[USRP-users] N210 DSP Group delay

Ian Buckley ianb at ionconcepts.com
Fri Dec 5 19:13:38 EST 2014


Over the years a lot of people have asked for a figure for the group delay of N210.
I finally had a reason to spend a few hours today putting together a formula to calculate the digital portion of it.


Glossary:
DecCIC - Decimation rate across the CIC filter
DecHB1- Decimation rate across the small Half Band FIR (HB1)
DecHB2 - Decimation rate across the large Half Band FIR (HB2)

For the N210 Rx path (Tx will be similar) ADC -> Packetization, sum the following:

Input register: 1
DC Offset Correction: 1
IQ Balance: 2
IQ Swap: 1
CORDIC: 21
Clip: 1
CIC Filter: 5+6*DecCIC
HB1: 7+6*DecCIC
HB2: 9+8*(DecCIC*DecHB1)
Clipping and Gain: 3

This is the number of 100MHz (10nS) clock cycles used in the DSP pipeline before packets are timestamped and packetized.

For example if you had configured N210 to produce a 1MHz sample stream, thats Decimation=100 which maps to : 
DecCIC=25, DecHB1=2, DecHB2=2. This makes group delay ~11.51uS

Another example. If you configured N210 to produce a 25<Hz sample stream, thats Decimation = 4 which maps to:
DecCIC=1, DecHB1=2, DecHB2=2. This makes group delay ~790nS.

You can buy me a beer at GRC2015 to say thanks….(Note: if if there is an error in the formula, I won't be buying reciprocal beer!)
-Ian





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