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| When I decided to build a signal
generator from junk parts, I thought that how hard can that be - almost
a piece of cake. After two months and several failed prototypes I wish
that I'd sold my car and bought a decent one instead of spending so
much time on this project. But now it finally works (somehow...).
Here's the schematic: |
| Three inverters in a 74HC14 hex
inverting schmitt trigger are used as a square wave oscillator.
Frequency is determined by potentiometer RV1 (coarse tune) and
adjustable capacitor VC1 (fine tune). That signal drives Q1, a class-C
amplifier. Q2 is a buffer and SW1 selects between high and low output
levels. RF choke, L1, is about 32 turns of enamel wire on a toroid. about 1,5 cm in diameter. It was in my junk box, I tried a couple of chokes, but they didn't make much difference. Because it would be very difficult to build a mechanical frequency readout I just added another inverter to drive an external frequency counter. With these component values the frequency range is from 1.1 MHz to 17.4 MHz. Fine tuning range with VC1 is about 1,2 MHz. You could add a band switch to throw in additional parts for higher frequencies. |
 |
I used a plastic variable
capacitor (from a transistor radio). An air variable would probably be
a better choice, but I didn't have any that would fit inside the box
(which is from a car stereo). The generator is shielded inside an aluminum box that I made from old TV-tuners (the lid is removed in the picture). There wasn't room for a power switch, so I left it out. You don't want to switch this thing off too often anyway - it needs some time to settle down. After 15 minutes or so it's stable enough for my purposes. |
| The generator needs a well
regulated power supply - particularly +5V. You should also place C2
near the Vcc pin of 74HC14. I haven't measured the power consumption,
but I think it should be less than 300 mA (?). I made the power supply
using 78-series regulators: |
 |
As you can see it still needs
some finishing work. After a cold start there's some drift, but then it
settles down (+/- 15 Hz short-time). The problem is that the signal is
not clean, it sounds rough. I haven't been able to cure that ( I don't
have an oscilloscope to see what's really happening here). But I can
live with it. |
| At this stage I really cannot
recommend to build this, except for experimentation. I might try to
finish this one day, but now I want to do something else (like finally
get the TS-510 working). |
