[R-390] Official specs

[Jim Whartenby]

Larry
What is the insertion loss of the DA-121?  That is what we really want to know, isn't it?
Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, October 27, 2024 at 08:40:26 AM CDT, Larry Haney <larry41gm2 at gmail.com> wrote:   

 Jim,  I agree with this posting of yours except for the 1st and last statements.  
1.  First you said: 'What has been overlooked is that there is an impedance transformation from 50 to 125 ohms.'  We are all very aware of this fact.  
2.  Lastly you said: 'To convert the SG voltage output into the voltage actually seen by the R-390, multiply the SG reading by 0.1235 or divide the SG reading by 8.097, either way works.'  That is not right at all.  You just went through a nice step by step explanation about how to determine the power loss, then you use that power loss ratio (0.1235) to determine the voltage seen by the 390.  Wrong, wrong, wrong.  The last 3 steps in your procedure are: 1. dB = 10 Log ^ (.00247watts / 0.02 watts), 2. dB = 10 Log ^ 0.1235, 3. dB = -9.083.  No real disagreement there.  The input watts to the da-121 = 0.02 watts, the output watts from the da-121 = .00247 watts, that's a 12.35% loss of power in watts, not voltage.  You can not use the 0.1235 power loss relationship to directly calculate the voltage loss relationship of the da-121 as you are doing in your last statement.
One way to correctly calculate the voltage coming out of the da-121 (Vout), would be to use the formula: 
Vout = Sqr rt (Pout (watts) x impedance (ohms)) 
Where Pout is the power coming out of the da-121 (in this case, 0.00247 watts) and impedance is the da-121 load impedance provided by the 390, 125 ohms.
Vout = Sqr rt (.00247 x 125) = 0.5556 Volts
     .00247 x 125 = 0.30875     Sqr rt  0.30875 = 0.5556     Vout = 0.5556 volts
Vout is what's going into the 390 (in this scenario).
Regards, Larry

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On Wed, Oct 23, 2024 at 9:35 AM Jim Whartenby <old_radio at aol.com> wrote:

What has been overlooked is that there is an impedance transformation from 50 to 125 ohms.  Any calculation that ignores this transformation is in error.  The only solution that accounts for different impedances is by looking at the respective powers at both input and output.

1 volt into the DA-121 gives 0.556 volts out.  Looking at the power-in verses power-out using the respective impedances:

Power = voltage squared / resistance
Pin = 1 volt ^2 / 50 ohms = 0.02 watts
Pout = 0.556 volt ^2 / 125 ohms = .00247 watts
dB = 10 Log ^ (Pout / Pin)
dB = 10 Log ^ (.00247watts / 0.02 watts)
dB = 10 Log ^ 0.1235
dB = -9.083

To convert the SG voltage output into the voltage actually seen by the R-390, multiply the SG reading by 0.1235 or divide the SG reading by 8.097, either way works.

Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy