Tuesday, December 11, 2012

Signal generator

Here's a little piece of equipment that was missing in the shack. Truth is I already had the Si570 signal generator but it doesn't go bellow 4Mhz and it's not very practical for fast frequency tune/change, so time to put together some components in the oven and see what came out...

This was a kind of the 3 R's project (reduce, reuse, recycle):
The frequency counter is from a "pound shop" radio (so it counts always 455Khz more), the on/off switch is courtesy of US Signal Corps (from a Bendix direction finding set), the coil select switch came with the "Speaky" kit that was replaced by another model, the select button was an ofter of a friend and so was the tune button.
The modulation on/off switch/pot was from a radio of my late grandfather.

Internally, another quality assembly by me....

..the BF's244 I don't remember were they came from but found them in an old box and were bought for something I forgot completely, the output plugs were recycled from old equipment and the variable cap probably bough in some ham fair long time ago.

The schematic:

The only addition to the schematic is a "volume" control in the audio input.
Coils are not specified in terms of value because all of them are different. It's just a matter of build them according to the scientific principle of placing in the circuit and see if it works...it will have overlap ranges.

Power (3V) for the frequency counter module is courtesy of 2 out of the 3 LED's that are in series with an 390 Ohm resistor, not the most elegant solution in terms of energy waste but works nice.

Unfortunately I still don't have the oscilloscope to check the wave form on the output, sure it's not perfect but no problem since it's for simple tests, I know also that I have a strong harmonic at 2f (the usual hum?!).
I'm still struggling to put the oscillator at VLF but will get there...anyhow from about 1Mhz to 31Mhz it works really nice.
I replaced the feedback cap from 33pf to 12pf but both work.

I still have to put more coils for other ranges and put some labeling...

Have a nice week!

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