Odd Erling N. Eriksen wrote:

Hi,

I am currently trying to debug somebody else's UHF telemetry transmitter
design. My primary concern at present is that the PA module (RF Micro-
devices 2117) datasheet states that ferrites of certain values should
be put in series with the bias voltage, power down control voltage and
RF input.

On the PCB I am evaluating, the ferrites have been substituted with
ordinary thick film SMD resistors of the same ohmic values.

My question, then, is the following: In what way is it likely that this
affects the operation of the PA?

-Nonlinear operation because of noisy supply voltage?
-Excessive intermodulation due to stray RF being let into the chip?
-Anything else?

I am not very experienced in applied electronics, I am afraid - the MSc.
studies I am currently undertaking focus (way to much!) on the
theoretical approach. Sigh.

Thanks in advance for any qualified guesses!


The MSc degree doesn't train you how to _be_ an engineer, it trains you
how to _learn how to be_ an engineer. Since technology is constantly
changing you have to retrain yourself constantly, so this is a good
thing. If they just taught you what you needed _right now_ you'd be
washed out and working in sales in less than five years.

Your follow-on is correct in that the ferrites are rated for their
operating frequency, and should have low DC resistance (if they're
surface mount they're not actually just beads, they're wires wrapped in
ferrite. The term is a holdover from through-hole where they _were_
actually beads slipped over component leads). So replacing them with
resistors means that the amplifier power lines are soft instead of
stiff, which could do all sorts of weird things.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com