I took a receiver design class at Rockwell-Collins when I was working for
them. The guidelines given at the class were no more than 70dB gain at any
single frequency. Any more than that, an instability can result. One of my
instructers in college told me that with the old school of design, allow at
least 1/2 inch of length for each 40dB of gain at a single frequency, and
even with those parameters, interstage shields would be necessary. One
technique that radio manufacturers is "egg crate" construction.............a
die-cast chassis with 1/16th inch walls separating each compartment that
contains a module is used.
The better RF generators also use this scheme. Examples of a receiver and RF
generator are the Racal 6790/GM and the Boonton 103D, respectively.
If you are still curious if your idea will work or not, I suggest that you
actually build the cascaded lineup and find out what happens. Sometimes, the
best way to learn about these things is to experience them yourself. I
remember the first cascaded amplifier lineup I built when I was going to
school. When I asked my instructor why the circuit was oscillating, he asked
me where the decoupling networks were. I asked him "decoupling what?". Oh,
that's what those 100 ohm / .01uF RC networks were that I used to see in
series with the collector/drain resistors. I used to jump out those 100 ohm
resistors, thinking that the extra collector/drain voltage would somehow
give the radio more gain!
It is always good to ask questions on the NG, but the best way to learn is
to actually build the circuit yourself. Nothing like the "school of hard
knocks".

Pete

wrote in message
ps.com...
When the answer is not clear for our doubts, we argue about it for
better explanation, until the doubts are solved. I think this is a
healthy way to get answers.




Wes Stewart wrote:
On 11 Oct 2006 12:13:20 -0700, wrote:

I asked what I don't know. I am expecting a helpful answer.

The problem was, you got some prior answers and then wanted to argue
about it.

The frequency is 70MHz.

Then, assuming some type of filtering around that frequency that's
more or less where your system will likely oscillate.

One reason superhetrodyne receivers were developed to replace TRF
receivers was to develop system gain at different frequencies so as to
minimize the chance of oscillation. Another, of course was to achieve
better selectivity that with older technology was better accomplished
at lower frequencies.

Even a single conversion receiver usually develops its gain at three
different frequencies: r-f, i-f and post detection.