I am working on a simple digital radio receiver and am confused by the difference between the LO to IF leakage vs. the 1LO - 0RF spur. These two tables in the datasheet of the Analog Devices HMC218B passive mixer are the source of my confusion.
The "Spurious Outputs" table lists the 1LO-0RF as being 3.4 dBc. Working from the test conditions at the bottom of the table, this means that the mixer would have an output spur at a frequency of 5.25 GHz 3.4 dBc below the power of the desired IF output, in this case 100 MHz. If the conversion loss of the mixer is 7 db (as is specified in the 2nd "Electrical Specifications" table), then for a RF power of -10 dBm the IF power at 100 MHz should be -17 dBm, so the power of the 1LO-0RF spur at 5.25 GHz should be -20.4 dBm.
If the LO to IF isolation given in the "Electrical Specifications" table is used to find the power of the LO signal at the IF port, one finds that the power of this unwanted LO at the IF port is -19 dBm. This calculation uses the specified LO to IF isolation from 4-6 GHz to be consistent with the frequencies used in the "Spurious Outputs" table.
The 1.4 dBm difference in 5.25GHz power at the IF port given by the "Spurious Outputs" table and the "Electrical Specifications" table is minor, but in my case the input RF power is around -50 dBm (@ 5.8GHz), not the -10 dBm that is used in these tables. This decrease in RF power significantly changes the calculated LO power at IF depending on which table is used to find the LO at IF power. For example, if the "Spurious Outputs" table is used in conjunction with the conversion loss from 4-6 GHz in the "Electrical Characteristics table" I find that the 1LO-0RF spur should be -60.4 dBm, 3.4 dBc below the -57 dBm IF output at the desired IF. However, if I use an LO power of 13 dBm (the LO power used throughout the datasheet), and I calculate the LO power at IF using the LO to IF isolation given in the "Electrical Specifications" Table, I find that the LO power at IF would be -19 dBm. Which method should I used to find the LO power at IF? Is there a fundamental difference between LO to IF isolation and the 1LO-0RF spur? Perhaps my understanding of dBc is wrong and is throwing off my calculations.
Thanks for your help.