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[SDR14] Re: System Gain
- Subject: [SDR14] Re: System Gain
- From: Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>
- Date: Fri, 21 Dec 2007 21:31:18 +0100
> My first approach is to just unplug the IF cable coming from the box. I
> am using an SDR-14 with the Linrad program. The 144mhz IF comes from the
> transverters and is divided in a 3dB hybrid. One output goes to a 1.2 dB
> nf, 20dB gain LNA, then through a 144 mhz bandpass filter, then to the
> SDR-14 input. The other output of the hybrid goes to the 144mhz input of
> my K2 IF radio.
I made a test on the direct input of my SDR-14 on 144 MHz.
An input of -90 dBm gives S/N=26dB in a bandwidth of 1kHz.
This corresponds to S/N=56dB at 1Hz which means that the
noise floor is at -146dBm/Hz. Room temperature is -174
dBm/Hz which means that the noise figure of the direct
input is 28dB.
Check whether the noise floor rises properly on the K2.
If it does, you just need more gain for the SDR-14. If
the noise floor increase is too small on the K2, that
unit also needs more gain and then it might be a good
idea to move the preamp to the transverter output because
you now have 10 dB loss in front of your 1.2 dBNF LNA
for a noise figure at the transverter output of 11.2 dB.
Could be a bit high...
The 28 dB NF of the SDR-14 might seem a bit high, but
since the analog bandwidth is 200 MHz at least while
the digital bandwidth is 33.333 MHz. This means that
there are at least 6 aliases that give contributions
to the noise floor.
When listening with the SDR-IQ you have 10 dB gain from
the transverters, cable(-7)->coupler(-3)LNA(+20) so the
NF that your transverter works against would be about
18 dB and I do not think it is designed for that.
Knowing the noise floor of the transverter (compute from
the nominal NF) you can find the noise floor at its output
by measuring the conversion gain. Then the noise floor
of what comes after should be at least 15 dB lower.
I would not be surprised if you need 15 dB more gain;-)
Leif / SM5BSZ
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