FM LNA with output voltage regulated

[Leiser]

Hi all!

I have to design a FM LNA with output voltage regulation.

The specs are:

output voltage of amplifier between 90 to 105dBuV for an input level from (90-amplifier gain) to 120dBuV.

Does anybody know how can I design this control?

At the moment I have a FM non regulated amplifier. Is there any way to add the regulation to this design?

Thanks a lot!

 

[Comcokid]

Sounds like what you want is a AGC controlled amplifer, and a level detector on the output of the amplifer controlling the amplifer gain. AGC stands for Automatic Gain Control. Usually these circuits are found within a receiver to maintain a constant demodulation level with varying signal strength.

 

[Leiser]

Hi,

I was thinking using a T-attenuator with pin diodes. Any ideas on a concrete part for the level detector?

Thanks

 

[zappedagain]

At 105 dBuV (-2 dBm in 50 ohms) you have a pretty strong signal so a limiter sounds like the right option. There are single chip solutions out there but first you'll have to define your frequency range and phase sensitivity. Motorola (On?), National, Analog Devices, and Maxim come to mind as potential sources. I'm sure there are other vendors too.

If you can get a diode limiter to meet your specs you'll have a cheaper solution. Are you in a 50 ohm system? If so your rms voltage (180 mV) is a bit low for a diode limiter.

 

[biff44]

Is it really FM? If so, you do not have to maintain the linearity in the gain. I would go with a fully compressed amplifier, and pad down the output to get the right output. Analog Devices log amp chips have a compressed FM output pin, for instance, for just such an application.

 

[Leiser]

Hi,

Yes I´m working in a 50R system. I have designed a non-regulated FM amplifier and now I need to add a block that limits the output. This block needs to be the simpler the better, as this is for an automotive application and cost is critical.

OIP3 is around 30dBm so linearity is quite important too.

Thanks in advance

 

[zappedagain]

"but first you'll have to define your frequency range "

315, 433, 866, 2400 MHz? Your intermediate frequency?

 

[biff44]

If you have an FM system, then OIP3 is totally irrelevant. FM is constant envelop modulation. Find out what type of modulation you really have and report back.

 

[Leiser]

Hi!

Well, sorry for not saying the frequency range.
This is a LNA for commercial FM radio bands in a car receiver. 87-108MHz. This amplifier goes just after the receiver antenna.

I do agree that FM is constant envelope but because the car moves then the amplifier will receive different power levels depending on how close to the emitter the car is. Hence the importance of OIP3.

 

[biff44]

I do not know if I buy that. A limiting amp can easily have 80 dB of dynamic range. Now, if you said that it was also going to be used for AM broadcast band reception, then that is a different story.

Obviously you can just put a amp chain in there with an AGC, and have a large output stage compression point. However, that will need a lot of supply current which may generate a lot of heat.

I have seen silicon bipolars set up in feedback mode so that the gain is reduced, but the OIP is greatly improved, while drawing little DC current.