ben-wpan/usrp/sps/20110306

Objective:

Reproduce sidebands observed in 200110303 and look for ways to reduce them.
In particular, examine the effect of the antenna tuning capacitor C16.


Test setup / changes:

4	original board, like in 20110303 (noise from WLAN I forgot to turn off)
4a	reflowed some RF vias; cleaning
4b	removed large solder deposits on RF; not cleaned
4c	cleaning after 4b
4d	0.5 pF at C16; cleaning
4e	1.0 pF at C16; cleaning
4f	C16 NC; cleaning
4g	resolder pin B1.1; cleaning
4h	replace cracked B1; switched to leaded solder; cleaning
4i	reflowed B1 at higher temperature; cleaning
4j	reflowed C8's balun side; cleaning

3	original board, like in 20110303
3a	wait 2 h (warm-up; USRP2 freshly booted)
3b	cycle (unplug/plug) atusb
3c	retract shutters (metal)

-------------------------------------------------------------------------------

Observations:

4	behaves as it did in 20110303
4a	no change
4b	no change
4c	no change
4d	no change
4e	no change
4f	major regression
4g	regression persists
4h	regression persists (slight recovery seen in 4h-0 may be due to
	temporary effects)
4i	regression persists
4j	regression persists; giving up

3	behaves as it did in 20110303; significantly cleaner
3a	sidebands are reduced
3b	sideband reduction persists
3c	~10 dB drop at lower end of spectrum

-------------------------------------------------------------------------------

Conclusions:

- build quality seems to be principal factor in RF performance variations
- a warmed up board performs slightly better than a cold board
- C16 has no noticeable effect
- reflection characteristics even of distant items in the environment have
  a strong influence on relative signal strength