On 19 Aug 2005 22:49:15 -0700, wrote:

I sucessfully hooked up two stages with no oscillation problems. I
added a 560 ohm load resistor across the output winding of the first
stage.

You could load the primary instead, values in the range of 10k to 80k
would be tried.

The LO oscillator is a separate circuit (JFET) and there are no
physical connections from the the oscillator to the mixer stage. It
seems to mix and produce the IF from just the radiation from the LO
capacitor to the station tuning capacitor.

Not uncommon, one circuit copuples to another due to adjacent
components. However, the Mixer may not be seeing the optimum
LO level for best gain or overload characteristics.

The IF modulated output is about 2 volts p-p on a strong station, 600
mV on a average station, and only 50mV on weak stations.

It's almost usable, but not quite. I need to raise the weak stations
another 12 dB or so. Also, the LO oscillator is leaking into the output
at the high end of the band. I get 100mV of noise from the LO toward
the high end.

Any number of factors here. More gain means more stanility problems.

Also, the LO oscillator is leaking into the output
at the high end of the band. I get 100mV of noise from the LO toward
the high end.

The LO could be noisy due to design error. The front end (loopstick
in AM BCB radios) may be tuned better at that end of the band and you
hearing (seeing) more noise. Or you are seeing better coupling of the
LO (increses with frequency) and better mixer gain. The MW range
(300kc to 3mhz) has a very high backgound noise both teresterial and
man made.
..
Is it a good idea to try and raise the gain of the 2 existing stages,
or try to add a third stage to raise the gain?

NO, they are only conditonally stable as is. That load on the one
transformer is both robbing gain and also giving a little stability.

I've noticed some transistor radios use only one IF amplifier stage
while others use 2.

Depends on perfomance. Those with one are pretty tepid in the
sensitivity department.

What are the advantages of the third stage if enough gain can be
obtained from just 2?

Many AM radios apply AGC to one or more stages to "level"
the strong and weak stations for even audio output and to prevent
later states from overloading. More stages allow for more applied AGC
and overall better perfomance.


Allison KB1GMX