Broadband (UHF) Tunable Channel Filter
I would like to design a Channel filter that is tunable along the full UHF band (470MHz-860MHz).
The channel bandwitdh is 8MHz. The stop band attenuation is around 37 dBc and the pass band is around 1.5dB.
The filter must accomodate around 100W CW power max.
Can you please share any design ideas to go ahead with the design?
Best regards. please let me know if i have to add more details
A question about the tune-ability -
a) Would this be a 'factory' tune or
b) are you referring to tuning in the field by non-technical personnel with no test equipment?
To achieve a passband of 8 MHz with 1.5 dB or less ripple with stopband attenuation of 37 dB in combination with easy tune-ability will not be a trivial solution, and on top of that a 100W power limitation - is that continuous (CW) or peak (as for a multi-carrier digital or vestigial-AM TV signal)? What crest factor if peak? ... one is approaching near-unobtainium with the combination of all these specs!
The best place to start is a filter design application; design a filter for the low end of the band, then design one for the high end of the band and look at what was necessary to be changed to cover from one end to the other ... again, just as a first 'exercise' towards a solution for this posed end-problem.
Hope this helps somewhat, although I did not provide a direct answer.
Jim
Added: References or examples of commercial filters showing the form and technology (Helical Filters), although only rated at 10 Watts:
TEMWELL CORPORATION
If this is a serious venture, it might be fruitful to study these lower-powered designs then draw-up models in the simulator of your choice and work towards meeting your specs 'on paper'. Just a thought ...
Other filters from sateng.com, but not transmit-rated (certainly not for 100 Watts):
http://www.sateng.com/downloads/3278.pdf
The bandwidth is too wide to realize your channel filter.
Normally "Coupled Coils" with "Resonating Capacitors" are used to realize channel filters to obtain low ripple and high rejection but UHF band is divided practically 3-4 region and
in according with these regions, the filters are specially designed.
But in any case, this filer may be realized by using "extreme-unusual components"
Hello,
Appreciate the comments and inputs.
By the way for additional technical details. The average power is around 10W rms and a Peak-to-Average of around 10 to 12 dB.
The tuning will be done in the field. For the whole UHF band, there are about 50 channels. The filter will be manually tuned to those channels.
Dear Bluebee:
The filter corresponding to your requirements was used in the old UHF television tuners. Usually two coupled coaxial quarter-wave resonators, tuned by a variable capacitor pair, did cover the full UHF TV band and was capable to transmit ~10 W of RF power.
I have made many similar filters; the best for you is to get such old TV tuner and add the output coupling loop instead of the original diode mixer. If you need, please write me to "jpolivka@spaceklabs.com", I have one spare here and can send it to you.
J.Polivka
hello Jiri,
thank you so much for your insightful advice.
can you please give me an idea on the characteristic of the channel filter like attenuation levels and the cut-off/passband frequencies?
yes please i'm definitely interested and very grateful if you could send me your spare one for me to work on.
i'll email you...
best regards,
bluebee
The tunable UHF-tuner band-pass filter covers 470-860 MHz, has a typical pass-band ~6-7 MHz /-3 dB and rejects ~20 dB on each side ~ 5 MHz from center. All those numbers vary as the filter is tuned but usually the selected TV channel passed well and the unwanted neighbour was well suppressed. For better parameters it would be possible to gang mechanically two such "tuners".
All newer TV UHF tuners are electronic, so only low-level signals can pass. The old ones used air capacitors; I guess that ~10W could pass without sparking.