Commercial DVB-T pass-band/notch filters: What we can learn
IntroductionLow power, UHF and VHF DVB-T pass-band/notch filters are commercially available at relatively low cost, ~US$750 that seem suitable for DATV. They seem a good off the shelf solution.
By examining such filters, it seems possible to see how they might work, giving some insight into possible home-brew.
The filters have two notch filters, one for each side of the signal, as per my earlier posts, to notch the TX skirts.
In addition, they have cavity pass-band filters to take out artefacts further out. A manufacturer indicates that the pass filter is a combline, but the mechanical construction suggests cavity filters with openings between cavities for coupling.
It seems possible to separate the notch and band-pass filters. For wideband UHF, two notch cavities and a pass-band filter. For 2m, two notch cavities and a single pass cavity may suffice.
Low-pass filters are still needed for odd harmonics in addition to a DVB-T filter.
Commercial filters DVB-T (UHF and VHF)The UHF first commercial filter seems to have five band-pass cavities and two notch cavities, one at either end. The input, notch and first pass resonator seem to share the same cavity, similarly for output. Three of the resonators are in individual cavities. On the top, RHS of the filter are the resonator tuning knobs.
From the response curves, the two sharp notches are evident to filter the skirts. This is similar to what I found in earlier posts on notch duplexers for DVB-T. However, one pair seems sufficient, something I have been working on, rather than 3 pairs in a duplexer.
How the notches work is not particularly evident. The connector has a loop coupling per the manufacturer's claim of DC to earth for lightning protection. There is a protrusion on the opposite side of the connector, the purpose of which is not evident. The notch and first pass-band resonator seem to be in the same cavity. Each resonator may be energised, one as a notch, the other as the first resonator of the pass-band filter.
The response shows the five minimums in SWR from the five cavities. The pass-band is shown without ripples, which seems a bit optimistic.
The response also shows the filter losses, less than 1 dB according to the specifications; quite remarkable!
The filter is meant to be a combline, presumably similar to the diagram from Piette 2010.
However, the cavities seem to have openings between them as the line of screws do not go all the way. It would seem to be similar to the band-pass filter from Piette 2010. It is not clear from the first drawing if there are screws to adjust coupling, but there seems to be another adjustment next to the resonator tuning.
From the mechanical design, it does not seem to be an inter-digital filter.
The filter is quite small,
Homebrew DVB-T TX filter?
Bernard Piette 2010 VHF/UHF Filters and Multicouplers: Applications of Air Resonators