Re: Dumb newbie has a question...
- From: "Ancient_Hacker" <grg2@xxxxxxxxxxx>
- Date: 13 Oct 2006 04:14:21 -0700
Dave wrote:
Originally posted to sci.electronics.misc, but maybe this is a better place
for it...
I am wanting to build a 10 MHz signal source to be used in troubleshooting a
preselector/RF amplifier I built to assist my shortwave radio in picking up
weak stations, and am wondering how feasable my current approach is. I
currently have a 10 MHz crystal oscilator on a chip
Whoa! If you mean you have a 10MHz canned oscillator, that's part of
the problem.
These generally put out a square wave, the exact opposite of what you
need.
I'd get a 10MHz crystal and put it in a one-transistor oscillator
circuit. Very simple circuit will do-- google the net for "colpitts
crystal oscillator", here's a good reference:
http://www.northcountryradio.com/PDFs/column007.pdf
Either the colpitts or the pierce oscillator will do just fine.
Don't try any of the logic gate oscillators, they'll have too many
harmonics.
Then assuming you have a 50Mhz or better scope, use a 10x probe and
find the spot in the circuit with the cleanest sine wave. This will
usually be on one end of the crystal.
Then since you need such a small signal, make a capacitor voltage
divider, something like 1000pF from ground to your output jack, then
maybe 5pF to the sine wave point.
That will give you a clean, low impedance, 5/1000'ths of the sine wave.
Put all that in a shielded metal box with just a BNC connector coming
out. Then build another little metal box with a 100 ohm carbon
composition potentiometer-- hot side of the pot is the input, gound
side goes to input and output grounds, the wiper goes to the output BNC
center pin.
That should give you a nice weak signal, adjustable over a considerable
range.
If the signal is still too strong, build another little 30dB attenuator
box, say a 50 ohm resistor from each BNC center pin to ground, and a
820 ohm resistor between the center pins.
If you're going to be doing a lot of this, I'd go to eBay and buy a
good attenuatorm like an HP 0-120db one. They're not too pricey.
Make sure the seller guarantees it hasnt been burnt out.
feeding into the primary
of a step-down transformer, the sescondary of which feeds into the primary
of a second step-down transformer. This gives me a .02 volt signal on the
second secondary, but it is trashy (tons of harmonics, from what I
understand about these things.) If I feed this into a 10 MHz tank circuit,
should that be sufficient to clean it up and give me a nice sine wave, or am
I just full of crap for taking this approach? I am (obviously) a mere
experimenter, with more time than money for accomplishing my goals. If this
approach is snake-bit from the start, what approach should I take? I am
trying to simulate a 200 microvolt signal arriving at the input to the
preselector/RF amplifier so I can track it through the circuit and find out
where it is failing. I am planning on adding a third step-down transformer
to the layout in an attempt to achieve this signal level, and if necessary
use a second tank circuit to clean it up one last time. Now that those of
you who understand these things have likely wet yourself from laughing so
hard, please do me the favor of sharing some of your knowledge and wisdom.
:)
Many thanks for any replies,
Dave
.
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