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I'm designing with LNA to receive a BW = 8 MHz signal (channel BW = 25 kHz), see block diagram:

enter image description here

There is a case where an in-band channel signal (or blocker) which is strong enough saturates the LNA input and then degrades the linearity of the channel of interest.

To avoid this case:

  1. I plan to use a coupler + power detector to measure the in-band power in this case and attenuate the LNA input power in case the blocker is too strong, so that the LNA operates in the linear zone (this also protects the LNA in case the maximum input rate is exceeded).

  2. Maybe use an RF limiter instead of attenuator to protect the LNA input. My question here is: when the in-band blocker is attenuated by the limiter (the limiter is not linear any more), what about the useful signal (at another channel)? Does the useful signal pass through the limiter normally or does it also suffer from the non-linearity of the limiter?

Are there other ways to avoid high-power, in-band blockers?

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    \$\begingroup\$ If the LNA is going non-linear...what about the poor mixer? It also sees that wide 8 MHz bandwidth, and a boosted antenna signal as well. In such a large-signal environment, the LNA is making things worse. \$\endgroup\$
    – glen_geek
    Commented Apr 4, 2023 at 15:23
  • \$\begingroup\$ I'm certain the limiter is going affect both your in-band interferer and the signal of interest. If you have a specific limiter in mind, link to the data sheet. \$\endgroup\$
    – SteveSh
    Commented Apr 4, 2023 at 23:38
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    \$\begingroup\$ Note also that in your option 1, both your power detector and the attenuator are going to attenuate your signal into the LNA. This will degrade the noise figure of your system. \$\endgroup\$
    – SteveSh
    Commented Apr 4, 2023 at 23:40
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    \$\begingroup\$ Finally, your best fix is a different LNA that can handle the interferer without going into compression. \$\endgroup\$
    – SteveSh
    Commented Apr 4, 2023 at 23:42
  • \$\begingroup\$ @SteveSh thanks for your comments. When the limiter is compressed, its insertion loss also increase, then the noise figure also degrades. So in either option 1 or 2 the degradation of NF can not be avoided, right? For limiters I was thinking about using this mini-circuits one: minicircuits.com/pdfs/CLM-83-2W+.pdf or this PIN diode: mouser.fr/datasheet/2/472/SKYS_S_A0002244728_1-2515377.pdf \$\endgroup\$
    – Pascal R.
    Commented Apr 5, 2023 at 10:20

1 Answer 1

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Today we were setting up a demonstrator with a AM/FM receiver we designed at my company. We were able to jam the FM reception easily by connecting an antenna to a signal generator producing FM-modulated audio sinewave.

The receiver has an automatic gain control loop, so the best it could do is increase the amount of attenuation to protect the frontend (an LNA with programmable gain, but no frequency discrimination within the band). However, we heard no music, only the tone we jammed it with, as expected.

If your signal is deterministic, like the UWB positioning systems that use a Gaussian monocycle, I can think of digitizing the signal and pass it through a correlator with a template of your signal in order to discern your actual deterministic signal from anything else.

Conclusion: Use the 1st option you proposed, an AGC loop: Sense the power level and regulate the attenuation accordingly. Define a maximum output level for your amplifier before compression (such that your amplifier is still in the weakly non-linear region) and attenuate whenever such a threshold is surpassed.

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  • \$\begingroup\$ thanks for your comments. If I understood well in your receiver example, the AGC you implemented can only protect the LNA in case of high level in-band noise/blocker by attenuating all in band signals (including desired freq.). So it seems no other options here in analog ways but using digital ways... \$\endgroup\$
    – Pascal R.
    Commented Apr 5, 2023 at 10:31
  • \$\begingroup\$ Does your AGC (with attenuator in front of LNA) try to make LNA work in its linear zone or just to protect the LNA below max rating ? \$\endgroup\$
    – Pascal R.
    Commented Apr 5, 2023 at 10:43
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    \$\begingroup\$ @PascalR. the AGC loop triggers (i.e. increases the attenuation by one step) the moment the maximum peak amplitude (defined by specification) is crossed. The LNA is required to be in the weakly non-linear region at this amplitude (i.e. 2nd and 3rd order sideband components increase by 2dB and 3dB, for every 1dB of input signal increase). \$\endgroup\$
    – Designalog
    Commented Apr 5, 2023 at 10:54
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    \$\begingroup\$ @PascalR. Noise degradation is unavoidable. In the receiver I designed, I had to comply with a maximum noise degradation for every so many AGC steps (e.g 1st AGC step cannot degrade input referred noise by more than 1dB, 5th step by 8dB, etc) \$\endgroup\$
    – Designalog
    Commented Apr 5, 2023 at 11:01

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