Don, For those who chase weak signals, any spurious signal generated inside the receiving system on the receiving spectrum is trouble. I operate with a 144-MHz receiver with -156 dBm sensitivity and add 21.3 dBi of antenna gain to that, so I hear VERY WELL.
I find most of the problem signals come from external to my station and are directly picked up by the antenna system. Nothing to do about that unless one moves to the far side of the moon. But any internal spurious or mixing product signal is a problem when you are working with such extreme weak signals. One asset of the preamp gain is sky noise is amplified by 25-dB which can cover up the weakest internal "birdies". At 144-MHz sky noise is approx. 200K min. but at 1296-MHz it is 10K, or less, so birdies can be more troublesome. 28.000-28.100 MHz is the sub-band that needs to be birdie free for VHFers, etc. as most transverters output there for the weak-signal frequencies. If you are one of the growing users of WSPR on HF, those frequencies are sensitive to interference from "birdies", as detection is -29 dB below noise in 2.5 KHz bw. HF sky noise really sets a limit on receiver sensitivity, though. 73, Ed - KL7UW ------------------------------ Message: 24 Date: Sat, 20 Aug 2011 23:28:15 -0400 From: Don Wilhelm <w3...@embarqmail.com> Subject: Re: [Elecraft] Tunable carrier (birdie) 20445khz Cc: 'Elecraft' <elecraft@mailman.qth.net> Message-ID: <4e507b4f.6070...@embarqmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Ron and I will have to just disagree with the definition of a "birdie" - and it is only a matter of definition. My definition is the result of multiples of the various signals and oscillator frequencies present in the receiver, and since they are always multiples greater than 1 of any signal (or oscillator), will always result in a fast tuning response. OTOH, there are unavoidable mixing products in any down-conversion receiver that will tune as a normal signal. The goal of the designer is to choose the IF frequencies to keep those spurious responses out of the bands of interest to the target users - in this case, the ham bands. So, if your definition of "birdies" agrees with Ron's, so be it - I will continue to refer to extraneous direct mixing products (those responses that do not produce fast tuning signals) as spurious responses. It is just a matter of definition. BTW, this is one of the advantages of up-conversion - those direct responses are so far away from the desired signal that they do not become troublesome, but up-conversion designs have their own share of troublesome problems. 73, Don W3FPR 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 50-1.1kw?, 144-1.4kw, 432-100w, 1296-60w, 3400-? DUBUS Magazine USA Rep dubus...@gmail.com ====================================== ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
