Hi,

I have built two mixers for a direct conversion receiver. One of them
a single transistor mixer, the other one is a single balanced diode
mixer. I already have AF amplifier and LO stages. I think both are
working fine seperately, but i can't hear anything when i connect the
LO and the amplifier to the mixers(either of them of course). Is there
any way that i can test these two mixers without test equipment like
an oscilloscope or a signal generator? I also don't have a RF probe
right now, only a multimeter. Actually i have another questions with
this receiver, too, but this is where i am stuck at the moment.

(LO is a crystal controlled one. I built a VFO, too, but didn't manage
to get it working, for now. )

Thanks in advance...

I forgot to mention;

I got diode mixer schematic from he http://www.vk2zay.net/article.php/46
and the bjt mixer schematic from he http://homepage.eircom.net/
~ei9gq/rx_circ.html#Mixers (first one).

Thanks again..

wrote:
Hi,

I have built two mixers for a direct conversion receiver. One of them
a single transistor mixer, the other one is a single balanced diode
mixer. I already have AF amplifier and LO stages. I think both are
working fine seperately, but i can't hear anything when i connect the
LO and the amplifier to the mixers(either of them of course). Is there
any way that i can test these two mixers without test equipment like
an oscilloscope or a signal generator? I also don't have a RF probe
right now, only a multimeter. Actually i have another questions with
this receiver, too, but this is where i am stuck at the moment.

(LO is a crystal controlled one. I built a VFO, too, but didn't manage
to get it working, for now. )

Thanks in advance...


And you have verified that the LO is working?

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) writes:
Hi,

I have built two mixers for a direct conversion receiver. One of them
a single transistor mixer, the other one is a single balanced diode
mixer. I already have AF amplifier and LO stages. I think both are
working fine seperately, but i can't hear anything when i connect the
LO and the amplifier to the mixers(either of them of course). Is there
any way that i can test these two mixers without test equipment like
an oscilloscope or a signal generator? I also don't have a RF probe
right now, only a multimeter. Actually i have another questions with
this receiver, too, but this is where i am stuck at the moment.

(LO is a crystal controlled one. I built a VFO, too, but didn't manage
to get it working, for now. )

Thanks in advance...

It would take a pretty lousy mixer to not get some mixing action. And
since you hve an audio amplifier following it, even weak mixing action
should provide a signal.

Hence the first step is to get a second local oscillator close to the
frequency of the existing one, and then you'll hear the beat note out
of the audio amplifier if things are working to some extent. If you
only have crystal oscillators on the same frequency, they might not be far
apart enough to get much of a beat note, but you'd be seeing some cycling
of the output of the mixer (because the oscillators are beating together
to generate an output so low in frequency that you can see it rather
than hear it).

Stop one of the oscillators, and if that cycling goes away then you
know things are working to some extent.

The advantage of crystal oscillators is that you can get some 4pin TTL
oscillators cheap, and they will be strong enough to check things out.
Their exact frequency won't matter at this point but wire them up
properly and you can be fairly certain that they will work, unlike
building up oscillators from scratch. Your problem might be that
the mixer isn't working properly so only strong signals will result in output.

Once you get some mixing action then you can move one oscillator away
from a direct connection to the mixer, and treat it like a "weak" signal
coming in through the antenna.

Make an RF probe, a coupling capacitor and a diode and a load resistor,
and connect it to your multimeter. (Make it a "probe" in that the
coupling capacitor should have short lead connect directly to the
rest of your probe circuit. You then touch the short capacitor lead
directly to the oscillator, rather than running a long lead between
the oscillator and the probe.) Since it's DC coupled, you won't
get a reading on the meter unless the oscillator is oscillating. YOu can
always verify by removing power from the oscillator, and when the needle
drops, you know the oscillator has been turned off. Of course, one has
to be careful because if you load down the oscillator too much, that
may kill the oscillation.

If you have a shortwave radio, tune it to try to find your oscillators,
to see if they are oscillating, and in the case of the LC oscillator,
where its frequency is. If you don't have an oscillator at a suitable
frequency, then you might not know if things are working if there's no
signal on that frequency.

Michael VE2BVW

-On Feb 6, 1:15 am, chuck wrote:

And you have verified that the LO is working?

I think it is, because when i turn it on with an alligator clip
attached to it's output, i hear a humming sound at the nearby receiver
- which is an envelope detector connected to a LM386 amplifier. It
means that it's working, right?

-On Feb 6, 7:20 am, (Michael Black) wrote:

It would take a pretty lousy mixer to not get some mixing action. And
since you hve an audio amplifier following it, even weak mixing action
should provide a signal.

Hence the first step is to get a second local oscillator close to the
frequency of the existing one, and then you'll hear the beat note out
of the audio amplifier if things are working to some extent. If you
only have crystal oscillators on the same frequency, they might not be far
apart enough to get much of a beat note, but you'd be seeing some cycling
of the output of the mixer (because the oscillators are beating together
to generate an output so low in frequency that you can see it rather
than hear it).

Stop one of the oscillators, and if that cycling goes away then you
know things are working to some extent.

The advantage of crystal oscillators is that you can get some 4pin TTL
oscillators cheap, and they will be strong enough to check things out.
Their exact frequency won't matter at this point but wire them up
properly and you can be fairly certain that they will work, unlike
building up oscillators from scratch. Your problem might be that
the mixer isn't working properly so only strong signals will result in output.

Once you get some mixing action then you can move one oscillator away
from a direct connection to the mixer, and treat it like a "weak" signal
coming in through the antenna.

Make an RF probe, a coupling capacitor and a diode and a load resistor,
and connect it to your multimeter. (Make it a "probe" in that the
coupling capacitor should have short lead connect directly to the
rest of your probe circuit. You then touch the short capacitor lead
directly to the oscillator, rather than running a long lead between
the oscillator and the probe.) Since it's DC coupled, you won't
get a reading on the meter unless the oscillator is oscillating. YOu can
always verify by removing power from the oscillator, and when the needle
drops, you know the oscillator has been turned off. Of course, one has
to be careful because if you load down the oscillator too much, that
may kill the oscillation.

If you have a shortwave radio, tune it to try to find your oscillators,
to see if they are oscillating, and in the case of the LC oscillator,
where its frequency is. If you don't have an oscillator at a suitable
frequency, then you might not know if things are working if there's no
signal on that frequency.

Michael VE2BVW

Actually, i first tought, that if i would connect the LO to the mixer
and attach a long wire to the antenna port, i could hear something
useful that is near the crystal frequency (7159 KHz) and determine if
the mixer is working or not. A funny thought i guess.

So back to your suggestion; I have a few more of these crystals and i
can build a second same oscillator for this test. I will also try the
cheap TTL oscillators you mentioned if the above test fails. If it
doesn't work too, i think i can be sure that the mixer isn't working.
(By the way, i've got the oscillator schematic from http://
http://www.geocities.com/raiu_harris.../osc-xtal.html this
page (the first one))

I would like to build a diode-ring mixer which is mentioned everywhere
on the net, but those toroids used in transformers are just too hard
to find here. That's why i used a ferrite rod, salvaged from an old
radio, in that diode mixer. Should using ferrite rods or other core
materials instead of toroids cause problems?

I build the RF probe you explained on a piece of PC board (got the
component values from the N5ESE's Classic RF Probe page) and when i
get the coaxial cable, that is needed to connect it to the multimeter,
i will test those oscillators with it, too. And i have digital meter
not an analog one. But both should work OK, i think.

And lastly, i don't have a shortwave receiver but when i go to buy
some stuff again, i will also get some crystals whose frequency is in
the AM band in order to listen for the oscillator signal in an AM
radio.

Thank you for your interest..

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...

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

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..