[Linrad] Re: First real EME tests with MAP65-IQ

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

Hi Gabriel,

> I made the changes you suggested and after setting the gain of the analog
> hardware by -10 dB I got the noise floor in the main spectrum at 20 dB.
> 
> However, when I switch off the 144 MHz preamp. the noise floor drops only by
> 10 dB, not by 15 dB as it seems it should be.  What is the reason of this?
First of all, the -10 dB has to be on the output side of the analog
hardware or possibly the input of the SDR-IQ.

I suggest you investigate the entire analog signal path.

Start by changing RCF output shift. Reduce it by 4 to temporarily
move the noise floor 24 dB upwards.

Then unplug the coax from your SDR-IQ and note the noise floor
of the SDR-IQ only. (If you did not temporarily change the RCF
shift, the level would perhaps be much less than one bit with
a totally incorrect result as a consequence.)

Then connect a dummy load to the input of the previous stage
and connect it to the SDR-IQ. Measure the new noise level.

Then move the dummy load further towards the antenna while
measuring the noise floor each time.

You may have preamp -> second RF amp -> 144 to 28 converter ->
28 MHz amplifier -> SDR-IQ. Maybe more, or you just have the
preamp, converter and SDR-IQ.

Generally, not enough noise change for preamp on/off is caused
by an inadequate fit between the preamp and what comes after it.
The preamp gain is not high enough to overcome the noise floor
of the second stage. More preamp gain or a better system noise
figure at the input of what comes after the preamp will be required.

With only 10 dB (10 times more power) for preamp on/off only
90% of all the noise you hear comes from the antenna/preamp.
That means that your noise floor is 100/90 times the ideal noise
floor you would have if all the noise was from the antenna/preamp.
1.111 is 0.45 dB so that is the S/N loss you currently have.

You could change back the hardware to 0 dB, but that would move
all signals 10 dB closer to saturate the SDR-IQ. The S/N improvement
would be rather small provided the attenuator is at the 
analog output side. In case you never have saturation problems
you may do this. To then set the level into MAP65-IQ you should
increase "First backward FFT att N" if possible. Only when
it is already at its maximum you should reduce first fft amplitude.

What to do to recover nearly all of the 0.45 dB you are missing
depends on what your noise measurements show.

73

Leif / SM5BSZ

 






 

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