Optimisiation procedure for IF and RF level settings

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MTG
Posts: 38
Joined: Sun Feb 12, 2017 6:07 pm

Optimisiation procedure for IF and RF level settings

Post by MTG » Wed Oct 03, 2018 3:39 pm

How do I go about optimisiation IF and RF level settings for best SNR?
I tried to do this with a RSP2 Pro some 18 months ago and had to give up and use SDR Console. However, I have now got an RSP Duo and this is not supported by Console yet.

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Roger
Posts: 217
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Re: Optimisiation procedure for IF and RF level settings

Post by Roger » Wed Oct 03, 2018 5:42 pm

MTG wrote:How do I go about optimisation IF and RF level settings for best SNR?
I tried to do this with a RSP2 Pro some 18 months ago and had to give up and use SDR Console. However, I have now got an RSP Duo and this is not supported by Console yet.
A few comments:
The latest version of SDR-Console 3.03 supports the RSPduo (available from https://www.sdr-radio.com/Software/Version3

The easiest way to setup SDRuno v 1.24 for best performance is to do the following:
a) Go to SETT in main window
- and select Low IF mode
- and check the IF AGC box so that the IF Gain is automatically adjusted to avoid A/D overload
- and select gain from the dropdown menu so that the slider shows RF gain instead of RF attenuation (like a typical radio)
perf.PNG
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b) Disconnect the antenna and start SDRuno
c) Use keypad to select the band of interest.
d) Set RF Gain to maximum and observe the noise floor on the waterfall
e) Lower the RF gain gradually and you will see the noise level increase on the waterfall. This is because the RSP receiver is becoming internally noisier (i.e. Noise Figure is getting worse) However as you lower the RF Gain the receiver is less subject to intermodulation distortion (IMD) and you will see fewer spurious signals (ghost signals). So there is a tradeoff between sensitivity and IMD performance.
f) Put RF Gain back at maximum
g) Connect your antenna and you will see the waterfall noise floor increase substantially due to galactic, atmospheric and man-made noise
h) Lower the RF gain until the waterfall noise floor just starts to get worse and then increase it by one notch. You have now got the best performance tradeoff.
i) In high signal environments you may see the A/D overload indication. In that case you need to reduce the signal level by lowering the RF gain. If you own an external attenuator you can also try to using that to reduce the input signal level.
j) You may observe the waterfall noise floor jumping up and down sometimes. This happens a lot on busy ham bands when strong signals appear and disappear. What is happening is the IF AGC is kicking in and this causes the waterfall to have a transient jump. You can go into settings and uncheck the IF AGC and this will stop but you will have to manually control the IF and/or RF gain.

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MTG
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Re: Optimisiation procedure for IF and RF level settings

Post by MTG » Thu Oct 04, 2018 8:05 am

Roger thanks for the reply. I will follow the procedure you have outlined.
However, for my application I do not wish to use AGCs - I am monitoring meteor scatter 24/7 and signals can have fairly large dynamic ranges that I do not wish to lose. The signals last for from a fraction of second to occassionally tens of seconds. There is just one frequency I set the SDR to and I use a bandwith of 2.5 kHz with "offset" USB. So it is the optimisation and interactions of the IF / RF gain for this kind of signal that is the issue I wish to understand.

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Roger
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Re: Optimisiation procedure for IF and RF level settings

Post by Roger » Fri Oct 05, 2018 12:35 am

MTG,

Could you please post the frequency band that you are looking for meteor activity and whether it has other signals that need to be considered if the AGC is turned off..

Roger

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MTG
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Re: Optimisiation procedure for IF and RF level settings

Post by MTG » Fri Oct 05, 2018 9:57 am

Roger,
A powerful space surveillance radar in France transmiits 24/7 on 143.050 MHz. I collect the Doppler shifted scatter from meteors illuminated by that radar. I tune to 143.048 to give me positive and negative shifts from that central frequency. I collect data 24/7.
Here is the RX window
SDRUno ScreenShot2.jpg
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Signals are in a relatively narrow band of +/- 300 Hz of that centre frequency. I offset the USB such that I am detecting in a window in a band band from 1 to 4.1 kHz as shown in the aux SP
SDRUno ScreenShot3.jpg
SDRUno ScreenShot3.jpg (81.65 KiB) Viewed 27863 times
The peak signal near 4kHz is a gps frequency (143.052MHz) and level signal (-75dBm) I inject into the rcx input to provide to calibrate my measurements. (I may change this when I use the duo in two channel mode and phase lock with a GPS) My data collection takes Uno output on a VAC to Spectrum Lab where a script detects signals in a +/- 300Hz band. The calibration is monitored in a seperate band.
Here is a Main SP window showing a meteor signal; this meteor signal varies between -15 and + 40 dB of the clibration level.
SDRUno ScreenShot4.jpg
SDRUno ScreenShot4.jpg (357.53 KiB) Viewed 27863 times
There is not usually any other signals in the band but my antenna is not ideally placed and I do occasionally get low level interference.

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MTG
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Re: Optimisiation procedure for IF and RF level settings

Post by MTG » Fri Oct 05, 2018 10:23 am

Roger,
I am going to be away for a week and thought I should add these two images showing the calbration, maximum meteor signal and noise levels from two systems - F using Funcube Dongle straight to Spectrum Lab and SP usinf RSPDuo, SDRUno , VAC into spectrum Lab. The latter shows recent twiddles to attempt optimisation.
Graph0.jpg
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Graph2.jpg
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Mike

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Roger
Posts: 217
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Re: Optimisiation procedure for IF and RF level settings

Post by Roger » Fri Oct 05, 2018 10:19 pm

I did not see in your screenshots where you injected a signal at -75 dBm. That is a strong signal and if your meteor bursts are up to 40 db stronger that means that you are receiving them at -35 dBm!!

Here is an update of my previous post that does not use the IF AGC.

The easiest way to setup SDRuno v 1.24 for best performance is to do the following:
a) Go to SETT in main window
- and select Low IF mode
- and uncheck the IF AGC box so that the IF Gain is manually adjusted . Set IF Gain to 59.
- and select gain from the dropdown menu so that the slider shows RF gain instead of RF attenuation (like a typical radio)
b) Disconnect the antenna and start SDRuno
c) Use keypad or manually select the band of interest.
d) Set RF Gain to maximum and observe the noise floor on the waterfall
e) Lower the RF gain gradually and you will see the noise level increase on the waterfall. This is because the RSP receiver is becoming internally noisier (i.e. Noise Figure is getting worse) However as you lower the RF Gain the receiver is less subject to intermodulation distortion (IMD) and you will see fewer spurious signals (ghost signals). So there is a trade-off between sensitivity and IMD performance.
f) Put RF Gain at minimum
g) Connect your antenna and you will see the waterfall noise floor increase substantially because there is a lot of internal receiver noise (poor Noise Figure) when the RF gain is very low. You would have seen this in step e above.
h) Connect your -75 dBm calibration signal
h) Increase the RF gain until the ADC Overload comes on and then reduce one step and the ADC overload will go off. . This will be around the mid-slider position with a -75 dBm input to the receiver (verified on my RSPduo at 144 MHz.)
I) Go back to SETT and reduce the IF gain to 30. This will give you approximately 35 dB of "headroom" (max -45 dBm receiver input).

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MTG
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Re: Optimisiation procedure for IF and RF level settings

Post by MTG » Sat Oct 06, 2018 3:12 pm

I use a 3 port directional coupler to inject the calibration signal: the antenna signal passes straight through the coupler to the output port; the directional port "mixes" with calibration signal 25 dB down, with the antenna signal and is isolated from the antenna by about 50 dB. The signal to noise ratio for the calibration signal at the Spectrum Lab end is about 30 dB and I use an SNR for the meteor signals of 17 dB minimum.

I now understand the way in which I should set up the radio to maximise SNR and prevent IMD. I will try this out when I get home. What sort of level (dBm at the input port) would lead to IMD?

Mike

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Roger
Posts: 217
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Re: Optimisiation procedure for IF and RF level settings

Post by Roger » Sat Oct 06, 2018 5:37 pm

MTG wrote:I use a 3 port directional coupler to inject the calibration signal: the antenna signal passes straight through the coupler to the output port; the directional port "mixes" with calibration signal 25 dB down, with the antenna signal and is isolated from the antenna by about 50 dB. The signal to noise ratio for the calibration signal at the Spectrum Lab end is about 30 dB and I use an SNR for the meteor signals of 17 dB minimum.

I now understand the way in which I should set up the radio to maximise SNR and prevent IMD. I will try this out when I get home. What sort of level (dBm at the input port) would lead to IMD?

Mike
Mike,

From your description the calibration signal has a level of -100 dBm (-75 dBm -25 dB coupler loss) at the input of the receiver. That means that the RF gain slider will be pretty close to the top in step h in my previous post. The ADC overload may not even flash with this level of signal. At maximum RF and IF gain the noise floor using USB 4 kHz. will be -136 dBm so if you inject a -100 dBm carrier the RSP2 SNR will be approximately 35 dB. If you reduce the IF Gain to 30 the noise floor will still be -136 dB because the Noise Figure of the receiver at maximum RF gain will be determined by the RF Gain LNA at the front of the receiver. You can check this on your RSP2duo. Here is a screenshot of mine.
snr.PNG
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With this setup (IF gain =30) you will be able to receive signals up to -65 dB without ADC overload. The minimum signal that you could detect (3 db above noise) is -133 dB if you had no external noise which is not realistic. Man-made noise and antenna gain might increase the noise floor observed on the receiver considerably. If the noise floor increases by more than 18 dB (-118 dBm on S meter) you will not receive the weakest meteor signals at -115 dBm (-100 dBm calibration -15). If you have a very quiet receiving site you could reduce the RF gain slightly which will give you a little more headroom from ADC overload for strong signals. If your are in a noisy area and need more headroom to avoid ADC overload then you can reduce the IF gain slightly.

Another issue is how to set the level of audio on VB cable. The best way is to set it up in SDRuno so that you don't clip. A good tool to use is the Audacity recorder (which is free). Audacity has a calibrated level meter. In Audacity you select the VB cable as input, start monitoring the input, connect your -75 dBm calibrated signal and then use the SDRuno volume control to get an audio level which is xx dB below 0. xx is the maximum meteor signal in dB above the calibration signal. Audacity is a great recording tool and has many features such as spectrum analysis, audio filtering, scaling etc.

You asked about IMD... Intermodulation distortion occurs at all input signal levels in any receiver due to non-linearity from mixers and amplifiers. At low signal input levels the spurious signals may be below the noise floor of the receiver. As the input signal levels increase the level of spurious signals due to IMD will increase. When designing receivers there are many factors to consider which is discussed in many textbooks and articles on the Internet. Users of RSP receivers will find that reducing the RF Gain from maximum will significantly reduce the level of IMD.

In most cases I suggest that RSP users enable IF AGC and reduce the RF gain (with the antenna connected) until the noise floor just starts to increase and then increase by one notch. This gives a good tradeoff between sensitivity, dynamic range and IMD. In this case I suggested another approach because you have a possible 55 dB (-15 to +40 dB) signal input range, no AGC and want the best sensitivity possible.

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MTG
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Re: Optimisiation procedure for IF and RF level settings

Post by MTG » Fri Oct 19, 2018 5:02 pm

An update for Roger
The tutorial on setting up the RF, IF and VAC was most useful - thanks. My calibration signal level is way lower than I thought. The output had been measured when I was using a different supply voltage for the gps unit; I then decided to run it at lower voltage to ensure no overheating. In fact this calibration signal at the tuner input as indicated by SDRUno was -115 dBm, so my meteor signal levels vary from -125 to -75dBm. Regardless of the actual levels, your procedure should be valid.

I tried using Audacity on the VAC and, even with the SDRUno audio gain set to maximum, the Audacity meter does not show output, I assume because the levels are below the -56dB or minimum . I use VB Audio Virtual Cables which have a utility that allows the cable input levels at around -57dB (not sure to what the dBs in Audactity and VAC are referenced)

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