The preamp had better amplify all things equally! If it doesn't then it is non linear and you have an intermod generator connected to your receiver. But if the preamp is driven hard enough by a strong signal(s) to drive it into compression then it becomes non linear. A condition that you want to avoid.
Amplifying noise along with signal is a common result of any amplifier. If the noise is already there at the site the preamp is not going to increase the signal to noise ration no matter how much or little gain it has. Once you have site noise that is higher than the noise figure of your receiver or preamp then any more or less gain or better noise figure front end will do nothing for you as to signal to noise ratio. In other words it won't help you hear any better. Adding a preamp to a receiver reduces the intermod rejection capability of a receiver by the amount of gain added by the preamp. A 10 db gain preamp reduces original IM spec of the receiver by 10 db. This is the main reason you want to add only enough preamp gain as necessary. There are two ways to increase intermod performance in a receiver. One is to reduce the bandwidth of the receiver front end to limit or reduce the total amount of energy reaching the receiver. This was commonly done by using the helical and other type of front end filters in older radios. The other way to increase intermod performance is to have higher dynamic range front ends in the receivers. This is the case with more of the modern receivers and is why you don't see as many with the multiple front end filters that the earlier radios had. The front ends are capable of handling more energy (greater number of signals entering) without overloading. The sharper the filter ahead of the receiver or preamp the less total energy enters the receiver. Here I am talking about off channel energy, the stuff you don't want. All signals that reach the receiver add together and reduce the total energy handling capability of the receiver. 73 Gary K4FMX _____ From: [email protected] [mailto:[email protected]] On Behalf Of Barry Sent: Thursday, February 05, 2009 11:52 PM To: [email protected] Subject: RE: [Repeater-Builder] Re: Hamtronics Helical Resonator Preamp or Advanced Research Preamp What he is saying is , a lower gain preamp aplifies less noise in relation to the signal so the audio sounds better _____ To: [email protected] From: [email protected] Date: Fri, 6 Feb 2009 04:47:11 +0000 Subject: [Repeater-Builder] Re: Hamtronics Helical Resonator Preamp or Advanced Research Preamp --- In Repeater-Builder@ <mailto:[email protected]> yahoogroups.com, "Joe Burkleo" <joeburk...@...> wrote: If for example the site has > a higher than normal noise floor a lower gain preamp will often times > amplify more of the signal and less of the extra site noise, where a > higher gain preamp may amplify both the noise and signal, giving you a > signal with more noise than you would like. Joe, scratchin' my head here... Would you be able to clarify the above statement for me? Laryn K8TVZ _____ Get what you want at ebay. Get rid of those unwanted christmas presents! <http://a.ninemsn.com.au/b.aspx?URL=http%3A%2F%2Frover%2Eebay%2Ecom%2Frover% 2F1%2F705%2D10129%2D5668%2D323%2F4%3Fid%3D10&_t=763807330&_r=hotmailTAGLINES &_m=EXT>
