See previous response. A transmission line needs a good match at each end to
control vswr-related reflection uncertainty. Assuming both antenna and EMI
receiver provide that, no extra attenuation is required.
--
Ken Javor
(256) 650-5261
From: Paolo Roncone <paoloc...@gmail.com>
Date: Saturday, July 15, 2023 at 11:17 AM
To: Ken Javor <ken.ja...@emccompliance.com>
Cc: <EMC-PSTC@listserv.ieee.org>
Subject: Re: [PSES] 6dB pad
Hi Ken and Tom
Thanks for your inputs. Yes, the 6 dB pad is at the receiver input, in addition
or not (depending on the signal level) to the built-in attenuator of the
receiver.
If the same 6dB pad is put at the antenna output instead of receiver input,
it's at the other end of the cable connecting antenna with receiver.
Is it preferable, or recommended, or right to consider the 6dB pad only at the
antenna output, rather than at the receiver input? If yes, why, considering
that the whole chain (antenna, cable, pad) is nominal 50 ohm?
Shouldn't the 6dB pad in any case improve the VSWR or return loss of the
antenna-cable-receiver chain? Maybe I'm oversimplifying the picture...
Anyway, assuming the antenna has a good (> 10dB) return loss or good (< 1.5)
VSWR, no need for any pad at all, or better (recommended) no pad at all?
Thanks
Paolo
On Sat, Jul 15, 2023 at 3:30 AM Ken Javor <ken.ja...@emccompliance.com> wrote:
But he is placing the pad at the receiver input in lieu of an internally
selected 10 dB, not at the antenna's output port.
--
Ken Javor
(256) 650-5261
On 7/14/23, 6:00 PM, "T.Sato" <vef00...@nifty.com> wrote:
On Fri, 14 Jul 2023 22:53:41 +0200,
Paolo Roncone <paoloc...@gmail.com> wrote:
> I'm having a discussion with my colleague about the use of a fixed 6dB
> attenuation pad at the input of the EMI receiver for radiated EMI in the
> range 30MHz - 6GHz.
> The pad I'm using is a Weinschel 6dB N-type 50ohm.
> My colleague says the pad is an unnecessary element in the measurement
> chain, because the receiver built-in attenuator (typically with 10dB
steps)
> and proper adjustments in the amplitude settings are enough.
...
For example in case of CISPR 16, CISPR 16-1-4 requires antennas to have
return loss higher than 10 dB.
6 dB pad at antenna output makes this requirement satisfied regardless of
return loss of the antenna itself, even for biconical and hybrid antennas
which will have very bad VSWR.
Regards,
Tom
On Fri, 14 Jul 2023 22:53:41 +0200,
Paolo Roncone <paoloc...@gmail.com> wrote:
> Hi all,
>
> I'm having a discussion with my colleague about the use of a fixed 6dB
> attenuation pad at the input of the EMI receiver for radiated EMI in the
> range 30MHz - 6GHz.
> The pad I'm using is a Weinschel 6dB N-type 50ohm.
> My colleague says the pad is an unnecessary element in the measurement
> chain, because the receiver built-in attenuator (typically with 10dB
steps)
> and proper adjustments in the amplitude settings are enough.
>
> The reason I'm using the 6dB pad is that - based on my EMI testing
> experience, the 6dB pad is a good trade-off between the need to avoid
> receiver overload and maintaining a good enough noise floor.
> Without the pad the noise floor is of course lower and everything is fine
> as long as the input levels are low enough.
> But in the majority of the test scenarios I'm working on the input levels
> are not so low and the 6dB pad is just enough to avoid triggering the auto
> built-in 10dB attenuation, that kicks in when the receiver attenuation is
> in auto mode, and that is oftentimes an overkill, raising the noise floor
> too much, especially in the 1-6 GHz range.
> I have the 6dB pad calibrated for cable loss once a year together with the
> N-cable connecting to the antenna. And also the cable calibrated without
> the pad.
>
> The most typical sources of overload are transient noise generated by
> motors (especially brush DC motors) and signals from radio modules like
> 2.4G wi-fi, Bluetooth and 5G wi-fi.
>
> In addition to overload protection, the 6dB pad improves the VSWR at the
> cable-receiver interface, as explained in C.J.Paul's EMC Introduction to
> EMC (John Wiley 2nd Ed.) Ch.7.
> Based on the above I think that, being a resistive network stabilizing the
> 50ohm termination impedance at the receiver end, the 6dB 50ohm pad is a
> good practice that can prevent overload or excessive auto-attenuation, and
> worst-case doesn't do any harm. Or, am I missing something here?
>
> I'd like to know if there is someone else in this community who has
> experience using fixed attenuation pads at the receiver end of EMI
> receivers/spectrum analyzers, or any opinion about it.
>
> Thanks a lot in advance for any feedback!
>
> Paolo
>
> -
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