Spectravue

Up
 

I am using Spectravue as SDR software because it gives me best performance for my goals.

I am using Spectravue for EME but also for tropo work and Airplane Scatter

Read my story using SDR for very weak signals by PA0EHG

The big advantage of Spectravue compared to other SDR software is that its simple in use and it has very good possibilities in showing extreme weak signals

I am using Spectravue version 3.15 in combination with a Funcube Dongle and a Funcube Dongle pro plus

I also tried other SDR software but in my opinion these have less possibilities in averaging or choosing very narrow Bandwidth.

This is a presentation of a signal with a signal to noise ratio of 4 dB in 12 Hz BW which is equivalent to -20 dB in 3 kHz

My settings for this are:

Using Combo display mode

FFT averaging : 5

FFT/BLK 8192 which gives a resolution Bandwidth of 12 Hz

Scale setting 1 dB/div, in this version is a bug and the display shows a scale of 2 dB/div

For signals even weaker or a carrier it's possible to increase the FFT Ave, sometimes I use 10 or 50

My standard setting for AS or EME is an FFT Ave of 5

When I am working EME and expecting very weak signals I reduce the span to 20 kHz

When working Aircraft Scatter I mostly use a span of 96 kHz

Noise measurements

For solar noise or moon noise measurements, Spectravue has a feature other SDR software don't have.

It is the Continuum mode and with this it is very easy to make accurate measurements on noise levels.

As we all know accurate measurements can be easily corrupted by small mistakes

The most important in measuring moon and or solar noise is to be sure that the SDR receiver is operating well within the normal linear mode

The Funcube dongle is not very good in dealing with strong signals, leading it very fast into compression or generating spurious products

On the other side one needs to be sure that the noise of the pre-amp used is clearly visible on the Spectravue display.

When switching on and of the pre-amp I would like to have a least 20 dB or more rise off the noise floor shown on the Spectravue display.

Be careful however, when the IF feeding the Funcube with high gain in front that the bandwidth of that signal is not extremely wide.

When I connected a PLLLNB with 60 dB gain and an IF running from 950 to 1950 MHz so 1000 MHz
 the total power from that 1000 MHz noise is already causing compression in the Funcube.
When the Funcube is entering non linear mode it is no longer possible to measure a small increase in noise power at the input.
I had to put a 30 dB attenuator between the PLLLNB and the Funcube to make good solar noise measurements.
I leave all the settings from the Funcube standard,  perhaps one could select less gain in the Funcube
but I have chosen to use an external attenuator to be sure that my Funcube is not entering compression.   

If one is using a transverter between the Funcube and the Pre-amp for instance a DB6NT transverter from 10 GHz to 432 MHz it is not needed to use an extra attenuator. The IF bandwidth of the transverter is limited to several MHz and the total power level entering the Funcube will not get it into compression.


For a better understanding lets see the difference:
If the transverter has a IF bandwidth of 5 MHz
The PLLLNB has an IF bandwidth of 1000 MHz
If both have 60 dB gain then the power level change from using a transverter compared to a transverter is
 10 log 1000/5 = 23 dB more power from the PLLLNB.
Hope it's clear that this large IF bandwidth can cause compression in the Funcube.
Even if the noise floor showing on the Spectravue screen is at the same level,
the Funcube has to deal with 23 dB more input power on its input.
So when using high gain, which I prefer, you better limit the IF bandwidth
to the Funcube or the other option use an external attenuator.

Set Spectravue in continuum mode for noise measurements

Be sure that the Funcube is well in its linear mode,
if you use a step attenuator and increase the signal by 10 dB the display level should also increase by 10 dB.
If the displayed level on Spectravue is increasing less than 10 dB there is a good possibility the Funcube is out of its linear mode.
You should add extra attenuation, but also check if the noise floor rises by at least 20 dB if you turn on the pre-amp.

Turning the antenna to a cold sky spot and then turning it to the moon or sun will give a rise of measured noise level.

On my 10 GHz EME equipment I measure:

With my 3 meter dish on 10 GHz I have 15 dB Solar noise or 2 dB moon noise

With my small setup a 50 cm dish I have 3.3 dB Solar noise and 0.1 dB moon noise

With my low cost setup with 64 cm offset dish I have 3.9 dB Solar noise and 0.2 dB moon noise

The moon noise measurements of 0.1 dB or 0.2 dB would not be possible using old noise meters

Example measuring Solar noise

The horizontal display is showing the noise power in time, you see the noise power rise because I am turning the antenna to the sun

Picture of solar noise with my 50 cm dish, look at the scale 0.5 dB/div

Example measuring moon noise

I started with the dish on the moon and at 10.04.03 i turned the dish away from the moon, The decrease off noise power
is the difference from moon to cold sky

Picture of moon noise with my 50 cm dish, scale is at 0.1 dB/div

If needed one can increase the FFT Ave level, normally I use 5 or 10

If the difference is relatively small one can increase the vertical scale.
Most of the time I use 0.5 or 1 dB/div but with the very small difference a scale of 0.1 dB/div will be helpful.