What is the low end of the RF input and output of the AD9371 when using an external LO? What is the physical limitation on the low end with external LO?
AD9371 can have external LO from 600 MHz to 12 GHz for an RF frequency range of 300 MHz to 6 GHz.
The transceiver consists of wideband direct conversion signal paths. TX & RX Center Frequency is 300 to 6000 MHz; with TX & RX Bandwidth (BW) up to 100 MHz. The data sheet specifies performance with 700 to 5500 MHz LO.
How is the 600 MHz to 12 GHz LO used? Is there an internal frequency halfer?
You are correct. The externally applied LO signal gets divided by 2 to generate I and Q LO signals.
Steve (and I) are trying to find out what the real limitation is, not what the datasheet is specified for. We did RT%M.
I don't think there is a compelling reason to use capacitive coupling on-die on the Rx chains before the mixer, since they are AC coupled in, but I'm no ASIC guru.
I'm pretty sure there is no reason to AC couple the LO on die. It's already AC coupled off die for 50% duty cycle, and it's 2X the frequency needed, so making I and Q LOs to run FET gates shouldn't use AC coupling on die.
The TX chains have some matching off die, but I don't know if they would use AC coupling on die.
Something at RF could be coupled using an on-die cap, but we have no way of knowing with out some more info from ADI.
The I and Q baseband channels should work the same, no matter what the LO frequency is, apart from RF/LO possibly leaking into the baseband chains.
So, the question is, what limits the lower working range of the chip?
A. capacitive coupling on-die, with small caps that won't work below 600MHz or 300MHz
B. Poor and uncorrectable or unpredicatble LO I/Q phase imbalance at low frequencies
C. Insufficient isolation between the RF / LO and the baseband signal chains on die.
D. Something else?
We're hoping to use the AD9371 for a family of products, with one application falling below the 300MHz cutoff.
If anyone in the know from ADI could chime in (private message is fine), we would really appreciate it.
Not looking for you to solve our problems (We can wring this out using a dev kit), just looking for some advice to see if it is somewhat feasible to use the 9371 below the lower frequency.
The limitation is coming from the signal path. We do not guarantee performance below the datasheet spec for process variations.
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