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20100305

MAX2611 LNA test circuit

Calibration: HP 8782A set to 150 MHz, -60 dBm Connected to FHS3, reading of -59.5 dBm

FHS3 set to 150 MHz +/- 5 KHz, RBW 1 kHz, VBW 10 Hz

• Dataset.000: Noise floor is -110 dBm when signal generator and amplifier are off.

• Dataset.001: Noise increases to -97 dBm at 150 MHz when LNA is switched on. Signal generator is off.

• Dataset.002: Signal generator set to -80 dBm. FHS3 reads -73.3 dBm.

• Dataset.003: Signal generator set to -60 dBm. FHS3 reads -53.4 dBm.

• Dataset.004: Signal generator set to -40 dBm. FHS3 reads -33.4 dBm.

• Dataset.005: Signal generator set to -30 dBm. FHS3 reads -23.2 dBm. Noise floor seems to have increased a lot as well, at least in the 5 KHz span.

• Dataset.006: FHS3 span changed to 3 MHz. Signal generator -60 dBm. FHS reads -53.4 dBm, noise around -82 dBm.

• Dataset.007: Signal generator -30 dBm. FHS reads -23.4 dBm, noise still around -82 dBm.

Note, these tests were performed without the optional RF choke - I didn't have an SMD part, so in the preceding tests I replaced it with a wire link.

Try an enormous 60 uH radial RF choke. Wire it up incorrectly (because it's so big and I can't see what I'm doing), and the MAX2611 starts smelling terrible.

But still works!

• Dataset.008: Signal generator set to -60 dBm. FHS3 reads -51.8 dBm. A slight gain improvement!

Check ID current. 1.382/56 = 24.67 mA. Should be a gain of 22 dB, according to the datasheet!

Try some other frequencies.

No, maximum response seems to be around 145 MHz. where I get -48.8 dBm - a gain of 11.2 dB.

Check the signal at the input to the LNA to see how much insertion loss is in the filter.

• Signal generator -60 dBm, FHS3 reads -67.5 dBm
• Signal generator -40 dBm, FHS3 reads -49.2 dBm

So the filter is losing about 8 dB. The gain of the amplifier must then be about 14.5 dB.

Will replace the MAX2611 with a new part, but will investigate why the filter is so bad first.