Friday, 30 September 2016

Duplexer DVB-T TX filter: Low bandwidths

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 other bands as such filters are relatively common, although less so with the notch going both ways. I will discuss this in another post.



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