Re: level and wave shape after balans mixer?

Ashhar Farhan <farhan@...>

On Wed, 30 Jun 2004, vdberghak wrote:

The LF parts works fine (a pitty the schematic is not corrected with
the voltage/output of the lm386).
the correct schmatics are now at i have run out
of bandwidth on ther is no way i can access those
files now. just before i ran out of bandwidth, i did manage to put a
redirect on the home page. yo probably have te BITX20 page directly
bookmarked. which is why you are not being redirected to the new site.

If I add a sinus LF wave to the
microphone terminal, the shape looks simular on the colector.
However, as soon as I connect the coil of the balans mixer to it, it
becomes a-symetrical (one side is going to be clipped).
you need a 10mV signal at the base of the mic. this is usually too low to
be measured with an oscilloscope. your best bet is to actually connect a
mic and speak into it. if you can whistle (keeping your breath away from
the mic), it is a single tone-test. an extended 'aaaa' looks like a
three-tone test to me.

the other side is clipeed, because you note, the diodes in the mixer will
clamp the output through the attentuator. this is an expected behaviour.

If I put a lot of LF signal in it (looks heavily distorted, then I
get a maximum output(after the attenuater) of about 0.2 mw. (in 50
ohm). After reducing the LF signal to a point it looks normal, the
output power is very very less....
Did anyone look at the osciloscoop the way I did? Anyone measured
the output?
it is best to check the balanced modulator on its own first, without the
mic connected. you should be able to null the carrier at mid setting of
the preset and the 22pf trimmer. if that works, then you can move to the
audio stage.

typically, keeping the audio output of the mic amp connected to diode
modulator, you should see similar waveforms at the modulator input (where
the two transformer windings are shorted) and the output (the attentuator
pad) when using the oscilloscope at audio frequencies (keeping the time
base to about 10mSec per div).

as you have pointed out, the distortion is due to the non-linear nature of
the diodes. To measure the RF component alone, you should measure the
output through a hihg pass filter. for quick and dirty measurement,
connecte an RFC across the output of the attenuator and then connected the

if you have already made the crystal fitler, it is best that you measure
and check the signal at the output Q12 (without the RF mixer connected).
there is little that can go wrong in the Q12 and Q11. the only caveat is
that depending upon the exact frequency of the BFO, you might or might not
get SSB. If you are sitting in the middle of the filter's passband, you
will get DSB with audio roll-off at 1-2khz. but the enveolope will be
there to see.

have you tested the receiver yet? it is usually the recevier that comes
alive first. it gives you a good idea of how things are working.

i am convinced that we need to add detailed test measurements for the
transceiver on the site. i have most of them in my log book. i will post
them soon.

- farhan

The distortion I can imagine can be caused by one of the diodes, but
such low output at that moment?
After the next amp (before the x-tal filter, the maximum output is
about 1 mw (in 50 ohm). The reason is that I measure in 50 ohm is
that the load is in the mw. meter.
Thanks in advance for your comments,
Chris PA3CRX

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