On Wed, 7 Feb 2007, Denny wrote:

Michael makes excellent points...

Sunday I was struggling with a simple crystal oscillator I had kludged
up on a protoboard for a quick project... I have been building
oscillators like this on and off for over 40 years, yet I could not
get this one to work... Tried different crystals... Used the LC meter
to check the caps and inductor... Used a VOM to trace the circuit and
measure the resistors... Had the oscilloscope and a receiver
monitoring the circuit for RF... I was just tearing my hair out... My
son got involved and he could not find a problem either...
As we sat there staring at the half dozen components on the protoboard
I commented that the bias voltages on the FET showed that it was
conducting, but not oscillating as if something was sucking the RF to
ground... On a whim I said to him, pull the RF bypass cap off the 5
volt DC line... (the LC meter had showed the cap as good and close to
the 0.1 mfd it was marked)...
Ahh that won't change anything because the RF choke blocks the rf from
getting that far, he said...
Well, just try it..
He reached over and idly plucked the DC bypass cap off the DC input
line and WHAMMO, instant loud, whistle in the receiver and the
oscilloscope showed a solid sinewave...
He looked at me, I looked at him, we both shrugged and he tossed the
cap into the wastebasket...
It's the little things that will drive you to drink...

I had one of these moments decades ago. Short version of a very long
story, and beginning doubts about whether reality exists or magic exists:
I built a circuit that didn't work but was supposed to work. Spent a LOT
of time. Finally, on an intuitive whim I added a component that should not
have done anything, but it made the circuit work. I told my boss, we
looked at each other, and we decided it was not worth spending more time
on and accepted the modification since what mattered was that the damned
thing work and we were not going to question black cats crossing our
paths. :-\


===== no change to below, included for reference and context =====

So, first verify that your oscillator is oscillating - receiver or
oscilloscope or RF probe to your VOM... It is not just some white
noise in the radio, but a solid whistle that indicates the oscillator
is working... As Michael suggested use a second oscillator as a BFO
if your receiver doesn't have a SSB or CW mode...
Next, put an audio signal into the audio amp and verify that it blows
the headphones off your head... Just suck some audio off a receiver
to feed it...
Once you KNOW these items work then try your mixer... Start with the
transistor mixer first...
Building or buying an RF probe for your VOM will be the most versatile
and cost effective tool you can make/buy for these projects..

GL - denny / k8do

I finally tested the circuit. I started with the transistor mixer as
you said. After being sure that the oscillators was working, i
connected them to the mixer and turned both of them on. When i turned
on the second one, i started to hear some strong low frequency
cycling, as you said before. When i turned off one of the oscillators,
the sound was gone. So that shows the mixer is working, i guess.

I have seen some crystal oscillator circuits with a trimmer connected
in series with the crystal. I thougt, that way i could generate some
higher frequency sound. I connected a trimmer to one of the
oscillators and it made the circuit output a real 'beat note'. I could
change the tone frequency with adjusting the trimmer. It could even
generate frequencies as high as 2 KHz. I didn't think that this small
trimmer could change the oscillation frequency that much.

I also experienced some strange things. While playing with that
trimmer, i found out that the mixer's power connection was lying on
the table. I connected it again, but it didn't change anything.
Circuit went on working as before. Is this BJT mixer capable of mixing
without power applied? And even the two oscillators were exactly same,
i found that one of them were more sensitive than the other one.
Touching the trimmer with a screwdriver changed the frequency like
400-500 Hz, while with the other one, it had no effect.

After then, as you suggested, i disconnected one of the oscillators
and moved it a bit far away from the mixer. I could still hear it from
1-2 meters away. I turned off the mixer to see if it works with this
situation too, but when i turned it off, i couldn't hear the other
oscillator even from 10 cm's. What about this?

I will test the diode mixer, too, but i have some problems with the
transformer right now. Until then, i will play with this one and see
if i can hear some real stuff with this. Do you think it is possible
with this simple setup, and with a long wire?

Thanks....

On Feb 8, 7:00 am, wrote:
I finally tested the circuit. I started with the transistor mixer as
you said. After being sure that the oscillators was working, i
connected them to the mixer and turned both of them on. When i turned
on the second one, i started to hear some strong low frequency
cycling, as you said before. When i turned off one of the oscillators,
the sound was gone. So that shows the mixer is working, i guess.

I have seen some crystal oscillator circuits with a trimmer connected
in series with the crystal. I thougt, that way i could generate some
higher frequency sound. I connected a trimmer to one of the
oscillators and it made the circuit output a real 'beat note'. I could
change the tone frequency with adjusting the trimmer. It could even
generate frequencies as high as 2 KHz. I didn't think that this small
trimmer could change the oscillation frequency that much.

I also experienced some strange things. While playing with that
trimmer, i found out that the mixer's power connection was lying on
the table. I connected it again, but it didn't change anything.
Circuit went on working as before. Is this BJT mixer capable of mixing
without power applied? And even the two oscillators were exactly same,
i found that one of them were more sensitive than the other one.
Touching the trimmer with a screwdriver changed the frequency like
400-500 Hz, while with the other one, it had no effect.

After then, as you suggested, i disconnected one of the oscillators
and moved it a bit far away from the mixer. I could still hear it from
1-2 meters away. I turned off the mixer to see if it works with this
situation too, but when i turned it off, i couldn't hear the other
oscillator even from 10 cm's. What about this?

I will test the diode mixer, too, but i have some problems with the
transformer right now. Until then, i will play with this one and see
if i can hear some real stuff with this. Do you think it is possible
with this simple setup, and with a long wire?

Thanks....

Sounds like the one oscillator is not working the way you think...

Yes, a transistor is a pair of diodes nose to nose, and can mix/
rectify without being powered.. But will need power to work as
intended...

Look, take a diode and a cap and a resistor and make an RF probe for
your VOM... Look in the ARRL handbook, or the RSGB, or just about any
handbook for this RF probe circuit..

Cheers ... denny

On Feb 8, 2:32 pm, "Denny" wrote:
On Feb 8, 7:00 am, wrote:



I finally tested the circuit. I started with the transistor mixer as
you said. After being sure that the oscillators was working, i
connected them to the mixer and turned both of them on. When i turned
on the second one, i started to hear some strong low frequency
cycling, as you said before. When i turned off one of the oscillators,
the sound was gone. So that shows the mixer is working, i guess.

I have seen some crystal oscillator circuits with a trimmer connected
in series with the crystal. I thougt, that way i could generate some
higher frequency sound. I connected a trimmer to one of the
oscillators and it made the circuit output a real 'beat note'. I could
change the tone frequency with adjusting the trimmer. It could even
generate frequencies as high as 2 KHz. I didn't think that this small
trimmer could change the oscillation frequency that much.

I also experienced some strange things. While playing with that
trimmer, i found out that the mixer's power connection was lying on
the table. I connected it again, but it didn't change anything.
Circuit went on working as before. Is this BJT mixer capable of mixing
without power applied? And even the two oscillators were exactly same,
i found that one of them were more sensitive than the other one.
Touching the trimmer with a screwdriver changed the frequency like
400-500 Hz, while with the other one, it had no effect.

After then, as you suggested, i disconnected one of the oscillators
and moved it a bit far away from the mixer. I could still hear it from
1-2 meters away. I turned off the mixer to see if it works with this
situation too, but when i turned it off, i couldn't hear the other
oscillator even from 10 cm's. What about this?

I will test the diode mixer, too, but i have some problems with the
transformer right now. Until then, i will play with this one and see
if i can hear some real stuff with this. Do you think it is possible
with this simple setup, and with a long wire?

Thanks....

Sounds like the one oscillator is not working the way you think...

Yes, a transistor is a pair of diodes nose to nose, and can mix/
rectify without being powered.. But will need power to work as
intended...

Look, take a diode and a cap and a resistor and make an RF probe for
your VOM... Look in the ARRL handbook, or the RSGB, or just about any
handbook for this RF probe circuit..

Cheers ... denny

I did it, but for some reason it just doesn't read anything. Doesn't
the above test show that both oscillators are working?

Thanks..

On Feb 8, 9:02 am, wrote:
On Feb 8, 2:32 pm, "Denny" wrote:





On Feb 8, 7:00 am, wrote:

I finally tested the circuit. I started with the transistor mixer as
you said. After being sure that the oscillators was working, i
connected them to the mixer and turned both of them on. When i turned
on the second one, i started to hear some strong low frequency
cycling, as you said before. When i turned off one of the oscillators,
the sound was gone. So that shows the mixer is working, i guess.

I have seen some crystal oscillator circuits with a trimmer connected
in series with the crystal. I thougt, that way i could generate some
higher frequency sound. I connected a trimmer to one of the
oscillators and it made the circuit output a real 'beat note'. I could
change the tone frequency with adjusting the trimmer. It could even
generate frequencies as high as 2 KHz. I didn't think that this small
trimmer could change the oscillation frequency that much.

I also experienced some strange things. While playing with that
trimmer, i found out that the mixer's power connection was lying on
the table. I connected it again, but it didn't change anything.
Circuit went on working as before. Is this BJT mixer capable of mixing
without power applied? And even the two oscillators were exactly same,
i found that one of them were more sensitive than the other one.
Touching the trimmer with a screwdriver changed the frequency like
400-500 Hz, while with the other one, it had no effect.

After then, as you suggested, i disconnected one of the oscillators
and moved it a bit far away from the mixer. I could still hear it from
1-2 meters away. I turned off the mixer to see if it works with this
situation too, but when i turned it off, i couldn't hear the other
oscillator even from 10 cm's. What about this?

I will test the diode mixer, too, but i have some problems with the
transformer right now. Until then, i will play with this one and see
if i can hear some real stuff with this. Do you think it is possible
with this simple setup, and with a long wire?

Thanks....

Sounds like the one oscillator is not working the way you think...

Yes, a transistor is a pair of diodes nose to nose, and can mix/
rectify without being powered.. But will need power to work as
intended...

Look, take a diode and a cap and a resistor and make an RF probe for
your VOM... Look in the ARRL handbook, or the RSGB, or just about any
handbook for this RF probe circuit..

Cheers ... denny

I did it, but for some reason it just doesn't read anything. Doesn't
the above test show that both oscillators are working?

Thanks..- Hide quoted text -

- Show quoted text -

Phin,
In a word, no...
You need to get that RF probe working and test it on a known RF
source... Seems someone in your area should have a crystal oscillator
or such, that you can use to prove the probe works... Order a crystal
oscillator from Mouser, DigiKey, where ever, (~$3.00) and use that as
a test source...
Then you need to check that both of your oscillators are generating RF
of roughly the same amplitude, and use a receiver to check the
frequency of each oscillator...
Did you test your audio amp?
Then and only then are you ready to trouble shoot the mixer...

denny / k8do