Re: VCXO measurements

On Sun, May 09, 2010 at 04:29:06PM +0200, Christian Vogel wrote: thanks a lot! You should consider to use the 13MHz signal. it's buffered and a simple divide-by-two of the 26MHz oscillator output. Anyway, from your data I would say: At DAC=500 we have 26 MHz At interpolated DAC=2750 we have 500Hz more So this means the voltage control slope is 500/2750 = 0.1818 Hz / LSB Converting this into ppb: 0.1818 /.026 = 6.993 ppb / LSB The TSM30-based algorithm seems to assume 4.9 ppb / LSB, and for reasons I don't remember we currently seem to use 9.76ppb/LSB (AFC_SLOPE in layer1/afc.c) Of course this value is highly dependent on the particular XO and its peripheral parts, so I'm not surprised to see that much difference. Based on this, the ideal value for AFC_SLOPE according to the measurements should be 229. I haven't tested that, this is just my thinking: 0.006993 * 32768 = 229 Also, to give you some idea what this means in terms of "Hz carrier frequency / LSB": Center of GSM 900 between UL and DL: (960+880)/2 = 920 MHz overall multiplication factor for mid-band GSM900: 35.38 resulting theoretical slope: 6.43 Hz / LSB Center of GSM 1800 between UL and DL: (1710+1880)/2 = 1795 MHz overall multiplication factor for mid-band GSM900: 69.04 resulting theoretical slope: 12.55 Hz / LSB Regards, Harald -- - Harald Welte <laforge(a)gnumonks.org> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)