Hi Ralph, W0RPK
I will write the formula in another arithmetical form:
NF = ( Vi x 10^-6 )^2 ) x 20 ) / (BW x S/N ) ) log ) x 10 ) + 174
dB 10
where:
Vi = input signal in uV applied over the 50 ohm RX input
BW = Band Wide in Hz (2400 Hz for SSB)
S/N = Signal to Noise ratio in power ratio = 10
Going to your YAESU FT-726 R the sensitivity is specified for 10 dB of (S+N)/N
and not for 10 dB of S/N so that we must convert first (S+N)/N into S/N with
the following formula:
S + N
S/N = ----------- - 1 = 10 log (10-1) = 9.54 in power ratio
N 10
Calculating the sensitivity for 2 meters SSB/CW less than 0.15 uV for 10 dB
(S+N)/N we have:
NF = ( 0.15 x 10^-6)^2 ) x 20 ) / ( 2400 x 9.54 ) ) log ) x 10 ) + 174 =
6.9 dB
dB
10
____________________________________
Calculating the sensitivity for 70 cm SSB/CW less than 0.15 uV for 12 dB
(S+N)/N we have:
S + N
S/N = ----------- - 1 = 10 log (15.84 -1) = 14.84 in power ratio
N 10
NF = (0.15 x 10^-6 )^2 ) x 20 ) / ( 2400 x 14.84 ) ) log ) x 10 ) + 174 =
5.01 dB
dB
10
CONCLUSION:
The sensitivity in 2 meters and 70 cm is the same 0.15 uV but in 70 cm 0.15 uV
are specified for 12 dB (S+N)/N and not 10 dB (S+N)/N as in 2 meters and so it
happen that the receiver is more sensitive in 70 cm
The formulas where published into an old issue of the Ham Radio Magazine early
about 1973 and I have to find it in my many papers after that I will send a
copy to you but I remember it very often ant this numbars are burned into my
brain HI HI !
I hope this helps and if you have any question please let me know.
73" de i8CVS Domenico
----- Original Message -----
From: Ralph Wallio, W0RPK
To: domenico.i8...@tin.it
Sent: Friday, October 04, 2013 4:48 PM
Subject: AMSAT-BB NF formula posting
Domenico --
Thanks for posting the NF formula. Your results for the ICOM IC-R10 cause me
to reconsider what I have been estimating
for my much older YAESU FT-726R. Specifications from the Yaesu Operating
Manual:
2m SSB/CW Less than 0.15uV for 10dB (S+N)/N
70cm SSB/CW Less than 0.15uV for 12dB (S+N)/N
I would like to make this calculation myself but I am having trouble
duplicating your results probably due to uncertainties
resulting from how the formula is formatted by AMSAT-BB.
Perhaps you have a document source to suggest that will resolve these
uncertainties.
TNX es 73 de Ralph W0RPK
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