The ARRL is publishing designs for simple phasing SSB exciters with 3-pole filters and filter-type exciters with 4-pole crystal filters so we can't count on DSP. Phasing transmitter kits have filters with at least 5-poles so they are somewhat better. These should be able to acheive 23 dB suppression 4 kHz from the carrier under any circumstances. There could be an exception for older AM transmitters or transmitters under 10W PEP. What is really needed is a rule that says 3rd order IMD must be at least 30 dB down.
73, John KD6OZH ----- Original Message ----- From: W2XJ To: digitalradio@yahoogroups.com Sent: Sunday, December 30, 2007 23:16 UTC Subject: Re: [digitalradio] Re: [illinoisdigitalham] Re: Power Mask for Bandwidth Rules - USA Modern filters that have been used in real equipment since the 80s can be -1 db at 3100 and down 25 db at 3.5 k with negligible overshoot and ripple in the 10ths of a DB. Chebyshev filters are not really the filter of choice for this, elliptic tilers with some custom tweaks are a better choice. They are in very common use in broadcasting. Today digital filter, common in current rigs, can do much better. A lesson to be learned from AM broadcast is that when emission standards were tightened, allowance in the standard was made for older rigs. That so called mask was then later used as a means to add digital carriers. There is a lot of interference created. A better approach would exempt transmitter built before a certain date but only for the AM mode. ohn B. Stephensen wrote: > An emission mask must accomodate AM so I looked at the speech amplifier and modulator chapter in the 1955 Radio Amateur's Handbook. It advocates up to 25 dB of clipping and no circuit has more than a 3-pole filter. The best that can be done today is a Chebyshev filter with 1 dB ripple and a 2.5 kHz cutoff frequency providing 23 dB of attenuation at 5 kHz and 27dB of attenuation at 6.5 kHz. Filters would be worse in 1955 as modern filter design methologies hadn't been invented yet. > > Only the outer portions of the mask should be defined in the regulations so that old equipment can continue to be used but hams with more modern equipment can be more efficient and use a larger percentage of the channel. The ARRL proposal of 9 kHz at -23 dB might be the best than AM'ers can acheive now. > > 73, > > John > KD6OZH > > >
