A Failure To Communicate! Those ominous 4 words have appeared in reports such as the 9/11 Commission Report, or in describing 20M Band Conditions and then extending even to an equipment breakd0wn. In short, these issues are the causal factors of a failure to communicate and not so much inside the communications itself.
The inside the communications factor can be seen in the current political environment where it is suggested that immigrants are eating your pets. It is clear the pet eating comments are to grab the headlines and not so much to identify any problem. The solution to that is November 5th.
I also believe those 4 words were used in a Paul Newman movie ~ Cool Hand Luke.
There is yet another aspect in the assumptions we individually make and then ask for something based on a not so clear assumed condition. I received another email last week that simply said send me the code. I wrote back that unless the person identified the specific project I was at a loss as to what to send as it was a failure to communicate. I have not heard back.
Without making an issue, the several requests for send me the unidentified code came from offshore where English is not the primary language. So that may be a factor but that is not an excuse!
The new phrase might be: If You Want Stuff You Have To Know Stuff. The knowing may be as simple as identifying specifically what you want.
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This raises a question (back on to the Techie Stuff). The question of the day involves gain distribution in a homebrew receiver and where should the gain stages be applied. This borders on how big is big?
But as we well know everyone wants a specific number if only to argue -- that is too big or too small. For the one or two sharp shooters in the blog readers set, your ears should perk up as you ready an argumentative response.
There are neat terms like MDS (minimum discernable signal) where you can quantify that a particular topology can copy a weak signal with the weakest identified as MDS.
Too much gain at certain parts of the signal train can cause a lot of noise to be amplified and carried along with the signal. This is especially true in front end RF Amplifier stages. Several commercial designs simply have no RF amplifier stages. This tends to work well below 20M but when you get beyond 17M -- the receiver almost sounds like it is on life support. No Noise BUT No Signals.
So, by picking a reasonable level of RF amplification walks the thin line of boosting the incoming signal without overly boosting the noise. A gain of say 5 to 10dB for the RF Amp would be a starting place.
But external noise is not the only noise as there is noise internally generated within the receiver itself. This drives what you use and how you use it. Running stages Class A would likely result in a different outcome than from those running in other classes of operation.
Analog Devices has an exhaustive paper on noise in a receiver and can be seen here.
Intermediate stage gain must consider that you want to assure you are not hitting the Crystal Filter with a sledgehammer. W7ZOI has often used a Pad after the post mixer amplifier so you are not really banging the crystal filter with signal. It also helps if you limit the bandwidth of the signals being applied to the filter especially those coming from the Balanced Modulator in a transmitter configuration.
A chance to make up for signal deficit lies with the audio amp stage. Not any old stage like a LM386, but a well-designed stage that delivers a significant amount of stage gain without a lot of noise in the side car is the goal. A homebrew audio amp using multiple gain stages is certainly a choice.
So, what is a number for overall stage gain? Such a number is a trap unless you also consider a clean sound coming out of the speaker. To hedge my answer the overall stage gain should be that required to copy weak signals while minimizing internal and external noise at the band of operation. Old wives' tales often suggested 100dB.
TYGNYBNT. Don't eat your pet.
73's
Pete N6QW
