how to use digital attenuators in AGC

[7rots51]

agc loops digital potentiometer

I want to use a 32dB 5 bit digital attenuator in an IF AGC loop,I use a RF detector(LOG or RMS) to convert RF to voltage,how use this voltage to control attenuator?
please give me some helps.

 

[madengr]

digital agc

Why don't you use an analog (voltage variable) attenuator?

 

[kspalla]

rf hittite attenuator agc circuit

Analog attenuators are closed loop in analog domain so the circuit may more likely unstable if not designed properly.

Digital attenuators are also closed loop with digital controller and the repose times are low spped.
There will be input level monitor and will decide in increasing or decreasing the attenuation level.

 

[madengr]

rms detector digital attenuator

Yes, but most analog AGC loops have a very low bandwidth so stability is usually not an issue; I have never had a problem.

A digital attenuator is fine for setting static power levels, or maybe if you are computing the power digitally. However if you are using a RMS detector chip then it seems overly complex to digitize it then control a digital attenuator, when you could use an analog attenuator and op-amp integrator. You can still control the set-point with a digital potentiometer or DAC.

Also, if you are using a true digital feedback loop then you can only move in 1 dB steps over that 32 dB dynamic range. If your feedback loop is continuously twiddling that 1 dB LSB to lock the power it's going to introduce quantization noise which maybe be quite severe depending on the type of signal you are working with You won't have this problem with an analog method. I have never built a digital feedback loop however just off the top of my head it seems like a lot of trouble for an AGC with 32 dB dynamic range (that's about the limit for a single stage analog loop).

 

[biff44]

ad8314

Are you under the delusion that just because digital control loops are "slower",that makes them stable???

Also, what do you do with the revceived data when the controller decides to change the state of the digital attenuator? Throw those packets away? What if the signal is varying often and you get a new digital command every millisecond?

 

[7rots51]

agc using digital potentiometers

for what applications digital attenuators is used?

 

[madengr]

make a digital attenuator

All sorts of places. Maybe in a T/R module for beam weighting. Maybe before the ADC in a spectrum analyzer so you can keep the signal to the full-scale of the DAC. Maybe before a PA to control output power.

As biff44 said, you have to be really careful about switching them in a comm system. Check the topology of the digital attenuator to make sure it does not drop-out during transitions. Even a small hiccup in the received signal that has been forward-error corrected can blow the whole link since the decoders must flush and restart. In fact you can't use step attenuators (rather you need variable attenuators) in a BER test setup since you can't drop the signal when testing at low Eb/N0.

 

[kspalla]

digital pot agc

I want to use a 32dB 5 bit digital attenuator in an IF AGC loop,I use a RF detector(LOG or RMS) to convert RF to voltage,how use this voltage to control attenuator?

IF AGC loop with RF detector and with out knowing the application (LOG or RMS)? Looks interesting solution.....

 

[7rots51]

agc digital potentiometer

I saw in Hittite 2008 selection guides in the block diagrams
that they show digital attenuators in first RF stage before first mixer !
does It mean using digital attenuators in first RF stage is not bad?

 

[madengr]

I saw in Hittite 2008 selection guides in the block diagrams
that they show digital attenuators in first RF stage before first mixer !
does It mean using digital attenuators in first RF stage is not bad?

We didn't say it's bad to use digital attenuators, but rather using them in a real-time feed-back control loop for an AGC is not going to be trivial. If you are looking for analog voltage-variable attenuators try M/A-COM and Skyworks.

 

[kspalla]

does It mean using digital attenuators in first RF stage is not bad?

KSPALLA: Attenuator before RF stage, means that the in[put power requirement for the system is too high and your LNA and/or the mixer stage is not surviving for IIP3.
Check the IIP3 of your system.

 

[7rots51]

my LNA OIP3 is +35dbm and my first mixer IIP3 is +23dbm,but after mixer I have SAW filter with MAX 0dbm input! how to protect SAW filter and other stages from strong signals and from saturation!

 

[kspalla]

There the problem.
How long the SAW filter can survive for large signal?
This determines the loop action to complete the attenuation.
Digital AGC is stable but takes longer time than analog AGC.
So go for AGC device in RF.

 

[7rots51]

I want to use hmc473ms8 voltage variable attenuator from hittie (4$). Is this better than digital att to control ? I use BAV99 diode or ad8314 for RF detection.

 

[mwmmboy]

I think you need to use and ADC to sample the voltage and then a logic circuit or a microcrontrolloler to process the sampled voltage and generate the digital word you need to set the attenuator. In this way you need implement a digital AGC.

I hpe this can help

 

[frannk]

If you sample the RF strength at the output of the digital attenuator, then you will need additional decision circuits, a clock, and a counter.