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Spectrum analyser in a high traffic strong signal environment
开云体育DW, ? You can confirm what the expected total power of multiple signals is by the usual formula:? Ptotal ?= ?P of one signal + 10Log N ?where N = number of identical powered signals.? In your example, 10 signals of -30dBm each produce a total combined power of -20dBm.? However, there’s a ‘gotcha’ lurking here for the unwary.? As @nj902 indirectly alluded, there may be other (larger) signals present outside the frequency band that your instrument is observing.? Observing off-the-air signals, this is likely the case.? The only way to be sure is to use a broadband power meter and then tuneable filters to only pass the signal band of interest to your SpecAn’s input. ? A quick and ‘accurate’ way to confirm ?that your instrument’s front-end is not suffering from overload (and IM issues) is to increase the input attenuation 10dB and observe that ALL signals reduce by 10dB. (You should insert another 10dB attenuation to be sure.)? If some, or all of the signals drop by more that 10dB – you’ve got IM and overload present ! ? As an aside, measuring the total power of the 3-30MHz HF spectrum ?with even a half-decent antenna is very instructive.?? With some Asian HF broadcast stations having ERP’s in the order of many megawatts using directional arrays, it’s no wonder many receivers suffer overload and image problems. ? ? Cheers, ? Kit |
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Lothar baier
开云体育Good pojnt and actually one that is often not considered , spectrum analyzers ARE wideband and as a result signals that fall within the bandwidth of anything before the mixer WILL affect the mixer even if they are outside of the selected center frequency and span ! There is HOWEVER one caveat to this , if you are using a microwave analyzer above 2.9GHz than things become less severe , this is because the MW path of your analyzer incorporates a YIG filter which not only suppresses signals outside the useable BW but also acts a limiter, granted the rejection is not as good as with a tuneable mechanical or cavity filter but its better than nothing ? Sent from for Windows ? From: [email protected] <[email protected]> on behalf of Kit via groups.io <kitscally@...>
Sent: Wednesday, March 2, 2022 5:54:46 PM To: [email protected] <[email protected]> Subject: Re: [HP-Agilent-Keysight-equipment] Spectrum analyser in a high traffic strong signal environment ?
DW, ? You can confirm what the expected total power of multiple signals is by the usual formula:? Ptotal ?= ?P of one signal + 10Log N ?where N = number of identical powered signals.? In your example, 10 signals of -30dBm each produce a total combined power of -20dBm.? However, there’s a ‘gotcha’ lurking here for the unwary.? As @nj902 indirectly alluded, there may be other (larger) signals present outside the frequency band that your instrument is observing.? Observing off-the-air signals, this is likely the case.? The only way to be sure is to use a broadband power meter and then tuneable filters to only pass the signal band of interest to your SpecAn’s input. ? A quick and ‘accurate’ way to confirm ?that your instrument’s front-end is not suffering from overload (and IM issues) is to increase the input attenuation 10dB and observe that ALL signals reduce by 10dB. (You should insert another 10dB attenuation to be sure.)? If some, or all of the signals drop by more that 10dB – you’ve got IM and overload present ! ? As an aside, measuring the total power of the 3-30MHz HF spectrum ?with even a half-decent antenna is very instructive.?? With some Asian HF broadcast stations having ERP’s in the order of many megawatts using directional arrays, it’s no wonder many receivers suffer overload and image problems. ? ? Cheers, ? Kit |
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In case anyone is interested, there is some amazing new technology that came out using some interesting magnetic materials that automatically filters out any frequencies above a certain threshold power level.? Imagine automatic notch filters that zero in on whatever signal are the highest.? You end up with all spikes in the frequency domain at or below the threshold.? I heard about this a few years ago; the company is called Metamagnetics. Maybe not relevant here, but it sure is interesting! Jim Ford
------ Original Message ------
From: "Lothar baier" <Lothar@...>
To: "[email protected]" <[email protected]>
Sent: 3/2/2022 6:15:24 PM
Subject: Re: [HP-Agilent-Keysight-equipment] Spectrum analyser in a high traffic strong signal environment
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Hi Jim:
Interesting.? It seems that the weak (desired) signal is attenuated about 4dB while the strong signal is knocked down to the limiting level.? See: The device makes use of circulators/isolators to maintain a reasonable VSWR. I looked since a simple diode limiter does the same thing, except it knocks down all signals and has a high VSWR. -- Have Fun, Brooke Clarke, N6GCE axioms: 1. The extent to which you can fix or improve something will be limited by how well you understand how it works. 2. Everybody, with no exceptions, holds false beliefs. |
开云体育IIRC, Metamagnetics considers the 4 dB the insertion loss.? In an interference-limited receiver, that would probably be acceptable. Jim Sent from my T-Mobile 4G LTE Device -------- Original message -------- From: "Brooke Clarke via groups.io" <brooke@...> Date: 3/3/22 2:11 PM (GMT-08:00) Subject: Re: [HP-Agilent-Keysight-equipment] Spectrum analyser in a high traffic strong signal environment Interesting.? It seems that the weak (desired) signal is attenuated about 4dB while the strong signal is knocked down to the limiting level.? See: https://patents.google.com/patent/US20190267960 The device makes use of circulators/isolators to maintain a reasonable VSWR. I looked since a simple diode limiter does the same thing, except it knocks down all signals and has a high VSWR. -- Have Fun, Brooke Clarke, N6GCE https://www.PRC68.com axioms: 1. The extent to which you can fix or improve something will be limited by how well you understand how it works. 2. Everybody, with no exceptions, holds false beliefs. |
开云体育I don't think so.? Seems like that would defeat the purpose.? Of course, linearity is an illusion after all, so some intermodulation is inevitable. Jim Sent from my T-Mobile 4G LTE Device -------- Original message -------- From: "Ted S via groups.io" <wr4t@...> Date: 3/4/22 2:24 PM (GMT-08:00) Subject: Re: [HP-Agilent-Keysight-equipment] Spectrum analyser in a high traffic strong signal environment Ted WR4T |
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Lothar baier
开云体育Diode limiters do create IM products and the same also applies to YIG filters which also act as limiters if a certain power is exceeded !It however doesn’t matter much in a practical sense as the job of a limiter usually is to protect a mixer or amplifier from being damaged? On Mar 4, 2022, at 17:54, Jim Ford via groups.io <james.ford@...> wrote:
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Kit