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.

http://forum.tvfool.com/attachment.p...0&d=1600732690

http://forum.tvfool.com/attachment.p...1&d=1600739602

http://forum.tvfool.com/attachment.p...2&d=1600740885

http://forum.tvfool.com/attachment.p...7&d=1600797177

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

http://forum.tvfool.com/attachment.p...5&d=1600708146

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.

http://forum.tvfool.com/attachment.p...6&d=1600711501

This was the setup:

http://forum.tvfool.com/attachment.p...7&d=1600712295

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.

http://forum.tvfool.com/attachment.p...9&d=1600731992

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.

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.

http://forum.tvfool.com/attachment.p...3&d=1601157632

http://forum.tvfool.com/attachment.p...4&d=1601157649

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.

http://forum.tvfool.com/attachment.p...5&d=1601157668

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

http://forum.tvfool.com/attachment.p...6&d=1601157695

http://forum.tvfool.com/attachment.p...6&d=1501113567

http://forum.tvfool.com/attachment.p...7&d=1501113723

http://forum.tvfool.com/attachment.p...7&d=1601159621

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

An example:

http://forum.tvfool.com/attachment.p...8&d=1603134499

http://forum.tvfool.com/attachment.p...9&d=1603134556

http://forum.tvfool.com/attachment.p...0&d=1603134589

http://forum.tvfool.com/attachment.p...1&d=1603134652

Testing for spurs:

http://forum.tvfool.com/attachment.p...2&d=1603135166

http://forum.tvfool.com/attachment.p...3&d=1603135946

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

http://forum.tvfool.com/attachment.p...4&d=1603136471

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.

http://forum.tvfool.com/attachment.p...7&d=1603204540

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.

http://forum.tvfool.com/attachment.p...2&d=1603392963

The RSP2 looks like it has a conductive coating inside:

http://forum.tvfool.com/attachment.p...8&d=1603213375

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, the Touchstone software does cover more than one channel and you can see changes in real time. With the RTLSDR Scanner, you don't see the scan until it is completed.

http://forum.tvfool.com/attachment.p...9&d=1603723275

http://forum.tvfool.com/attachment.p...0&d=1603723331

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.

http://forum.tvfool.com/attachment.p...5&d=1600708146

My amateur Videos

I didn't use a preamp for this scan. The channel 16 signal (first strong signal on the left) measured -6.3 dBmV (-55.1 dBm) with my Sadelco DisplayMax 800 signal level meter:
https://www.dropbox.com/s/no068tnzs0...45-43.mp4?dl=0

https://www.dropbox.com/s/no068tnzs0...45-43.mp4?dl=1

When I used a 17 dB preamp (CM7777HD Amplify, low gain setting), more attenuation before the dongle was needed.
https://www.dropbox.com/s/18g3b36dmv...27-48.mp4?dl=0

https://www.dropbox.com/s/18g3b36dmv...27-48.mp4?dl=1

I use VLC Media Player.

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/

But, I think it would be better to buy an SDRplay RSP1A and use the RSP Spectrum Analyser software.

When I used the 17 dB preamp with the SDRplay RSP1A and the RSP Spectrum Analyser software, I was able to adjust the Display Scale settings for a proper display:

http://forum.tvfool.com/attachment.p...4&d=1604280723

The ADC is the Analog to Digital Converter in the SDR.

http://forum.tvfool.com/attachment.p...6&d=1604282907

The channel 16 signal input to the RSP1A with the 17 dB preamp measured -35 dBm. I don't usually use the preamp; just wanted to see how the RSP Spectrum Analyser software would handle the stronger signal.

When the RSP1A is used with the SDRuno software, there is an overload warning if the gain is set too high. It did flash when the gain was set to MAX when using the 17 dB preamp. To avoid damage to the RSP1A, the input signal should be kept below 0 dBm (49 dBmV).

http://forum.tvfool.com/attachment.p...7&d=1604284579

Export Data Airspy R2?
 

Hi rabbit73, or anyone familiar with Airspy.

Do you know if there is software for the Airspy R2 that exports logs of spectrum scans in .csv format (or something similar)?

Interested in software for Airspy R2 that can log and export amplitude vs frequency, similar to RTLSDR Scanner.

The purpose would be to save signal data sets (many scans) for later comparison using Excel.

Thanks.

Are you familiar with rtl-sdr dot com? not sure I can post a url here -
https://www.rtl-sdr.com/big-list-rtl...rted-software/ has a large list of software.

I use with SDR# for tuning as well as Orbitron and WXtoimg to download satellite weather.

Also to check out signal strength from my TV antenna.

Rabbit started me on this journey. I am sure he will have other ideas for you.

Nice list of software, thanks.

RTLSDR Scanner is shown in the list. Used it several years ago with the least costly version of RTL-SDR dongle. RTLSDR Scanner with its export feature, worked fine for the specific application.

Looking for something similar that will work with Airspy R2.

Not sure how much, if any, of the software in that list, will work with the Airspy R2.

.

I have no direct experience running on Airspy , but as far as I know, it should. Why not give it a try and see what happens?

Sorry, none that I know of.

Haven't tried it for a couple of reasons:

1. Do not yet possess Airspy R2.
2. Haven't seen on Internet where anyone has successfully used the combo.

The reason for Item #2 could be because the RTLSDR Scanner was designed to be used on the more common inexpensive RTLSDR devices. There appears to be considerable hardware differences between the lower cost devices and the Airspy.

Thanks, rabbit73.

.

The only software that I know of that will produce CSV output is for the RF Explorer SA and the V1.1 RSP Spectrum Analyser for the SDRplay RSP1A:

http://forum.tvfool.com/attachment.p...3&d=1606249794

http://forum.tvfool.com/attachment.p...4&d=1606249811

Interesting, thanks for the info and images.

Seems the Airspy has such a good reputation, that there would be logging/export software available for it.

As we discussed earlier in this thread, interested in high dynamic range obtainable with either SDRplay RSP1 or Airspy R2.

Tend to favor the Airspy R2 over the RSP1 because it seems to display less spurs (indicating better dynamic range at least in spectral regions where spurs occur).

However, also would like to log and export data.

The specific high dynamic range application does not necessarily require logging, and the logging application does not necessarily require high dynamic range.

Therefore, an option could be to use Airspy R2 for high dynamic range tests and the low cost SDR dongle with RTLSDR Scanner software for logging.

.

I wonder if the Bandscan approach would be suitable for any of your needs.
https://rabbitears.info/tvdx/all_tuners
Map takes a while to load.

https://m.rabbitears.info/index.php?...10311B6D&tno=0

https://rabbitears.info/tvdx/one_tun...0311B6D/tuner0

http://forum.tvfool.com/attachment.p...6&d=1606272557

Yes, thanks for your thoughts on this topic, that approach could be quite helpful.

Having capability for logging signal quality over longer periods of time (at least 24 hours) would help in evaluating the performance hardware design changes.

Think most contributors to RabbitEars Bandscan use SiliconDust HDHomeRun tuners.

Recently purchased one of those tuners for the purpose of recording signal strength and signal quality.

According to Trip at RabbitEars, seems the software that produces the web display is not set up to export raw data to a file format that could be conveniently imported to Excel.

A person who developed a Windows version of that software said it would take some time, but he could add the option to make the export.

.

That is true. Here are Trip's instructions for setup:
Joining the Live Bandscan
https://www.rabbitears.info/static.p..._live_bandscan

It is possible to modify the graph for clarity:

http://forum.tvfool.com/attachment.p...7&d=1606317606

Yes, in many cases, the graphical images could be helpful.

Having the raw data to analyze, when looking for relatively small performance differences (say between two different antenna preamp combinations), could help to ascertain differences of less than one dB.

FYI:

Have designed, built, and tested several low-noise preamplifiers in combination with custom high gain antennas.

It is somewhat straightforward to evaluate relative noise figure of amplifiers, or the relative gain of antennas.

But more difficult to evaluate the performance of a particular preamplifier and antenna combination. The degree of impedance match or mismatch between amplifier and antenna can affect both noise figure and effective antenna gain.

Further complication is (in KY) receiving stations at 100 miles distant, there are large variations in signal levels vs time.

It is difficult to ascertain performance differences, especially when the performance difference may be a dB or even less.

.

It would be necessary to compare then two systems at the same instant because OTA signals constantly vary in strength. I have seen OTA signals vary as much as 10 dB in 30 minutes.

I have more confidence in the results when using a constant strength transmitted test signal for a comparison.
Very true. Keeping the preamp close to the antenna will minimize the mismatch loss. When you compare the two systems, that will tell you which has the superior performance.

Low noise preamps can make a difference on UHF if the ambient noise is below -100 dBm, but they don't seem to make much difference as compared to average NF preamps on VHF because of the higher ambient noise level on VHF, especially VHF-Low.

http://forum.tvfool.com/attachment.p...9&d=1600731992

That would be good if was practical. For accuracy, the two antennas would need to be located at the same physical place. In many cases at UHF, a separation of a foot or so, and the two antenna combos can receive different signals.

Yes, at 100 mile path have seen variation of 10-15 dB in a minute or so.

That can allow measurement of signal strength related to antenna gain.

But doesn't address realized noise figure, or realized signal quality.

In general, short transmission line minimizes transmission line losses.

Transmission line losses contribute directly to signal loss & degrade overall system noise figure.

However, even with virtually no transmission line, mismatch loss between antenna and preamp may still be present.

Mismatch has two main effects:

1. Signal loss which is is a result of reflection and reduces realizable antenna gain.
2. Degrades realizable noise figure of preamp.

The noise figure of the first active device in a preamp is dependent on the active device seeing a specified termination impedance.

Low noise preamps rarely have an input impedance of *real 75 Ohms*.

An antenna rarely has an impedance of real 75 ohms.

A preamp only has a specified noise figure when loaded with a specified complex impedance.
Preamps are usually tested for noise figure when terminated with 75 Ohms real impedance.

In reality, impedances of antennas are not real 75 Ohms and preamps input impedances are not real 75 Ohms.

So when combined;

- The realized antenna gain is less than optimum and
- The realized noise figure of the preamp is not as predicted.


-----------------
*real 75 Ohms*
-----------------

rabbit73, mainly for others benefit, realizing you know this:

Impedances generally are complex numbers, consisting of a real part and an imaginary part.

Usually denoted a R+jX or R-jX.

Where R=real and X=reactance, either inductive (+) or capacitive (-).

For perfect 75 Ohm match, the real part must be 75 Ohms and the reactance or imaginary part must be zero Ohms

.

That is true because the wavefront is non-uniform. I have seen a difference in the scan of a channel when moving the antenna a foot horizontally.

http://forum.tvfool.com/attachment.p...8&d=1606331232

But then, how is it possible for you to make a valid comparison of the performance of the two systems? Have the two antennas on a sliding carriage that moves each antenna into the same location, or what? Otherwise, how can you possibly get a comparison like you would on a lab grade antenna range? On a range, you would use a constant strength test signal and place each antenna, in turn, in the same location.

The closest I got to the ideal was to switch the two antenna locations, but I really wanted to be able to slide them sideways.

http://forum.tvfool.com/attachment.p...0&d=1469629130

The white wall was just a background for the photo. The actual comparison was done at a location with OTA signals that were as constant as I could find. I used an A/B switch for a rapid comparison.

http://forum.tvfool.com/attachment.p...0&d=1606334826

"But then, how is it possible to make a valid comparison of the performance of the two systems? Have the two antennas on a sliding carriage that moves each antenna into the same location? "

would splitting 1 feed to the 2 systems work? Then reverse the feeds to compensate for output differences of the splitter and average the results?

There *must* be something wrong with this idea (lol!) It is just too simple, and I am too ignorant!

But very interesting discussion!