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20110822

A bunch of tests conducted on the ADRF6850

Test setup:

R&S 100 - 1000 MHz signal generator
R&S FSH3 3 GHz spectrum analyser
ADRF6850 evaluation board
PIC microcontroller used as SPI configuration programmer
Centre-tapped transformer converts differential output to single ended for FSH3

jw1 - LO 140 MHz, LOMON off, VGAIN = 60dB, NO INPUT

Heaps of noise is present at the output when gain is as high as it will go!

jw2 - LO 140 MHz, LOMON off, VGAIN = 0dB, NO INPUT

Noise is greatly reduced when gain is at the minimum. Some remains at 5 and 10 MHz.

jw3 - LO 140 MHz, LOMON off, VGAIN ~ 30dB, input - 146.7 MHz @ -70 dBm

Gain is set to an intermediate level. 5 and 10 MHz spurs are visible.

Spurs: 5 MHz = -60 dB (6 dB above noise floor) 10 MHz = -61.8 dB (4 dB above noise floor)

jw4 - LO 140 MHz, LOMON off, VGAIN ~ 60dB, input - 146.7 MHz @ -70 dBm

Gain set to maximum. Signal is visible, but there are lots of spurs, as much as -31.5dBm @ 5 MHz

Conclusion - probably best to set gain to a more conservative level e.g. 40 dB - the data sheet claims that minimum noise figure is achieved at gain = 40 dB.

jw5 - LO 140 MHz, LOMON off, VGAIN ~ 40dB, NO INPUT

Gain set to 40 dB. Some spurs are visible.

jw6 - LO 140 MHz, LOMON off, VGAIN ~ 40dB, 146.7 MHz @ -70 dBm

RF input is switched on. Spurs are unchanged.

Let's investigate the effects of a different LO frequency, e.g. 145 MHz

 Code:
 [0xd4 0x07 0x00]
 [0xd4 0x06 0x2a]
 [0xd4 0x03 0x01]
 [0xd4 0x02 0xed]
 [0xd4 0x01 0x09]
 [0xd4 0x00 0x7b]

jw7 - LO 145 MHz, LOMON off, VGAIN ~ 40dB, NO INPUT

Spurs have shifted somewhat, though 10 MHz remains.

jw8 - LO 145 MHz, LOMON off, VGAIN ~ 40dB, 146.7 MHz @ -70 dBm

With RF input (now downmixed to 1.70 MHz)

Need to calculate if these are below the detection threshold of the ADC:

With 1V input range, maximum signal = 1 Vpp = 13 dBm

With ENOB = 14, ADC has a resolution of 86 dB - therefore minimum signal is -73 dBm (though an additional 20 dB gain stage is used)

With ENOB = 12, resolution is 74 dB, giving minimum signal of -61 dBm

(using http://www.maxim-ic.com/tools/calculators/index.cfm/calc_id/maxim_sinad )