IF amp filter and gain

[jordens]

• This should read "0 dBm ..."
  

• At the sum mixer product (of >20 MHz) it's OK and beneficial to use very low impedance. That makes the mixer happy and also helps with suppressing recification in the opamp. Let's make a 10 MHz Chebyshev pi-filter:

  • DNP R22A,B
  • R62A,B = 49R4
  • R63A,B = 0R
  • C27A,B, C28A,B around 560 pF
  • L8A,B around 1 µH

• IMO the IF amp gain should be increased from 3 to 10 for all use cases:

  • Only the double-saturated use case of the mixer will get anywhere near 0 dBm output. All other use cases will need to stay significantly below (like -6 dBm).
  • We have the in-amp on Stabilizer that can cope fine with +-3V. Using low gain there also makes it faster and less noisy overall.

[jordens]

We could also consider a proper anti-aliasing lowpass here.

[jordens]

And I would like to consider a better noise match to the 50R mixer IF impedance. The FET opas are cool for high impedance sources but that's not what we have here. DC offset isn't a big problem since it's typically comparable to the mixer DC offset (~0.5 mV) and its temperature coefficient. Rectification to me seems to be the same effect as what the mixer itself would be limited by.

[gkasprow]

@jordens where would you put such LC anti-alias filter? Shall I add more LC sections before the opamp? I can add another placeholder for LC section.

[gkasprow]

In terms of an opamp, what about ADA4896/7

[jordens]

Let's go for this filter:

• Plase that filter between mixer IF and opamp. The only resistor should be a RL = 50 R termination but after the filter. Keep C60 at the opamp input. R21=0R. Remove R9 traces (potential for annoying positive feedback). Make the opamp with gain=20 (to also lift it over the INA noise which is best case 18 nV/rtHz), cap its gain to 1 MHz.

• Make sure the mixer sum products (5-500 MHz) see low impedance (< 50R, can be capacitive). Shunt-first Chebychev helps here, but let's put a 33 pF in parallel to that first big shunt C.
• ADA4896/7 looks good to me in gain=20

[gkasprow]

I got rid of the notch filter and other options to make space

[jordens]

Perfect