A low noise and good input matching VGA design

[pursue]

I want to design a differential VGA, the input differential resistance is 100 Ohm, and the NF is below 6dB, the gain step is 0.5dB, the gain range is from -10dB to 50dB, the output Vpp is 2V, also the P1dB is large enough too, the bandwidth is 50MHz-100MHz with 1dB gain variation, is that possible?

I read some papers, also saw some products, I think, may be three or four gain stage must be used, every stage is consisted by wideband operational amplifier with feedback, the first stage may be coarse tuned stage, for example, -10dB, 0dB, 10dB, 20dB,30dB,40dB,50dB, the later stage is 0.5dB fine tuning step with 10dB gain range.Can someone give some good advice, thanks.

1、The circuit is simulated in Cadence, but I am confused with the simulation of the specification.
2、If I want to see the input matching performance, the SP must be used to see the S11, right? but the circuit is differential, when the Balun is used in the input port, the input resistance is differential 100 Ohm, that is the differential input port is a 100 Ohm resistor, but the input resistance of the later feedback OP also have effect with the resistance, how can I simulate the input matching in the case.
3、As the input of the circuit have some resistors, the noise may be bigger than 6dB, is there any good advice with the low NF?
In the OP simulation, the AC characteristic is used to see the gain with the input resistance is infinity. Here, as the input resistance must be 100 Ohm differential matching, if I want to see the gain, how to set the input port, the psin port, or just the port? the AC, or SP, or some other characteristic will be seen.
4、The coarse gain stage and the fine gain stage must be used, but the bandwidth will be decreased as too much gain stage is used, what can I do if the bandwidth is 50MHz-100MHz with 1dB gain variation.

Can somebody help me, thank you all~

 

[BigBoss]

First of all I should say that Gain step in dB on a VGA is not a easy task.Because gain is not linear by changing current or something in VGA.
Second thing, a 0.5dB gain increments can be obtained by using a attenuator on inter-stage so NF won't be affected too much. ( This is a regular design for almost all manufacturer ) but I'm not sure that you can catch 60dB dynamic range.You say it should have 2Vpp output swing over on which impedance ???If this impedance 50 Ohm ( single ended ) , the output power will be +13dBm@P1dB compression point ( roughly ).This is not a easy task for especially non experienced engineers...

 

[vfone]

Most of the high-linearity VGAs use an integrated step attenuator in front of a fixed gain amplifier, as for example LMH6521 from TI:

https://www.ti.com/lit/ds/symlink/lmh6521.pdf

Advantage using a step attenuator in front of the fixed gain amplifier is not only to keep good linearity, but also to improve input/output match, and the gain flatness vs frequency.
Unfortunately in VGA case there is nothing to improve noise figure, other than using low noise figure active components. Generally VGA's are placed after the LNA and mixer, and its noise figure doesn't affect too much the system noise figure.

 

[pursue]

Most of the high-linearity VGAs use an integrated step attenuator in front of a fixed gain amplifier, as for example LMH6521 from TI:

https://www.ti.com/lit/ds/symlink/lmh6521.pdf

Advantage using a step attenuator in front of the fixed gain amplifier is not only to keep good linearity, but also to improve input/output match, and the gain flatness vs frequency.
Unfortunately in VGA case there is nothing to improve noise figure, other than using low noise figure active components. Generally VGA's are placed after the LNA and mixer, and its noise figure doesn't affect too much the system noise figure.

This product give a gain range of 31.5dB, it uses a step attenuator in front of a fixed amplifier, why does this structure achieve 60dB gain range, what's the difficulty about that. how to guarantee a good input match and a good NF equivalently. thanks.

 

[D.A.(Tony)Stewart]

For a 2Vpp output or 66 dBmV and a gain of 50dB with a minimum high video signal of 50dB SNR or a noise floor of -34 dBmV.... or 20uVpp with 100MHz BW is possible but unrealistic requirements . WHat is your application?

Making a wideband ultra-linear 50 Ohm usually employs differential cascode video amplifiers with variable gain using current mirror controlled emitter current.

Usually sync pulses can be used for AGC so you don't need step size gain rather active clamped and agc using the sync pulse height is for calibrating the gain.

What is your application?