Understanding receiver sensitivity (12db SinAD) in uV mesarement.

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by N8MJS, Aug 12, 2015.

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  1. N8MJS

    N8MJS Ham Member QRZ Page

    I was comparing the receiver sensitivity of a Yaesu 2900r vs a Kenwood TM281A.
    The 2900r says
    Sensitivity (12dB SINAD):
    Better than 0.4uV
    The Kenwood 281A says
    Sensitivity (12dB SINAD)
    Less than 0.18µV

    Based on the specs and my limited understanding it appears the Kenwood would have the better receiver.
    I'm basing this on the lower the input voltage needed to achieve the given level of SINAD, the better the receiver performance should be.
  2. WA8ZYT

    WA8ZYT Ham Member QRZ Page

    .4 microvolts is -115 DBM and .18 is -122 DBM, so a 7 db difference in sensitivity.
  3. WB3BEL

    WB3BEL Ham Member QRZ Page

    Like most things, amateur radio equipment documents including product brochures are a mix of specifications and advertising.

    For the most part these documents contain very little which can be considered guaranteed parameters. The documents are littered with phrases like "typical" or "expected" or "nominal".

    One measure of receiver performance is as you mention receiver sensitivity. It is not the only metric or maybe even the most important metric depending on the way the equipment is expected to be used. It measures the weakest signal that can be received at a specific signal to noise ratio in the absence of noise or interference and when operated into a matched antenna transmission line system. If you live where there are a lot of strong signals, having the most sensitive receiver may mean you can not receive the signals you want because the interference is dominating. Other specifications like dynamic range or adjacent channel suppression or third order intercept point indicate how well a receiver can handle strong signals.

    In the case of the two radios you mention it might be good to look up the archived QST test results of these units performed by the ARRL lab. These are available online to ARRL members. The 281A was reported in the May 2012 issue. The 2900r was reported in the Sep 2010 issue.

    For the sensitivity of the 2900r the ARRL lab measured 0.2uV for 12 dB SINAD. This is 6 dB better than the "spec" you mentioned.
    The 281A sensitivity was 0.14uV.

    So the sensitivity of the units measured in the ARRL lab was slightly better for the 281A versus the 2900r by about 3 dB.
    The interference handling specifications as measured by the ARRL lab also look a bit better for the 291A.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sensitivity is measured in a closed, shielded laboratory environment and unless that's how you're going to use it (very unlikely) doesn't mean much.

    Once you connect an antenna, receivers with untuned front ends like these rigs are bombarded with so much junk -- including in-band and out-of-band -- they cannot use most of the sensitivity they have; too much sensitivity causes overload, IMD, adjacent channel rejection problems and all kinds of ills we'd usually be better off without.

    It's the one parameter I'd completely ignore, unless the application was using an extremely narrow beamwidth antenna system aimed at the sky, such as e.m.e.
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    You should be aware that there exist two ways of expressing sensitivity in microvolts; one which uses the emf behind the output impedance or "soft microvolts", which is European standard and some Japanse manufacturers use it, and another which uses the p.d. or "hard microvolts" which is the potential difference over the receiver input.
    This convention is mostly used in the US.
    The "hard microvolt" figure looks 6 dB better than the equivalent emf, which is one of the reasons that it is commonly used in advertising.
    It is quite likely that the ARRL lab measures sensitivity using the "hard microvolt" standard, which makes both transceivers very similar.
    In the vast majority of the terrestial applications for FM equipment nowadays external noise will also be the limiting factor for weak signals.

  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You'd think so; actually in most part of the U.S., radio interference, desensitization and intermodulation products caused by strong local signals both in-band and outside the band are usually the limiting factor.

    Didn't used to be that way when 2m rigs covered "only the 2m ham band" and had zero coverage outside the amateur band, so they used multiple helical resonator front ends. Today, almost none of them do that and they all receive well outside the band, making them prime targets for horrendous problems if you're line of sight to paging systems, commercial repeaters and all sorts of stuff.
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    The proliferation of strong adjacent band signals and bad RF selectivity certainly makes it worse, but the problem is still there even if there is very good filtering ahead of the receiver RF stages. I have seen some measurements made by PMR developers of the urban and suburban noise temperature in the 160 MHz band,
    starting in the late 60's and continuing for some 3 decades.

    The noise temperatures in the suburban areas were in the 300-500 K range in the 60's increasing to more than 2000 K in the 90's. 2000 K is 8 dB over the thermal noise, - 174 dBm/Hz, corresponding to a noise floor of -125 dBm in a typical FM bandwidth and a 5 dB receiver noise figure. Compared to the -129 dBm without the external noise contribution. A "desense" of about 6 dB caused by external noise contributions is about the amounts that are routinely measured when dealing with modern marine VHF base stations.

    A most unfortunate development for us who grew up in an environment where the sensitivity of a VHF receiver with an antenna pointed to the horizon by definition still
    was limited by thermal noise.

  8. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    At H.F. frequencies, SENSITIVITY is just about the EASIEST thing to achieve, and is a very poor indicator of overall performance in real world conditions. Since about 1960, every H.F. receiver out there has far more sensitivity than can possibly be used....and that's WITHOUT an optional front end pre-amp
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    True enough, but what he listed are 2m FM rigs.

    Also more sensitive than they'd ever need to be unless you're truly in the middle of nowhere with no other VHF signal sources around in any direction.
  10. N8MJS

    N8MJS Ham Member QRZ Page

    I'm a ARRL member and cannot find the online section for ARRL lab results on the ARRL website.
    I would be most interested in finding this section for further information.

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