Signal Quality and the RF Front End

538

RF Front End

Did you know that every digital system has what is termed as an “RF Front End”?

They do! The RF front end refers to the signal reception portion of anycommunication system – both analog and digital systems. The RF front end consists of the antenna and the cables that deliver the signal to the receiver. With satellite the antenna is the dish, and with OTA broadcast it is the TV antenna.

Each individual’s in-home system performance is determined by how well the signal is received at the RF front end. It is the integrity or quality of the arriving signal delivered to the receiver that will determine the performance of any digital system.

The goal of a properly designed system is to maintain the integrity of the signal as it is received, decoded, and presented. Everything about these systems (performance, reliability, and picture quality) is dependent on the quality of the signal you have to work with.

The measure of performance of a communications system is called signal quality or signal fidelity. Signal quality is a simple relationship of signal power to noise power. It is called, signal-to-noise ratio or SNR.

See attached

Signal quality (SNR) meters are found in many television receivers and are typical with satellite systems. The value usually expressed on signal meters is not signal strength, but signal quality, or SNR. Some receivers have both signal strength and quality readings.

I hear many of you saying, “We are using digital systems and BER (bit error rate) is what counts.” Truly, BER is the term applied to describe digital system performance. What we need to realize is that BER is a function of SNR. In other words, SNR determines BER. They are tied together and cannot be separated. (I’ll post “Interpreting BER?” next.)

See attached

As SNR increases, BER decreases (good). As SNR decreases, BER increases (bad). What we have failed to realize is that as the SNR decreases, the number of errors and the rate of errors increases. The greater numbers of errors that result from lower SNR all have to be corrected or covered by the FEC (forward error correction) algorithms – the coding and decoding of the signal.

The greater the amount of errors that need to be corrected in a bit stream, the further from the original quality will be your end result. FEC corrects or ‘diminishes’, or hides errors to the best of its ability. For it to correct errors there needs to be enough good redundant information to replace or correct the corrupted bits. In the absence of all the right stuff, FEC uses its tricks and tools to interpolate (guess) the correct information from what’s left. The more errors you have means the more guessing the FEC has to do. Every guess is one step further from the best quality available.

There are two ways to affect the signal quality or SNR. The first way is to increase signal strength. The signal quality (SNR) will be increased and the BER greatly decreased, by even a small increase in the signal strength. The second way to increase signal quality is to reduce noise. Either of these actions will result in greater signal quality and better performance, but the “biggest bang for the buck” is increased signal strength.

Don’t fall for the “as long as you have lock” mentality. First maximize your signal, then pay close attention to proper installation to further preserve the signal received. Every bit of noise injected by lack of grounding, poor connections, or improper materials, robs you of just a little more of the all important signal quality.

10 Comments
  1. Fringe Reception says

    highdefjeff wrote:

    … “Signal quality (SNR) meters are found in many television receivers and are typical with satellite systems. The value usually expressed on signal meters is not signal strength, but signal quality, or SNR. Some receivers have both signal strength and quality readings.” …

    … “There are two ways to affect the signal quality or SNR. The first way is to increase signal strength. The signal quality (SNR) will be increased and the BER greatly decreased, by even a small increase in the signal strength. The second way to increase signal quality is to reduce noise. Either of these actions will result in greater signal quality and better performance, but the “biggest bang for the buck” is increased signal strength.” …
    ——————————————————————

    THANK YOU for these MOST informative postings and furthering my edumacation. :hail:

    In my difficult reception location (it was almost hopeless before digital, primarily do to multipath) I have found imperically, that signal strength is KING and if I somehow overload my sets’ front end, that signal can be attenuated (hasn’t happened yet). Your explanation confirms exactly how I have accomplished receiving HDTV here and its contrary to another engineer’s advice which was “try a paperclip”.

    Secondly, I am limited to the onboard diagnostic tools on my Sony Bravia, which report ‘signal strength’, S/N ratio and bit error. Do you know if these “measurements” are accurate or of any value? That’s what I have worked with, with much success, but the above mentioned engineer does not believe the onboard reports are of any value. Thanks in advance,
    Jim

  2. highdefjeff says

    Thank you for the compliment, Jim. It is my pleasure to help.

    You wrote: “Secondly, I am limited to the onboard diagnostic tools on my Sony Bravia, which report ‘signal strength’, S/N ratio and bit error. Do you know if these “measurements” are accurate or of any value?”

    These instruments are of the ultimate value, as your results indicate. Their accuracy may be contested, but as long as they give you any relative change (you want up for signal strength and or SNR, or down for BER) then you have all you need. Those indicators are your gauges of system performance and they are included because these values have direct relationship to the function of your system.

    Any increase that you can make to signal strength will increase SNR (S/N) and decrease BER. Any reduction in noise that you can achieve (best connections, proper wire, single wire, grounding, etc.) will also increase SNR and decrease BER, but won’t affect signal strength. You can’t change or affect BER without affecting SNR first (BER is just the translation to digital terms).

    In terms of BER and SNR, only a small increase in SNR (most easily achieved by increased signal strength) results in dramatic decrease in BER. The problem is that the converse is also true. Only a slight misalignment of a dish or antenna will greatly increase BER, and that is especially not good using the “as long as you have lock” mentality.

  3. ProjectSHO89 says

    Jeff,

    Don’t overlook EVM and multi-path as contributors to the degrading of signal quality.

    SNR, by itself, is not what counts. It’s the combination of SNR with the over-all quality of the signal that must be considered. I can take a crappy signal, amplify the heck out of it so it has great strength and SNR numbers on my Sencore, and it will still be crap with lousy BER and peak-valley indications and won’t decode. It’s the GI-GO situation.

    Further, the latest chip-sets are getting better in dealing with the effects of multi-path such that they can still recover from signal deficiencies that would have resulted in no reception only 3 or 4 years ago. One would hope that there will be further advances in this area.

  4. Fringe Reception says

    So, it seems my Summer antenna work at home is now vindicated according to highdefjeff and thank you!

    Go for the “carrier” and get it as strong as possible. So, what’s new about that??? It seems logical and the same theory since my low band (ham) and VHF days … (except for one local engineer’s DTV ‘paperclip’ suggestions …)

    Again, I have achieved receiving over 30 OTA channels at my ‘impossible’ location by literally going after their signals with hi-gain multi-bar home-built Yagis – (no paperclips included, nor batteries).

    My home-brew antennas work for me and signal strength VS noise is KING! Currently, ZERO bit errors on every station I receive, same as it was this summer. Rock solid.

    Q: Are any others here aside from EV and myself CURRENTLY building and or testing antennas here?

    Thanks and best regards (73).
    Jim

  5. IDRick says

    I build DIY mclapp 4-bays. EV is using a similar design to the mclapps. I test my antennas using an APEX 502 converter box which has both signal strength and signal quality meters. All my stations have 100% Signal Quality. The signal strength meter is indeed measuring signal strength and is highly correlated with margin to video dropout (amount of attenuation to cause dropout). Signal quality remains at 100% until the margin to dropout drops below 6 dB. See attached graph.

  6. rabbit73 says

    Secondly, I am limited to the onboard diagnostic tools on my Sony Bravia, which report ‘signal strength’, S/N ratio and bit error. Do you know if these “measurements” are accurate or of any value? That’s what I have worked with, with much success, but the above mentioned engineer does not believe the onboard reports are of any value. Thanks in advance,
    Jim

    Jim:
    You made a smart move when you bought the Sony Bravia, because you not only got a DTV but its diagnostics screen that gives very useful information to solve reception problems. I have made a comparison of the Sony Diagnostics Sceen with a CECB and a STB using my SLM and an attenuator:
    AVS Forum – View Single Post – The Official AVS Antenna Topic!

    As far as I’m concerned, in order to do better you would have had to buy a Sencore SLM1456CM which also gives MER that tells you how close you are to the “cliff.”

    1. Fringe Reception says

      Jim:
      You made a smart move when you bought the Sony Bravia, because you not only got a DTV but its diagnostics screen that gives very useful information to solve reception problems. I have made a comparison of the Sony Diagnostics Sceen with a CECB and a STB using my SLM and an attenuator:
      AVS Forum – View Single Post – The Official AVS Antenna Topic!

      As far as I’m concerned, in order to do better you would have had to buy a Sencore SLM1456CM which also gives MER that tells you how close you are to the “cliff.”

      ——————————————————————
      Rabbit, thank you. +++

      Before purchasing our Bravia, I spent many hours online comparing different ‘sets’ and I ultimately decided to buy the Bravia. My research and our purchase was not based on the name brand at all.

      As you probably already know, at my location it was virtually impossible to receive any analog TV since long before I was born. Nevertheless, using specifically designed antennas and attenuators, ultimately, I was the only person in this area that could receive decent OTA analog TV. All others gave up and bought (payed month after month after month) … for cable.

      Digital happened and WOW! In spite of being told by one online Engineer to stick a paperclip in my F socket … um … I have established 30+ feeds, using antennas. Imagine that!

      Thank you for verifying the onboard Sony tools I used and continue to use are not “fluff” and they actually have value. They were all I had to work with from the start of this endevor and so far, the results are pretty darned good!
      Jim

      1. rabbit73 says

        Rabbit, thank you. +++…..Thank you for verifying the onboard Sony tools I used and continue to use are not “fluff” and they actually have value. They were all I had to work with from the start of this endevor and so far, the results are pretty darned good!

        You’re welcome, Jim. I’m glad to have been able to add a vote of confidence for your choice.

        Before purchasing our Bravia, I spent many hours online comparing different ‘sets’ and I ultimately decided to buy the Bravia. My research and our purchase was not based on the name brand at all.

        I too didn’t decide by brand. I am fascinated by instruments that measure and couldn’t resist the diagnostics screen.

        …..at my location it was virtually impossible to receive any analog TV since long before I was born…..ultimately, I was the only person in this area that could receive decent OTA analog TV…..Digital happened and WOW!…..I have established 30+ feeds, using antennas. Imagine that!…..

        Your research and the application of what you learned have paid big dividends in good reception in a difficult location and have given you the satisfaction of a job well done. You have earned the right to say: “I didn’t know it couldn’t be done, so I did it!”

        As mentioned by highdefjeff, your Sony SNR measurement is a good substitute for BER measurement and it gives an early indication of the deterioration of BER that doesn’t show as increasing errors (when measured after correction) until the last moment because the FEC is correcting as many errors as possible until it fails:
        Digital Terrestrial – Signal Measurement

        MER (modulation error ratio, which is similar to SNR) also gives an early indication of deteriorating BER, but it is only available on the more expensive measuring instruments.
        BER VS MER in attachment.

  7. highdefjeff says

    Jeff,

    Don’t overlook EVM and multi-path as contributors to the degrading of signal quality.

    SNR, by itself, is not what counts. It’s the combination of SNR with the over-all quality of the signal that must be considered. I can take a crappy signal, amplify the heck out of it so it has great strength and SNR numbers on my Sencore, and it will still be crap with lousy BER and peak-valley indications and won’t decode. It’s the GI-GO situation.

    Further, the latest chip-sets are getting better in dealing with the effects of multi-path such that they can still recover from signal deficiencies that would have resulted in no reception only 3 or 4 years ago. One would hope that there will be further advances in this area.

    Project,

    Thank you for reminding us that with digital processing, GI-GO applies. This has gotten lost for some of us because of the “all or nothing” mentality of those marketing digital television, and because of the inadequate and incorrect information that is taught to satellite installers.

    It was not my intent to overlook EVM (Error Vector Magnitude) or multipath effects, but to provide a basic understanding of the factors that can be affected by the home user to benefit their reception and performance.

    Multipath is certainly part of the signal degradation equation but, to my knowledge, cannot be changed at home. EVM is, very basically, another measure of signal quality and of best use to design engineers. What I attempt to do is to simplify digital science to understandable information that can be utilized in the field resulting in the best quality end product.

    When you say “SNR, by itself, is not what counts. It’s the combination of SNR with the over-all quality of the signal that must be considered.”, I take some exception. To be clear, SNR (Signal to noise ratio) is a wholly complete measure of signal quality in the home, just as EVM is complete as a signal quality measurement useful to design. In reality, when we view EVM in terms of BER, we get the same performance curve that accompanies viewing SNR in relation to BER.

    Since the tools that are likely to be in the hands of the installer or home enthusiast are a signal quality meter and maybe a signal strength meter, SNR is what we have to work with. And with SNR there are only two components of the equation. The first is signal power (that we can great effect) and the second is noise power (that we can somewhat effect). Whatever ISN’T signal can be called noise. Actually, whatever degrades signal is broadly called noise.

    Regarding amplification, it must be understood that increasing a poor quality signal gives you more poor quality signal, just like you said. Amplification increases both signal power and noise power. Amplification won’t necessarily increase the quality of your signal. What’s more, the addition of the amplifier will increase the noise in the system which will adversely affect signal quality (SNR). Amplification doesn’t give the same beneficial effects as increasing signal strength (power) or reducing noise power. What amplification can do is increase a small amount of ‘quality” signal to a usable level. This is why amplification is used to overcome long cable runs that attenuate signal, not to increase quality.

    1. Don_M says

      Please stick around and hold that thought: We might need you to explain these principles again the next time someone writes in to ask why, at 20 miles with LOS and an over-sized antenna on the roof, he loses stations every time he adds yet another amplifier onto his antenna system!

Leave A Reply

Your email address will not be published.