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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-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..