Hi Brett, your questions are interesting but they requires many words to clarify them... I will try to give you simple answers and may be other friends will add replies.
Message: 22 Date: Wed, 02 May 2007 11:38:44 -0400 From: Brett gazdzinski <[EMAIL PROTECTED]> Subject: RE: [Elecraft] here is how to "get a feel" for a roofing filter To: elecraft@mailman.qth.net Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; charset=US-ASCII Is there no way to put dsp at/as the first filter? > Yes, more or less. This is High Performance Software Defined Radio solution where you connect the ADC to the antenna, then a DSP is handling all the demodulation of signals, manage the filters and notches, once this works (software) is completed the voices or dit dah willa come out from an audio amplifier and on to the loudspeaker(s). > K3 is on the hardware and software transmitter and receiver group. What are the limitations of the dsp, frequency, filter quality? Is there a problem getting dsp to work at say 455 khz? > Putting the DSP (including ADC and DAC) at 455kHz is not a problem as the key is the ADC frequency limit. There will be not too much difference between havin the IF at 455kHz or 15kHz as the major work will be done by the software application. What would be the result of a real good variable filter at the 8.215 Mhz IF just after the mixer and NO dsp for filtering? (K2?) > The result will be a K2 with a higher IF. DSP is a powerfull stage (including the software) to generate filters at audio level. No ringing with narrow filters and soft to straight attenuation skirt better than xtal filters. The 2nd IF of 15 khz is audio, is it not? > NO. certainly if you have a good ear you can listen to something... It is an electrical Frequency like the ones used for submarine communications and other services. At what point does the audio spectrum end and RF begin? > The RF (IF) ends inside the Analog to Digital Converter (ADC)... then you have digital signals handled by the DSP (and SW). The audio start at the output of the Digital to Analog Converter (DAC) I notice that the K3 info says the dsp is in the 2nd IF, but the 2nd IF is high pitched audio? > Even if the 15kHz is a frequency that has an equivalent in the audio, it is a real IF and the conversion to this second IF is done by a second mixer converting from 8.250MHz to 15kHz, more or less like an SSB/CW demodulator. This modern stuff sure spins my head, the homebrew receiver is a single conversion to 455khz, no rf amp, good filter just after the mixer, wide IF chain, detector. It seems to work real well, and is basically the old tube table top radio (all American 5?) with a good filter, s meter, and a digital frequency readout. > I know what you mean ... I am an analog chap and do not like a PC in my radioshack ... but I am fiddling with it, on and off to learn what SDR, Software Defined Radio con do ... today we can play with an analog mixer that converts from HF to audio...yes the stuff that comes out from the loudspeaker ... and then you have a BIG DSP ASSY ... the Audio Card (Stereo) a Big PC and a Software application that may make you crazy to look at the PC screen ... Soon all this type of SDR will become obsolete like a valve receiver because the receiver will be nearly 100% all digital ... the only "analog" stuff will be the antenna, the bandpass filters and the loudspeaker ... plus a BIGGGG and Powerfull PC. > At this point you will have to change operating technique as you will be able to operate with several stations and your roofing filters will be your BandPass Filters ... our good friends at INRAD will retire and we will be able to buy second hand surplus xtal filters for a couple of dollars (I hope .. hi) > I hope I was able to clarify at least partially your right questions > BTW, if you can fidlle with a soldering iron and have a K2 I can send you a working front end mod (1st mixer replacement) that can improve it to an IP3 of +27dBm , from a +15dBm value, and the addition of a xtal homebrew roofing filter at 4.9xx MHz. 73 Gian I7SWX Brett N2DTS __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com