18, 2010

Pinout paper

Every time I solder anything (with transistors) I have to look for the transistors pinout's, so, and since transistors are almost the same references in all that is radio related schematics I decided to place near the soldering iron a small piece of paper with the pinout's drawing... that avoids me to power the computer just to look a part pinout I forgot to draw.

Thursday, June 17, 2010

AD8307 Power meter

One of the problems faced when testing circuits is to know the power level in certain points.... I already did an uncalibrated simple power meter (see: ) but for very low level measurement I needed something else, so, built also an uncalibrated (yet) power meter based on the AD8307 chip. It's a mix from the datasheet and from the circuit in EMRFD (Experimental Methods for RF Design), the AD8307 part of the diagram is from datasheet and the opamp parts is from EMRFD (placed an 68K resistor in series with my 100uA meter in the original shcematics is an 6K8 for 1mA meter)
Don't have my exact shcematic since I allready closed the box but you can see some similar designs here:

and here:

The front panel: meter, output for a DVM and on/off switch (without button for now)

Inside, se on the back, the rf blindage for the AD8307 chip and the professional batery holder.... the box was from an homebrew swr and I didn't removed all the parts.

here's the LM358 part of the schematic, sory for the lousy photo.

Monday, June 14, 2010

Standard plug for equipment power

Now that the warm weather arrived, time spent in the shack must be kept to the minimum (it's hot).
Today decided to rearrange the power cables to the homebrew equipments. Since they all run on 12v direct current (DC) and power consumption is low (mostly qrp power) found this little nice plugs that fit quite nicely, previous I was runing some cables connected directly to the equipments.

Here the "male" inner terminal

The plugs can be placed in the back panel of the equipment but I opted to put a dropout cable in the back and then the plug them.

Still I have to put some protection on the battery circuit, am thinking in testing an, house electrical installation, standard 230v automatic circuit breaker for 25Amps (they are cheaper than the marine 12V circuit breakers), they work by the heating principle so the only thing that counts is the current passing, the 230v marking should only regard isolation capacity....hope it works...

Friday, June 04, 2010

VHF VFO for PLL use

I decided to make an VFH FM transmiter based in PLL control, the PLL PCB will be manufactured in an outside company and sent to me by
I have an audio clip but in "amr" from the mobile phone. Will try to put latter on. Belive me, it sounds better than most transceivers I have seen....