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AN2463 Application note
STw8009 Spectrum of CVBS video output
Introduction
This document shows the action of the video output analog filter (330pF/2.2uH/330pF, symetrically loaded with 75R), with a cut-off frequency close to 10 MHz. For both PAL and NTSC video standards, spectrum has been captured with and without filter. The video source is the chosen internal color bar pattern of the STw8009, in order to be compared at customer level if required. Figure 1. Internal color bar pattern of the STw8009
This typical frame, shown in Figure 1, contains a maximum of energy around the color burst frequency for a given standard (4.43 MHz in PAL, 3.57MHz in NTSC). The 27 MHz sampled system creates energy around this frequency with two bands that are image of video baseband, with intrinsic sin(x)/x filtering. As energy around sampling frequency can be up to 31 MHz (27 MHz + video band), the upper limit of frequency analysis is rated to 40 MHz. Assuming that no TV set is able to handle a signal that is more than five times the upper limit of video band, there is always an anti-aliasing filter very close to the video input of every digital TV set, on the other side, analog TV sets featuring limited frequency band, so in any case, from an electrical point of view, the analysis limit of 40 MHz is suitable.
Reference documents
STw8009/STw8019 datasheet AN2347 - Getting started with STw8009 and STw8019 AN2462 - STw8009/STw8019 PCB guidelines
March 2007
Rev 1
1 /5
www.st.com
Setup description
AN2463
1
Setup description
Tests have been performed on the STw8009 application demo board (see AN2462), driven by the ST ARM DBB control board for I2C commands. Results have been recorded on an R&S FSU spectrum analyzer. As this device is dedicated mainly to RF (using 50R input impedance), a 25R serial resistor has been added, in order to have 75R real load for STw8009 output. The DBm values given are not absolute, they are as measured on 50R across 75R load. The captured screen copies show how the remaining energy level is relative to the in-band signal with and without low-pass filter.
1.1
Selected video standard PAL BDGHI
Figure 2. Green trace and blue trace schematic diagram
* RBW 10 kHz * VBW 10 MHz
Ref
10
10 dBm
Att
35 dB
SWT 400 ms
Marker 2 [T1 ] -77.75 dBm 27.051282051 MHz Marker 1 [T1 ] -19.88 dBm 4.423076923 MHz
*
A
0 1 RM * VIEW
-10
1
-20 3 RM * VIEW
-30 PRN -40
SWP
-50
30 of
30
-60
-70
2
-80
-90
Center
20 MHz
4 MHz/
Span
40 MHz
Date:
7.NOV.2006
16:37:06
2/5
AN2463
Setup description The maximum in-band energy is around PAL color burst frequency (4.43 MHz); out-band energy around sample frequency is around 27 +/- 4.43 MHz and theoretically reproduced and sin(x)/x attenuated every multiple of 27 MHz.
Blue trace is CVBS video output spectrum with low-pass filter. 27 MHz is no more detected; remaining image energy is still present, but more than 40 dB attenuated. Green trace shows spectrum without filter.
Conclusion: the filtering effect is to reduce unwanted energy by at least 30 dB.
1.2
Selected video standard NTSC M
Figure 3. Black trace and green trace schematic diagram
* RBW 10 kHz * VBW 10 MHz
Ref
10
10 dBm
Att
35 dB
SWT 400 ms
Marker 2 [T2 ] -77.97 dBm 27.051282051 MHz Marker 1 [T2 ] -19.74 dBm 3.589743590 MHz
*
A
0
-10 2 RM * VIEW 3 RM * VIEW
1
-20
-30 PRN -40
SWP
-50
30 of
30
-60
-70
2
-80
-90
Center
20 MHz
4 MHz/
Span
40 MHz
Date:
7.NOV.2006
16:40:48
Maximum in-band energy is around NTSC color burst frequency (3.57 MHz); out-band energy around sample frequency is around 27 +/- 3.57 MHz, and theoretically reproduced and sin(x)/x attenuated every multiple of 27 MHz. Black trace: CVBS video output spectrum with low-pass filter. 27 MHz is no more detected; remaining image energy is still present, but more than 40 dB attenuated. Green trace shows spectrum without filter and gives the same conclusion as above: the filtering effect is to reduce unwanted energy by at least 30 dB.
3/5
Revision history
AN2463
2
Revision history
Table 1.
Date 21-Mar-2007
Document revision history
Revision 1 Initial release. Changes
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AN2463
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