This is a nice mental thought experiment. Tune two identical receivers to the same pass band and listen on both. If you sum the outputs together (neglecting the noise generated inside the receivers themselves like we have on a noisy 40m band), the two outputs will be the same. The band noise goes up 2x, the desired signal goes up 2x.
Think about what you are proposing: In order for there to be 3 dB SNR degradation, somehow the receiver would have to "know" what "stuff" is "signal" and what stuff is "noise" and treat them differently. A receiver (other than cases like noise blankers) does not do that. The receiver translates both band noise and desired signal to audio. Thus, as long as the signals external to the two receivers swamp the internal noise of the receivers, summing the two outputs of the two receivers driven from the same antenna will not help or hurt the signal to noise ratio. How can it hurt the SNR? How can the audio output **not add** for the desired signal and **add** for the noise? The desired signal adds the same as the undesired band noise add. I was once fooled by this thinking as well. It helped me a lot to understand that to a receiver everything is a signal. The desired station is signal. The undesired band noise is also a signal. Thus adding the output of two receivers together gets 2x "signal" and also 2x "noise". Both will increase by 6 dB, but the ratio of "signal" to "noise" is unchanged. Like I said before, the only potential signal to noise improvement happens when the receiver internal noise starts masking the external desired signals, in which case all external signals double in voltage (both signal and band noise), a 6 dB gain, while the internal noise from the two receivers will not be the same, adding in an uncorrelated manner, seeing only a 3 dB gain. Thus we can see a net signal to noise improvement since the receiver noise component will increase less than the external signal and band noise increases. Receiver noise being a limiting factor happens either under very quite band conditions, or when a very poor antenna is being used. - Dan, N7VE -----Original Message----- From: Kok Chen [mailto:[EMAIL PROTECTED] Sent: Monday, November 17, 2008 12:55 PM To: Elecraft Reflector Cc: Tayloe Dan-P26412 Subject: Re: [Elecraft] Re: K3: listening to both rcvrs - Reduced receiver noise floor On Nov 17, 2008, at 10:06 AM, Tayloe Dan-P26412 wrote: > Band noise from one receiver at > any instant in time will look exactly like band noise from the second > receiver. That is true if the two receivers are tuned to the same passband and you are using an identical antenna for the two receivers. In the case of receiving split, you are not looking at the same noise entering the two receivers. In this case, by combining two different passbands, the desired signal would only come from one receiver but the summed noise would come from both receivers, dropping the SNR by 3 dB. An easy test is to subtract two receiver outputs (assuming the receivers are phase coherent). You should get a "reasonable" null (sky noise and signals are nulled away, leaving just the receiver noise and any gain/phase mismatch in the two passbands) when looking at the same antenna. When you tune one receiver away, the noise level should rise. Come to think of it, it is an easy DSP experiment by looking at the output of two complex mixers using different (numerical) local oscillators. 73 Chen, W7AY _______________________________________________ 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
