You and I are of the same "Ham generation" Don. In the 1950's I acquired a "dream receiver": a National HRO-5. It featured two tuned RF amplifier stages ahead of the mixer, two I.F. amplifiers after the filter and two stages of audio delivering several watts of power to an 8-inch speaker!
I, too, equated "sensitivity" with the HRO's ability to fill the room with noise. After learning a little bit about proper gain distribution in a receiver I fashioned a step attenuator that I put between the antenna and the antenna jack to cut down on the overall signal strength reaching the first stage. It was amazing how much the attenuator reduced the background QRN without cutting the signals down nearly as much, especially on 80 and 40 meters. What was happening was that very strong signals well off of my desired frequency (e.g. shortwave broadcasters around 40) were overloading either the RF amplifiers or the mixer. The result sounded exactly like normal background QRN, but it wasn't. Adding attenuation made weak signals jump up out of the noise level. The loss of overall gain was easily made up for by simply turning up the volume. That led to an ongoing interest in receivers. My old correspondence includes many schematics and notes exchanged with Doug DeMaw and Wes Hayward, much of it about direct-conversion and phasing receivers. I've also followed the impressive work by Rick Campbell, but I've never had the pleasure of working him or corresponding with him. Of course, modern receivers like the K2 has are, by and large, much more tolerant of strong signals than receivers of only a few years ago, thanks to improved mixers and amplifiers. I think that's given a lot of operators a false sense of security. They don't think to try turning the preamp off (or turning the attenuator on) and turning up the AF gain when digging for a weak signal, but when a strong out-of-band signal is managing to cause overload somewhere in the front end, that can make the weak signal much easier to copy. Ron AC7AC -----Original Message----- Ron, you are completely correct - if you can hear the atmospheric noise at all, the receiver will hear signals right down to the noise level. Once that amount of distributed gain is achieved, the next most important characteristic of a receiver is the dynamic range which is an indicator of how strong a signal can be received before the receiver blocks and distorts (reducing the ability to hear weak signals in th evicinity of strong ones). When I was a young ham (that was back in the youth of SSB), I did get the impression that there was a relationship between how loud the receiver was and how sensitive it was. After some study and some receiver design, and more years of experience, I discovered that those old thoughts relating sensitivity and 'loud' were indeed quite different. One can always add more amplification and make it louder, but making the noise louder is not necessarily a good thing - one must make the signal louder and not the noise to improve the reception of weak signals. If you want an extreme descrition of how a really quiet receiver can sound, Rick Campbell KK7B often has an apt description in his writings about his phasing receivers. 73, Don W3FPR _______________________________________________ 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
