Question about measuring RF IC mixer
I have a question regarding measuring an RF front end IC. It includes an LNA and a Mixer both differential input and output. The LNA input is matched to 100 ohms (differentially).
The initial design was aiming at designing the VGA too, thus the output matching of the mixer wasn't taken into consideration. But due to some issues, only the LNA and mixer were fabricated. Creating the PCB, it was needed to have a balun at input (2.14 GHz) transforming from 50 to 100 and it is present from many companies (my choice is TDK). The problem comes from the mixer output (0-60 MHz, Direct conversion receiver). The mixer gives reasonable gain (4 dB) at high resistances (5-6 kohms) and when terminated by (100-300 ohms differentiallY) the gain drops to lower than -7 dB.
I thought of using a buffer to transform from this high impedance and in same time to transform to single ended (buffer + balun).
My questions are:
1- does the buffer as an idea works in that case ? does it degrade linearity, NF or Gain of whole receiver ? (of course it will affect linearity as it is the last stage, how to avoid that) ?
2- Can anyone recommends an off-shelf buffer IC for that purpose ?
3- What I saw in different electronics retailers websites, that there are a lot of buffers that their data sheets say "It can drive current xx at 75 ohms " ? does this mean it will be matched to 75 ohms or 50 ohms or wha t? I will use spectrum analyzer with output impedance of 50 ohms so matching is really important to me.
thanks a lot for your help in advance.
Regards,
Mostafa
1. If I were you, I would do some link budget and calculation to see the influence of adding a buffer.
2. You should give your system specifications.
3. Yes, that's matched to 75 ohms, but there are many buffers matched to 50 also.
I have used the AD830 some time ago for smaller bandwidth receiver, it can be configured as differential-to single ended buffer. You can try something similar with wider bandwidth to cover the whole 60 MHz although it wont be easy to find a good part.
To avoid limitation on receiver linearity, you should choose a part with some margin (reasonable margin is 10 dB) on IP3 respect to the target OIP3 of your receiver, gain=1 and NF usually not an issue (but also for this parameter you should check it).
These buffers has a low output impedance, so you can configure it with gain=2 and use a series 50 Ohm resistor at buffer output. You will have a total gain=1 and output impedance 50 Ohm.
I hope it can help.
Mazz