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[Linrad] Re: First real EME tests with MAP65-IQ
- Subject: [Linrad] Re: First real EME tests with MAP65-IQ
- From: "Gabriel - EA6VQ" <vhfdx.net; ea6vq.1@xxxxxxxxxxxxxxxx>
- Date: Mon, 6 Apr 2009 11:43:46 -0000
Thanks for the detailed comments Leif,
I have separate RX/TX lines and I only have one preamp on the tower. Once in
the shack the RX line is split to the TS-790 and to an 144/28 MHz converter
that is connected to the SDR-IQ, so that I can receive with both receivers
at the same time.
I will make the tests you suggest and see what can be improved.
73. Gabriel - EA6VQ
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> -----Mensaje original-----
> De: linrad@xxxxxxxxxxxxxxxx [mailto:linrad@xxxxxxxxxxxxxxxx]En nombre de
> Leif Asbrink
> Enviado el: lunes, 06 de abril de 2009 9:41
> Para: linrad@xxxxxxxxxxxxxxxx
> Asunto: [Linrad] Re: First real EME tests with MAP65-IQ
> 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.
> Leif / SM5BSZ
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