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Old 21-Sep-2020, 5:43 PM   #61
rabbit73
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Using an Attenuator to Measure the Strength of a Digital TV Signal, Part 2

Using an Attenuator to Measure the Strength of a Digital TV Signal, Part 2

I ordered a Mediasonic HW-150PVR and a Toner 0-20 dB Variable Attenuator to see how they would do at estimating the strength of a TV signal.

I had previously used a 0-10 dB step attenuator, but they are expensive; I was lucky to find a good used one. I'm trying to find the least expensive equipment to do the job.

I had also used an Antennas Direct 0-20 dB Variable Attenuator, but they have been discontinued along with their FM filter.









Since the AD ATT-1 was no longer available for others to buy, I made a calibration chart for the Toner attenuator:



I don't know why the factory pasted the label on upside down.

I put a black mark on the knob and on the housing to be able to count the number of turns as I rotated the shaft counterclockwise with a screwdriver to increase the attenuation to dropout.



This was the setup:

Code:


                    Fixed       Variable                      SDR
Ant > 25 ft RG6 > Attenuator > Attenuator > 4-Way Splitter > HW-150
                    20 dB    at 1-1/2 turns                   SLM*
                                  5 dB                   75 ohm terminator
* SLM is Sadelco DisplayMax 800 Signal Level Meter


To find the signal strength of Channel 20, I added the attenuator setting to the average tuner dropout point of -85 dBm:
-85dBm + 25 dB = -60 dBm

To confirm the method, my Sadelco DisplayMax Signal level meter reading was -11.9 dBmV = -60.7 dBm

(Since I used a 4-Way splitter, the signal before the splitter was 7 dB stronger. I put a 75 Ohm terminator on the unused port because without it, the outputs weren't equal.)

This method works best on UHF. On VHF, where the noise level is higher, dropout will be higher than at -85 dBm.



The Thermal Noise Floor for a 6 MHz wide TV signal is about -106 dBm.

The Noise Figure for the average tuner is 6 dB:
-106 dBm + 6 dB = -100 dBm

To that we must add 15 dB to allow for the minimum required SNR for the TV signal:
-100 dBm + 15 dB = -85 dBm

However, if the ambient noise is stronger than the internal noise of the tuner, the tuner Noise will be buried in the ambient noise.
Attached Images
File Type: png Toner Attenuator Calib Curve3.png (66.3 KB, 156 views)
File Type: jpg Toner 0-20 dB Variable Attenuator and 20 dB Fixed2.jpg (70.5 KB, 150 views)
File Type: jpg MediasonicHW-150PVR9-19-2020CH20SNR15.JPG (137.2 KB, 220 views)
File Type: png Minimum Indoor Signal Levels_5.png (81.2 KB, 146 views)
File Type: png AD ATT-1 Calib2.png (112.5 KB, 167 views)
File Type: jpg Variable T-Pad Attenuator.jpg (75.5 KB, 146 views)
File Type: jpg Piher Variable Attenuators2.jpg (105.7 KB, 142 views)
File Type: jpg T-Pad Attenuator Values5.jpg (59.5 KB, 137 views)
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Last edited by rabbit73; 22-Sep-2020 at 5:54 PM.
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Old 26-Sep-2020, 10:01 PM   #62
rabbit73
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Review of Toner 0-20 dB Variable Attenuator

I ordered two of the Toner attenuators in case one was defective. One of them had a coax connector with insulation that hadn't been completely removed, making it difficult to insert the center conductor of the RG6 coax.





I removed the excess insulation with a drill bit held in my hand; the coax was then able to be properly inserted.

I still haven't figured out why they would paste the label on upside down.



It's important to check the female coax connectors to see if they make good contact with the center conductor of the coax:







Attached Images
File Type: jpg Toner Attenuator Connector OK.jpg (195.0 KB, 100 views)
File Type: jpg Toner Attenuator Connector NG.jpg (220.2 KB, 99 views)
File Type: jpg Toner Attenuator Label Inverted.jpg (183.5 KB, 98 views)
File Type: jpg Testing Coax Connectors.jpg (115.0 KB, 102 views)
File Type: jpg TestingCoaxTip3_1.jpg (93.7 KB, 99 views)
__________________
If you can not measure it, you can not improve it.
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Last edited by rabbit73; 26-Sep-2020 at 10:44 PM.
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Old 17-Oct-2020, 1:14 AM   #63
rabbit73
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Quote:
Originally Posted by tripelo View Post

Would like your thoughts on dynamic range of Airspy vs. SDRplay RSP1.

Thanks
I did some additional tests with the SDRplay RSP1A and the RSP Spectrum Analyser (British spelling) software.

It seems that it is necessary to properly set the Reference level of the RSP Spectrum Analyser. If you set it too high, part or all of the signal will be buried in the noise.

The RSP Spectrum Analyser tries to simulate what a real spectrum analyzer does. You have to be careful with the Display scaling settings to get a correct representation of SNR. Manual:
https://www.sdrplay.com/docs/RSP-Spe...lyser-V1.1.pdf

Setting RSP Spectrum Analyser Reference Level

Quote:
Reference Level

A common error is a complaint that signals with a 40-50 dB SNR as displayed on SDRuno, shows a signal barely breaking through the noise floor on the analyser. In almost all cases, AGC is enabled on SDRuno, and the LNA gain reduction slider set at a low gain reduction value resulting in a lower noise floor. IF gain reduction will be whatever the AGC system sets it at.

Conversely, the analyser is usually set at a high reference level of somewhere between -30 to+10 dBm, has a very high noise floor and sees little or no signal above the noise. This is usuallycaused by the user attempting to use the reference level control to position the trace within the display area, instead of using the range and offset controls provided for this purpose.

The reference level should initially be set to around -80 dBm, while ensuring the ADC is not overloaded and spurious signals are not generated. This will give maximum sensitivity and a lower noise floor. Increasing the reference level towards 0 dBm will result in an increasing noise floor, and depending on signal level, may result a lower S/N ratio. Use the range and offset controls to scale and position the display as required.
An example:









Testing for spurs:





Then I decided to just wrap the plastic SDR enclosure in foil and turn On the Clock spur removal



CONCLUSIONS:

It is important to properly set the Reference level to display the correct SNR.

The RSP1A has a plastic enclosure that is letting strong signals through. I had hoped that the inside of the plastic enclosure had been sprayed with a conductive paint that would act like a shield, but apparently that wasn't done.



The aluminum foil that I put on the outside of the enclosure, and contacting the ground of the SMA antenna input connector, acted as a substitute shield.



The RSP2 looks like it has a conductive coating inside:

Attached Images
File Type: jpg Ref level -30 dBm3_1.jpg (135.7 KB, 25 views)
File Type: jpg Ref level -40 dBm3_1.jpg (136.1 KB, 23 views)
File Type: jpg Ref level -50 dBm3_1.jpg (138.0 KB, 25 views)
File Type: jpg Ref level -60 dBm3_1.jpg (137.3 KB, 24 views)
File Type: jpg Ref level -60 dBm 75ohmR3_1.jpg (149.6 KB, 24 views)
File Type: jpg Ref level -60 dBm 75ohmFoil3_1.jpg (125.4 KB, 25 views)
File Type: jpg Ref level -60 dBm 75ohmFoilEnclSpurRemovOn3_1.jpg (130.1 KB, 27 views)
File Type: jpg RSP1A-Inside3.jpg (131.5 KB, 22 views)
File Type: jpg RSP2-Inside3.jpg (147.2 KB, 21 views)
File Type: jpg RSP1AfoilShield.jpg (107.4 KB, 16 views)
__________________
If you can not measure it, you can not improve it.
Lord Kelvin, 1883
http://www.megalithia.com/elect/aeri...ttpoorman.html

Last edited by rabbit73; 22-Oct-2020 at 6:57 PM.
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Old Yesterday, 2:42 PM   #64
rabbit73
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Join Date: Aug 2012
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Quote:
Originally Posted by tripelo View Post
Have noticed that (spurs) in the low cost RTL devices. Most of the time not a problem if one is aware of their presence.
I found another scanner that can be used with the RTL-SDR.COM V3 dongle.
http://rfexplorer.com/touchstone/

The resolution isn't too good and there is no way to adjust the gain of the dongle. It is necessary to insert an attenuator before the dongle to obtain the correct representation of SNR. But, it does cover more than one channel.





The amount of attenuation must be found by trial-and-error. It helps if you have a TV that will tell you the SNR of a known signal.

The Toner attenuator shown above could be used.

I didn't use a preamp. If I had used a preamp, more attenuation before the dongle would have been needed.

If you want to be able to adjust the gain, you would need to purchase the Rational Waves RF SPECTRUM ANALYZER SOFTWARE:
http://rationalwaves.com/rationalwaves/

http://nutsaboutnets.com/supported-rf-analyzers/
Attached Images
File Type: jpg Touchstone 10-25-2020 VHF-High_1.jpg (98.0 KB, 10 views)
File Type: jpg Touchstone 10-26-2020 UHF 10dBAttn_1.jpg (108.2 KB, 11 views)
__________________
If you can not measure it, you can not improve it.
Lord Kelvin, 1883
http://www.megalithia.com/elect/aeri...ttpoorman.html

Last edited by rabbit73; Yesterday at 5:49 PM.
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