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!

--
73 de LB1LF

Odd Erling, ARK

This message transmitted on 100% recycled electrons.

Odd Erling N. Eriksen wrote:

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

-First you post, then you start thinking...

I assume the ferrites are ordinary ferrite beads - not some special kind of
resistor. The ohmic value written in the datasheet then, is the impedance
at the operating frequency, whereas the DC resistance of same will be near
zero ohms.

Thus, the ferrites are not as much for preventing noise from entering the
PA chip, as they are to prevent noise from _leaving_ the PA.

Am I right?

--
73 de LB1LF

Odd Erling, ARK

This message transmitted on 100% recycled electrons.

Odd Erling N. Eriksen wrote:

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

-First you post, then you start thinking...

I assume the ferrites are ordinary ferrite beads - not some special kind of
resistor. The ohmic value written in the datasheet then, is the impedance
at the operating frequency, whereas the DC resistance of same will be near
zero ohms.

Thus, the ferrites are not as much for preventing noise from entering the
PA chip, as they are to prevent noise from _leaving_ the PA.

Am I right?

--
73 de LB1LF

Odd Erling, ARK

This message transmitted on 100% recycled electrons.

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

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

Tim Wescott wrote:

The MSc degree doesn't train you how to _be_ an engineer, it trains you
how to _learn how to be_ an engineer.

-I know, I know - I just wish that we could spend more time in the lab
trying to figure out the practical implications of the theory we had
learned; don't get me wrong. Thanks for your input, by the way.

--
73 de LB1LF

Odd Erling, ARK

Do infants enjoy infancy as much as adults enjoy adultery?

Tim Wescott wrote:

The MSc degree doesn't train you how to _be_ an engineer, it trains you
how to _learn how to be_ an engineer.

-I know, I know - I just wish that we could spend more time in the lab
trying to figure out the practical implications of the theory we had
learned; don't get me wrong. Thanks for your input, by the way.

--
73 de LB1LF

Odd Erling, ARK

Do infants enjoy infancy as much as adults enjoy adultery?

Odd Erling N. Eriksen wrote:
Tim Wescott wrote:


The MSc degree doesn't train you how to _be_ an engineer, it trains you
how to _learn how to be_ an engineer.


-I know, I know - I just wish that we could spend more time in the lab
trying to figure out the practical implications of the theory we had
learned; don't get me wrong. Thanks for your input, by the way.


I don't know how much time they have you spending in the lab, but I home
you get _some_. Some lab work is essential, if for no other reason than
to bring to you, as forcibly as possible, the fact that lab work and
theoretical work are two entirely different things. On the other hand
if all they do is train you to be a jumped-up technician then they
haven't served you very well.

--

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

Odd Erling N. Eriksen wrote:
Tim Wescott wrote:


The MSc degree doesn't train you how to _be_ an engineer, it trains you
how to _learn how to be_ an engineer.


-I know, I know - I just wish that we could spend more time in the lab
trying to figure out the practical implications of the theory we had
learned; don't get me wrong. Thanks for your input, by the way.


I don't know how much time they have you spending in the lab, but I home
you get _some_. Some lab work is essential, if for no other reason than
to bring to you, as forcibly as possible, the fact that lab work and
theoretical work are two entirely different things. On the other hand
if all they do is train you to be a jumped-up technician then they
haven't served you very well.

--

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