A bunch of tests conducted on the ADRF6850
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