MARK 5 MEMO #029
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
HAYSTACK OBSERVATORY
WESTFORD, MASSACHUSETTS 01886
28 August 2006
Telephone:
Fax:
To:
Mark 5 Development Group
From:
Alan E.E. Rogers
781-981-5407
781-981-0590
Subject: DBE performance vs number of FIR and interpolator taps.
The effect of reducing the number of taps in the FIR filter of the polyphase filter bank is
primarily a reduction in the useable bandwidth of each channel as a result of the degraded shape
factor. On the other hand reducing the number of taps in the interpolator Reduced the inband
image rejection. In memo #18 Hans and I suggested 16 taps per sub filter for the PFB and 16
taps per sub filter for the PFB and 16 taps for the interpolator. Figure 1 shows the performance
of the PFB in “mode 3” with 15 32 MHz channels. (Channel 0 is lost due to aliasing – see memo
#18). The colors indicate the filter function and the black show the strength of the aliased
images. The following table lists the performance as a function of the number of taps.
#PFB taps
16
8
8
8
4
2
# Interpol taps
16
16
8
4
4
2
Correlation loss %
0.5
1.3
1.5
3
5
30
The overall sensitivity loss of the PFB is the sum of the correlation loss from the table above plus
the loss of 1 out of 16 channels (3.2%) plus a small added loss due to the filter shape function
which narrows the effective bandwidth and results in another approximately 1%.
-1-
8/28/2006
0
-10
-20
dB
-30
-40
-50
-60
8
24 40 56 72 88 104 120 136 152 168 184 200 216 232 248 264 280 296 312 328 344 360 376 392 408 424 440 456 472 488 504
freq (MHz)
Fig 1. PFB with 16 taps per PFB filter and 16 taps per interpolation filter.
0
-10
-20
dB
-30
-40
-50
-60
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70
freq (MHz)
Fig 2. Expanded view of PFB response shown in Fig 1.
-2-
8/28/2006
0
-10
-20
dB
-30
-40
-50
-60
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70
freq (MHz)
Fig 3. Expanded view of PFB response with 8 taps per PFB subfilter and 8 taps per interpolation
filter.
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8/28/2006