Gain for Delta44

["Leif Åsbrink" <SMTP:LEIF.ASBRINK@xxxxxxxxxxxxxxxx>]

Hi All,

I found this question from OE8FNK:
> Q1: What is the total amplification needed for a 
> 2m Receiver into a Delta 44? 
The Delta44 saturates for 10V peak to peak when set to minimum gain.
This is 3.5V rms.

When talking about gain at RF we usually mean power gain.
At audio frequencies we talk about voltage gain.

This can be confusing because the input impedance of the Delta44
is very high so it does not consume much power.

To avoid any confusion, assume a 50 ohm resistor is connected
across the Delta44 audio input. 3.5V rms is then 0.25W = +24dBm.
(nearly all the power goes into the resistor)

The Delta44 noise floor is at about -146dBc which becomes -122dBm
across the 50 ohm resistor. The noise figure is thus 174-122 = 52dB.

For a noise figure of 0.4 dB at the antenna input you will need 65dB
gain if the noise figure of the preamplifier is 0.2dB. There will
probably be some noise from various other sources than the preamplifier
and the Delta44 so you will need some additional gain to overcome
that. You might also want to come closer to the ultimate 0.2dB
noise figure limit given by the preamplifier in this example.
I would guess you arrive at 70 to 75 dB gain in total.


The NF of the Delta44 itself is actually much lower than 52 dB. 
If the soundcard were properly matched (transformer) to 100 
kiloohms or whatever the input impedance is, it would se perhaps
25 times more voltage and have a NF in the order of 25 dB.

73

Leif / SM5BSZ



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