) 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