[Linrad] Re: system gain

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hi Dave,

> The measured nf and system gain quoted are at the IF output connector on 
> the box at the top of the tower...
Yes. That is how I understood it.

> >> From there, the 144mhz IF travels through about 7 dB 
> >> of loss to a 3 dB splitter.
> > Now we have 10 dB loss in total.
Ooops. The converter is at the antenna you wrote. I assumed 144 to 28
converter...

> The 7 dB loss is the connecting feeedline from the top of the tower to 
> the shack--it is quite long as the box is 185 feet above ground on a 200 
> ft tower which is located 250 feet behind the shack.
> 
> Yes--there is a total of 10 dB loss presented to each input--the IF 
> radio and the SDR-14 side.
OK. But the frequency is 144 MHz?

> >> One side of the splitter goes to the IF 
> >> radio,
> > Let me guess that the noise figure of the IF radio is 5 dB.
> > That would give a noise figure at the other end of the
> > cable of 15 dB. The composite system noise figure would 
> > be 1.04 dB
> The input of the IF radio is 4 dB nf with unity gain. It is a DEMI 
> 144-28 transverter board mounted inside the K2 transceiver.
OK. 

> >> the other side to the .6 dB nf, 18 dB amp, and then into the 
> >> SDR-14 and on to the computer.
> > At the antenna it looks like 10.6 dB NF and 8 dB gain. The composite
> > noise figure becomes 0.82 dB with an associated gain of 33 dB.
> > This is then connected to the SDR-14 direct input with a noise
> > figure of 37 dB. The compsite noise figure for your Linrad
> > part would be 5.7 dB. Totally unacceptable.....
> This is where I am getting confused. The input to the SDR goes through a 
> .6 dB nf, 18 db LNA (with T/R relay control to protect it and the 
> SDR-14) Wow--I didn't know the SDR 14 direct input had that bad of a nf....
Sorry, I was too quick when checking. I did not make sure that the
RCF output shift was small enough. The direct input is not quite as bad,
the NF is more like 31 dB (on 28 MHz) and about 32 dB on 144 MHz.
That is still too high so you need some more gain.

> So--comparing the two systems...The IF radio system seems to have almost 
> a 5 dB better NF than the SDR system?  Am I figuring this correctly?
Yes. Close to. (32 dB rather than 37 dB NF makes the difference a bit 
smaller)
 
> That is about the difference I think I am seeing  :-((  Perhaps if I put 
> a higher gain stage in front of the SDR it would improve the system nf?
It would surely help. You would need something like 15 dB gain.
(With a very good output IP3 in case you want to preserve the dynamic range.)

> > You should use the 0.1 to 30 MHz input which has a noise figure
> > of about 17 dB. Then your system noise figure would be 0.91 dB.
> > Probably just a little better than your IF radio.
> > 
> > If I were you, I would move the 0.6 dB NF amplifier to the other 
> > side of the splitter and possibly insert an attenuator in front
> > of the IF radio. The system noise figure for Linrad would then be 
> > 0.84 dB for Linrad and the same for your IF radio if the attenuator
> > is selected to provede a NF of 17 dB.
> >
> 
> This would call for a significant redesign of my system. Moving that 
> splitter is the problem. The splitter also passes the 17 dBm Tx signal 
> back to the transverter. Unfortunately I do not run a two cable system 
> up the tower for TX and RX. I will have to think about it.  Thanks for 
> your help.
Well, the difference is not very large compared to what you will see
with one additionla amplifier on the SDR-14 input.
Maybe a good idea would be to connect the SDR-14 input with a splitter
on 28 MHz inside the K2 transceiver? Perhaps at the output of the 28 MHz
RF amplifier of the K2 itself. In effect that would move the 3 dB loss 
of the splitter further down the signal path to where the level is higher
and a 3 dB loss would have less influence.

73

Leif / SM5BSZ



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