What is the lower frequency limit for the IF output of ADRF6650, and can the mixers and IF chain handle DC?

Question

Accepted Answer

The output circuit cannot operate down to dc. Pull-up inductors are required at the output of the fixed gain amplifier that comes after the mixers. These are pins 15, 16, 55 and 56 (note that there is a typo in the datasheet; these pins should be referred to as "MIX_PU…" but in some instances, they are referred to as “MUX_PU…”). The size of these inductors is going to frequency limit low frequency operation. A value of 220nH is mentioned. At 50 MHz, this has an impedance of 70 ohms which is high enough for a 50 MHz signal to see a high impedance when looking into the inductor (this is what you want). If you want to operate at lower IF outputs, a larger inductor size will be needed. This should help to reduce the gain roll-off vs IF that you see in Figures 7 and 8. But you cannot go all the way down to dc. You can dc couple the output of the IF DGA (pins 18, 19, 52 and 53). This is discussed in the datasheet.

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ADRF6650 : FAQ