[HBR] HBR2K -- Chapter 14 -- Large Signal Performance, Part 6

[hbr@mailman.qth.net]

Mike Feher wrote:
> Hate to be the "Devil's Advocate" here, And, posssibly you answered this
> in a previous post. In either case, could you please remind of the
> bandwidth that you are using to measure your noise floor?

A lot of the reason for posting all that stuff is to get other thoughts 
and viewpoints.   If I knew what I was doing, I'd have finished the 
project six months ago.    Since this is in large measure a learning 
experience, please advocate!   

I measure the NF in the time-honored ham way -- with an AC 
voltmeter on the audio output; actually, across the primary of the 
output transformer.   I set the audio gain for a convenient reading on 
noise alone -- say 1 volt -- then apply the signal and adjust it for triple 
the reading -- 3 volts in this case.   That would be roughly a 10 db 
ratio of (signal+noise)/noise.   

So the measurement bandwidth is set mainly by the audio bandwidth 
-- probably 5 kcs or so -- and the bandwidth of the filter in use, 
usually 2.4kcs.  

Such a casual approach can cause a few db error when comparing to 
measurements made on other sets and would show the HBR2K 
unfavorably if the other set measurement was made with a narrower 
bandwidth;  the effect would be 3 db/doubling, right?  Measuring at 
4kcs would make the NF 6 db higher (worse) than measuring at 1 
kcs bandwidth.

Of course my noise isn't uniformly distributed over the 4kcs -- there's 
more in the 2.4kcs corresponding to voice intelligence because most 
noise (now) actually comes from the mixers and must get through 
the filter. Only the 3180kcs IF noise (generated after that filter) is 
broadband enough to cover the audio section bandpass in a uniform 
fashion.   

(In very high quality receivers it is common to include a second 
selective filter at the end of the IF chain right ahead of the detector to 
take out the out-of-bandwidth IF generated noise but the added 
complication didn't seem worthwhile for this project.)

I think the method is good enough for measuring my progress.   And 
the absolute answer it gives squares with my subjective impression -- 
this receiver is 'getting there' but it is still too noisy.   I expect that 
when a really good set is adjusted to comfortably receive signals on 
even a fairly quiet band (say, 20 when things are good -- does 
anyone remember the last time that happened?) if you then 
disconnect the antenna, the set goes essentially silent -- you might 
not know it was on.   HBR2K at its current state gets considerably 
quieter, but no one would yet call it 'silent.'    

Walt Hutchens
KJ4KV