Dumb newbie has a question...

[Dave]

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 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

 

[Ancient_Hacker]

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

 

[Dave]

Wow. Thank you. I had no idea that my canned oscillator was part of the
problem. I'd thought about building a Colpitts, but thought I would take
the easy way out. First problem.

Thank you very much for the input. I've already got the metal box etc, so
all I'll have to do is pull the stuff I have on the perfboard off, and start
over. Should not be a problem.

I do appreciate your assistance. (And understanding.)

Dave

 

[Ancient_Hacker]

I've already got the metal box etc, so
all I'll have to do is pull the stuff I have on the perfboard off, and start
over. Should not be a problem.

You may need to use separate boxes for the oscillator and attenuators.
You're asking for such a large stepdown one box may not provide enough
isolation.

 

[Dave]

You may need to use separate boxes for the oscillator and attenuators.
You're asking for such a large stepdown one box may not provide enough
isolation.

Hmmm. Hadn't thought about that. No problem. Can do.

One question- the Colpitts oscillator in figure 7 at the website you gave me
the link to has a tuning capacitor (2-20 pf). Is this to "trim" the output
to exactly what you want? Just curious. I slept through oscillators in
school. :\

One more question: figure 8 at that same website shows an inductor in
parallel with the shunt capacitance of a crystal. How can I tell what the
shunt capacitance of a crystal is? Hook it up to my DMM? (which measures
capacitance).

Thanks much...

Dave

 

[Michael A. Terrell]

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 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

Make a 10 MHz bandpass filter to remove the harmonics from the canned
oscillator. You can make a nice filter with a seperate 10 MHz crystal,
as well.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Ancient_Hacker]

One question- the Colpitts oscillator in figure 7 at the website you gave me
the link to has a tuning capacitor (2-20 pf). Is this to "trim" the output
to exactly what you want?

It will trim the frequency very very slightly, like a KHz or less.

Figure 8...

I'd suggest not using figure 8 circuit, Figure 7 is simpler and doesnt
require those funny tuned coils.

How can I tell what the
shunt capacitance of a crystal is? Hook it up to my DMM? (which measures
capacitance).

Well it's going to be a very very few pF, which is probably below the
capability of your DMM.

 

[Dave]

It will trim the frequency very very slightly, like a KHz or less.



I'd suggest not using figure 8 circuit, Figure 7 is simpler and doesnt
require those funny tuned coils.



Well it's going to be a very very few pF, which is probably below the
capability of your DMM.

Gotcha. Was looking in the Allied catalog at their XT49S series of
crystals, and notice the specify a "load" and an "ESR Max (ohm)" value for
each of these crystals. Any info on what these refer to?

Thanks again,

Dave

 

[Dave]

Gotcha. Was looking in the Allied catalog at their XT49S series of
crystals, and notice the specify a "load" and an "ESR Max (ohm)" value for
each of these crystals. Any info on what these refer to?

Thanks again,

Dave

PS: the "load" is in pf (20 to be exact).

Dave

 

[Ancient_Hacker]

Gotcha. Was looking in the Allied catalog at their XT49S series of
crystals, and notice the specify a "load" and an "ESR Max (ohm)" value for
each of these crystals. Any info on what these refer to?

Don't worry about it. I've never seen a crystal that wouldnt oscillate
nicely.

 

[Dave]

Don't worry about it. I've never seen a crystal that wouldnt oscillate
nicely.

Okay, thanks. Yoiur help is greatly appreciated.

Dave