How to control this RF Power Amplifier chip?

[dralthi]

Hi folks,

I'm working on a project for class and we're building a RF transmitter. We have to use this part:
http://www.skyworksinc.com/uploads/documents/SKY85402_11_202120I.pdf

We have the circuitry almost built, but we're wondering, how do we control the power output? For example, we want to be able to change power output from +0 dB to +28 dB with DIP switches. I don't see any control circuitry for it, nor do I see any way to input that information. But it seems like it can be done??

Any help would be greatly appreciated from any the gurus!

[Mechatrommer]

you cant. thats no RF transmitter, thats a RF amplifier, the one after the RF transmitter stage. so whats your transmitter? because thats where you adjust the gain...

[dralthi]

Actually we have to use a DSP chip to modulate a BPSK signal..so on the RF Amplifier, how do we control the Tx power?


The block diagram is:

DSP ---- [SuperHetrodyne Mixer stuff] --- digital attenuator ---- RF PA



The digital attenuator is: http://www.psemi.com/pdf/datasheets/pe43503ds.pdf

The superheterodyne is being designed by another team.

[xygor]

Control the output power by controlling the input power.

[donmr]

Isn't that what the "digital attenuator" is for?

[Earendil]

You can use VDET to determine the output power level. And you can use the digital attenuator to set the output power. This is usually done in a control loop. To a certain extent you can also change the output by changing the amplitude from the DSP's DAC.
However if you need to ask questions like this on a forum than you probably shouldn't use this circuit to actually transmit. Use a dummy load or connect your receiver directly through a coax cable and a fixed attenuator.

[dralthi]

Thanks for the help. We're connecting the output to a Spectrum Analyzer....

So, yes, I'm going to see if there is a DAC in the DSP that can be used.

[Howardlong]

Thanks for the help. We're connecting the output to a Spectrum Analyzer....

So, yes, I'm going to see if there is a DAC in the DSP that can be used.

I would assume (but stand to be corrected) that you're filtering your BPSK in the digital domain with some kind of matched filter, maybe root raised cosine or some such? Anyway, if you are, then there will be a DAC of some sort (maybe even PWM) wih an LPF before your modulator. You can therefore read the DET with an LPF and ADC connected to you DSP and use that to create an ALC loop.

As a matter of interest, is your modulator a quadrature modulator or are you using a simple double balanced mixer to generate the BPSK?

[Mechatrommer]

So, yes, I'm going to see if there is a DAC in the DSP that can be used.

i was wondering what the heck. you already gave the answer to your own question in the OP in reply #2.

how do we control the power output? For example, we want to be able to change power output from +0 dB to +28 dB with DIP switches.

DSP ---- [SuperHetrodyne Mixer stuff] --- digital attenuator ---- RF PA
The digital attenuator is: http://www.psemi.com/pdf/datasheets/pe43503ds.pdf

and someone tried to put that on the water...

Isn't that what the "digital attenuator" is for?