On Apr 30, 12:59 pm, bigorangebus wrote:
I'm a professional electronics engineer (not rf enginneer though), and
what I really find frustrating about radio projects is the
uncertainty!

I spend hours designing and analysing filters. Designing oscillators
and minimising harmonics, noise etc. Then I cheerfully commit it to a
PCB (I tightly laid out surface mount PCB to minimise layout
inductances etc).

Nothing ever works as simulated. Transistors never have the rf gain
as SPICE seems to suggest.

And local oscillators...well carefully chosen components just get
thrown out the window to be replaced by more and more random changes
in component values.

Oscillators are very tricky to design using tools like SPICE. Most
simple oscillators by definition build up in amplitude until the
device goes nonlinear and loop gain becomes one. SPICE models outside
the linear range are often very poor.

And a very important factor for oscillators, is that we often require
that the oscillator start reliably at all corners of the gain/
temperature space, but SPICE models usually sit squarely in the center
of gain/temperature space. SPICE models for crystal parameters are
largely nonexistent or very proprietary and again don't cover all
corners of gain/temperature space.

So I wonder, what would be your top tips for someone moving into the
radio design arena? Are there hidden secrets that nobody tells and
the books omit? I'm not thinking about PCB layout here, more things
like, are simulation programs of any use (if so which ones) and what
kind of design procedures can result in predicatable results?

Canned crystal oscillators are a good choice if they really, really
have to start reliably. Many handbook recipes for oscillators are not
necessarily reliable starters.

Simulation for oscillator design was a laughable concept when I first
learned SPICE in the 80's. Today we have more CPU power, but the
models are the weakness.

If you are working in the VHF/UHF range, amplifier design usually
starts with a transistor model for the frequency range. It's rare to
have to get into SPICE.

Tim.