Abstract

This page will be used to summarize all the left over issues on the IF board's design side. We will continuously update the progress/decision on current issue and new issues in future.

Why

Short version of the issues and solutions:

1. IF splitter. (Frequency range doesn't match)

•    The problem:
◦    We will use one IF band splitter each board to split VCO output to drive two IQ mixers‘LO port. Our IQ mixer (IQ0255-M from Marki) is working from 2GHz to 5.5 GHz. We want the splitter to work in the same range. But none of the surface mounted splitter we found meets exactly the frequency range: e.g. there is GP2Y1+ (1.5GHz to 4.4 GHz) and GP2X1+ (2.8GHz -7.2GHz).

•    Suggested action:
◦    Based on the data tested by Mini-circuit for splitter out of working range performance: For GP2Y1+, it designed to work at 1.5 GHz to 4.4 GHz. And up to 5.5 GHz, the loss is within 5.5 dB; For GP2X1+, it designed to work at 2.8 GHz to 7.2 GHz, but at 2Ghz, loss is upto 12 dB which is too high. I think we should order GP2Y1+
2. Amplifier from VCO to IQ mixer LO port. (amp reach power compression)

•    The problem:
    Our passive IQ mixer require 13dBm – 16 dBm power level for the LO port. The VCO only output -4dBm to 5dBm, and there is at least 7.2 dB loss due to 2 switches and splitter. We currently use one HMC313 (gain 17 dB) just before the mixer to amplify the signal. For the min 13 dBm requirement of LO port on mixer, HMC313 must has >=15.2 dB gain when input power is -2.2dBm. This input power is already in the compression range, and makes HMC313 may not has upto 17 dB gain as labeled: for input power of -2dBm, most likely we will get 14 dBm at output at 3 GHz. And could be worse for higher/lower frequency, which will make us not be able to maintain 13dBm at IQ mixer’s LO. And there will be other issues like distortion (output third order intercept (IP3)) when running close to the amplifier compression point.

•    Suggested action:
    Use 2 amplifiers to do this: I choose an amplifier with high output P1dB: 22dBm from minicircuit PSA-0012+ or PHA-1+, but with relatively lower gain: average gain is 12.5 dB or 11.5 dB. And if we output 15dBm to LO, we are 7 dB away from 1 dB compression. In this case, we will have to combine that amplifier with a HMC311 (gain 14.5 dB) amplifier.
3.Talk with Ferenc Marki about IQ mixers. (Power compression & distortion)

•    The problem:
1.    We want to know more about the power compression for IQ mixer from Marki. However, They didn’t study the performance of IQ mixer with different input power (they just have input 1dB compression label for IQ0255L is 4 dBm, for IQ0255M is 6 dBm). We can use their input 2 tone 3rd order interception point (IIP3) to have a general idea about the level of inter-modulation distortion(IMD)or harmonics. And when input power close to compression, its performance going to be worse than that (but they don’t has any data on this). If the IMD/IIP3 is a very big issue, they can provide even higher power version of IQ0255. E.g. IQ0255S, will give much better P1dB or IP3 performance, but require much higher LO power (20dBm) 
For IQ mixer’s LO port: IQ0255L require 10-13 dBm; IQ0255M require 13-16 dBm. It doesn’t matter what the power level of LO as long as it is in that range. Higher LO power won’t hurt, but lower will give bad performance. 
And how clear the LO signal (in terms of harmonic or IMD) will matter for the performance. In our case, max input power for IQ0255 is -2.8 dBm: 8.8 dB away from 1dB compression of IQ0255M

•    No action required

4. Baseband switch

•    The problem
    The ADG902 is baseband switch. The place we need them are: DAC output I; DAC output Q; ADC input I; ADC input Q; Clock output (to choose from onboard or off board clock). ADG902 is just a on and off switch, we need 2 for each place above to make it be able to choose from A to B or from A to C with good isolation. So we need total 10 of them per board. We have trouble to find ADG902-LFCSP, they are either no longer available or require large amount of min order from AD or Digikey etc.

•    Suggested actions 
There are two alternative options:
    Can we switch the package for ADG902 to MSOP, if it is ok. Those part are in stock and can be ordered in small amount.
    If LFCSP is required, there is some alternative part, e.g. ADG918/919, they are in stock with same price (~$1.25 each) and ok for small amount. Actually ADG918/919 has Single Pole, Double Throw, Could this save us half the switch needed? Is there any drawback for our application to use this part?

How

Details of the above problem and solution: Summary_of_leftover_issue_about_IF_board_design_v2.pdf
Related datasheet: related_datasheet_IFdesign.zip