Topband: dynamic range

[John Kaufmann]

Jim Jarvis wrote:

> 1)  What's s9? I've got references indicating 50uV or 100uV.
> 	The front end of a decent receiver is spec'd at .1uV 
> 	sensitivity, making 100uV a reasonable 'full scale' signal. 
> 	100uV plus 60dB is....gee...100V! A very large signal, 
> 	indeed.  

Correction:  100 uV plus 60 dB is 100 mV, still a strong signal but a long way from 100V.  Decibels (for voltages) are defined as

dB = 20*log(V2/V1)

> 
> 	Going the other way...9 wiffy-units*6dB is 54dB to get to
> 	S0.  A mere 30dB down from 100uV brings you to .1uV, or
> 	the receiver noise floor.  Where'd the other 20dB go?
> 	Or could it be the wiffy-unit problem, again?  

Correction:  30 dB down from 100 uV is about 3 uV.  See above.


> 
> 2)  Who among us has a receiver which is calibrated in dBm?
> 	Who among us trusts the logarithmic calibration of their
> 	wiffy-unit meter, really?  And over how many decades of 
> 	its range? (regardless of indicated units.)

The S-meter is still quite useful for measuring signal level differences if you have an external step attenuator.  You add step attenuation to equalize two different signal levels on your S-meter and read the difference as the amount of attenuation you added in.  I built a very simple step attenuator many years ago from a simple Handbook circuit.  It has resolution down to 3 dB, which is good enough for most measurements.  You can easily build in lower resolution, if needed.


> 
> 5)  Interesting if the new Tentec claims -140dBc/Hz noise output.
> 	Add 10dB for an amp..still impressive, and well below the level of
> 	atmospheric noise, if it proves true.  

The Orion spec is indeed very impressive.  (My IC-781, which I'm replacing with a new Orion, has phase noise that measures at about -105 dBc/Hz for close-in spacings).  However, to compare phase noise to atmospheric noise, you have to reference a specific input signal power level.  If the input signal is strong enough, then phase noise may be audible.  

73, John W1FV