Problems with AGAF DATV board set

[WA8RMC]

I have been experimenting with the AGAF boards and have a number of possible problems and questions.

1. The MPEG encoder board seems very susceptible to power supply noise. My power source is a 13.5 volt switching supply with about 5 to 10 millivolts of ripple. Even with that, I found it necessary to source each PCB power supply and ground lead directly from the supply. Connecting the MPEG encoder supply to the transmitter supply and then to the power supply resulted in system lockups. Bypassing each supply at the MPEG board and transmitter board connectors improved things. I used 47MF tantalum caps. If I isolate the MPEG board power with another supply, it goes away. (The issue is NOT coming from the supply itself). I do not have schematics so I cannot troubleshoot farther but the transmitter board does have supply glitches at power up so it seems like the problem starts there.

2. The MPEG board seems to lock up on power up about half of the time. I usually have to remove power from the MPEG board till the other boards stabilize then connect the power to the MPEG board. That usually starts normal operation. Unless I find the cause of this, I will have to include a "reset" button that interrupts power to the MPEG board momentarily after everything else powers up. This problem may be related to the noise issue above.

3. I have the system output on 1260 MHz at a 4167 symbol rate. Only one of my two different receivers will recognize the signal. One of the receivers reports a 95% signal strength and 100% signal quality but a blank video screen. My other receiver captures the signal video and sound immediately. Until I identified this, I couldn't tell where the problem was, transmitter or receiver. Any idea as to what I can try so the "malfunctioning" receiver will receive the signal? Also, sometimes when I interrupt the signal by disconnecting the antenna, the "operational" receiver reports "low signal" until I have it re-scan the inputs. Then it locks onto the signal with 95% signal strength.

4. Is there any way to reduce the symbol rate below 4167? 4167 is too high because it requires a wide bandwidth. I have another board set running at 3125 with 2 channels of video and occupies about 4 MHz of spectrum. The AGAF boards take up about 5 MHz. The other board set (D-ATV boards from the Netherlands) have been running continuously at our repeater for about 3 years now with no interruptions).

5. Is there new software available for the AGAF boards? If so, how can I download it? I assume a JTAG port is available for this but it is not identified.

6. I viewed the 23 cm signal output with a spectrum analyzer. I notice sidebands 16 MHz above and below the main signal with the same flat top as the main signal but about 45 dB below the main carrier. Is this normal? If it is, I may have problems for our receive frequency is 1280 MHz and within the passband of the upper sideband. A filter in the transmitter output would be necessary but then linearity may be a problem due to filter characteristics.

7. The existing board set operates on 1260 MHz. I plan to change the crystal to operate on 1280 MHz. I assume re-tuning of the coils in the up converter will be required. Can I just re peak them for maximum output or is there a special sequence needed?


That is all for now. I have a power amp ready to test with the system when everything else is resolved but only one problem at a time. The power amp will output about 10 to 20 watts with a 10 mw input so I'm anxious to do linearity testing to find out just how much non linearity can be tolerated for a given receiver.