lna gain variations vs input signal power

I am designing an lna and mixer for zigbee receiver. my lna gain has varitions between 26.9db for -91dbm input power to 29db for -20dbm!my lna is fixed gain apparently!. but it's gain variations isn't constant.it's gain variations file has been attached here. my lna's output impedance is 772ohm. I think it is ubnormally very high.it's nf is 2.6db ,ip3=3.5dbm and p1db=-25dbm ip3 and p1db difference is very big!!I think it's linearity has problem but i don't know the problem arises from where.other parameters of lna such as s parameters, power and stability is good. my lna's structure is common sorce common gate cascode source inductive degeneration. also I want to know how much power does the active gilbert mixer can receive typically so that it has no distortion ?
thanks.
View attachment lna gain.doc

Hello,

Did you measure your lna in-circuit, or with a defined source and linear load? Did you change attenuator settings, do you see power supply current variations during the measurements, do you notice hysteresis effects?

If there is no change in power supply current, I can hardly imagine that gain changes 2 dB at a 10 dB step at around -80 dBm.

For the Gilbert Cell, not possible to say from the info. Maybe measurement and/or simulation may give you an answer.

hello

my lna input is a port with sinusoidal sigal(frequency=2.4gig) and a 50ohm resistance. and the output is a port with dc or sin(there was no difference between them in simulation results) and 772ohm resistance(output impedance of lna that will be matched with mixer input impedance).attenuator. what do you mean?
the power supply current is constant over all of gains and input signal's power.I don't know how to consider the hysteresis effects with rf spectre?
my lna input circuit is a dc blocking capacitor and an gate inductor.the output is a tank and dc blocking capacitor.what do you think about the output impedance of lna?isn't it too high?
about the active mixer.is it possible to increase the input signal amplitude up to 400mv?!I have no enough vision about it?

Have you actually checked the accuracy of your signal generators attenuator and your measuring device. Also if you are using a wideband measuring device, you may be indicating the sum of various mixer products.There does not seem to be any law linking the gain with level, I would have thought that at your low levels the mixer should be linear, with compression at high signal inputs. Perhaps there is some parametric mixing going on as well?
Frank

Hello,

I did assume results were from actual measurement, not simulations..... Did you do your gain simulation for the LNA alone, or with the mixer as load?

In real measurements, attenuators can be used to show specified source or load impedance to a circuit when the actual source or load is not known, or not what you want. Off course they are also used to attenuate signals to avoid overload to measuring instruments and circuit. If you change attenuation and the quality of the attenuators is no good, you can get gain steps (like in your situation).

I don't know your circuit exactly, so I cannot say whether 772 Ohms is something to expect or not. It seems high to me (given the 2 GHz). I don't know RF Spectre, but you may experiment with accuracy settings. In your case it looks like a simulation accuracy issue. Are you sure you did correct voltage to dB conversion (in case of manual calculation)?

I hope that some Spectre experts will reply.

It seems you are right. I adjusted some of accuracy setting and my variation decreases from2.4db up to 1db but I don't know whole of the primary accuracy setting adjustments of rf spectre so I'm not sure all of variations and problems is from the accuracy setting.my mixer has this problem also. but in the case of mixer it's too vigorously.I have to say that my mixer have no problem in hspice.in rf spectre it's gain changes with both of if frequency and input amplitude.it's ip3 so far has reached up to 1.5dbm.