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Showing posts from September, 2016

Duplexer DVB-T TX filter: Low bandwidths

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Duplexer DVB-T TX filter; Low bandwidths: 2 and 1 MHz.
Introduction The duplexer DVB-T TX filter was adjusted to see if it would work at low bandwidths, specifically 2 and 1 MHz at 70 cm. It achieved this easily, but with a small increase in losses.
Testing The duplexer was original adjusted for a 7 MHz bandwidth with a spectrum analyser and tracking generator. I only moved the lower frequency notch, one side of the duplexer.




At the request of a USA operator, wanting to work DX, I readjusted the filter for a 2 MHz bandwidth. This was possible, with with little effect on losses.



For interest, I adjusted it to a 1 MHz bandwidth, again possible, but with slight losses.


Discussion The filter works surprising well at the lower bandwidths. I thought losses may have been higher.

The losses are not a major issue as the DVB-T amplifier can be driven a little harder to make up for them, and possible even more because of the filter.

Notch cavity filters could be used, in principle, for DVB-T in…

Testing a duplexer as a DVB-T TX filter-Wow!

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Testing a cheap Chinese duplexer as a 70cm DATV DVB-T 7 MHz TX filter- Wow! Introduction In my last post I described a cheap Chinese duplexer re-tuned as 70 cm DATV DVB-T 7 MHz TX filter. The duplexer uses notch cavity filters, six in all. The notch filters have a much sharper edge, compared to a band-pass filter. The sharp notch seems suited to the vertical edges of a DVB-T signal.

I initially check the signal source, a HiDes camera with direct DVB-T output at 1080P. I was a little surprised at the spread, but the filter cleaned it up well. This would indicate the need for a filter before the main power amplifier.

I pressed on with just one filter and tried it at the output of the amplifier, a 10 W device, from Darko OE7DBH, using a RA60H4047M1 60 W module. Even with the indifferent input, the filter was able to reduce the spread to -60 dB and give a clean 10 W output.

The notch duplexer/filter seems to overcome some of the major hurdles with DVB-T amplifiers and warrants further inv…

A 70cm DATV TX filter using a cheap Chinese duplexer

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A 70cm DATV DVB-T 7 MHz band-pass filter using a cheap Chinese duplexer Introduction DATV transmitters for DVB-T are notorious for "spread" outside the channel, to the point that keeping it 30 dB or more below the signal becomes a limit for power output, typically 10 W out of a 70 W module amplifier.
Even with -30 dB spread, it is desirable to have a band-pass filter before further amplification or transmission. Usually an interdigital filter is used, but they are either expensive to buy or a bit difficult to build.
VK4JVC suggested using a cavity filter duplexer instead. I tried a four cavity notch duplexer, but the pass-band losses were too high, more than 20 dB. I had bought a cheap, ~A$100, Chinese Jiesai duplexer, but had put it aside as the response looked bad. After try other duplexers (notch and pass-reject types), I tried the Chinese one again, this time successfully.
The result is that the Chinese filter seems to provide a good pass-band for the 7 MHz DVB-T DATV s…