----------
>From: Muriel Bittencourt de Liz <mur...@grucad.ufsc.br>
>To: Lista de EMC da IEEE <emc-p...@ieee.org>
>Subject: Information about EMC receiver
>Date: Fri, Jul 21, 2000, 1:59 PM
>
>
> Hello Group,
>
> There are some doubts about some parameters of an EMC receiver that I'd
> like to understand better:
>
> - IF bandwith: What it mean/is? How do I set it up? If I have a HF
> signal, what IF is better? Or if I have a low freq. signal, what is the
> better IF?
ANSWER: IF bandwidth usually refers to the bandwidth of the final stage of
down-conversion (just before detection) in a super-heterodyne receiver. How
you set it up depends on the receiver. Older receivers had a fixed
bandwidth (varied with tuned frequency but not user selectable) just like an
AM/FM/SW receiver. Later models had NB/WB settings, or switch selectable
options of maybe two or three filters. Nowadays spectrum analyzers offer a
spectrum of bandwidths limited at the low end only by local oscillator
residual FM, and at the high end usually at or above 1 MHz, typically these
will be soft key selectable. Use that bandwidth which most closely models
the radio receiver protected by the emission limit. In the case of
commercial emissions tests, these have been defined by CISPR to be 200 Hz
below 150 kHz, 9 kHz from 150 kHz to 30 MHz, and 120 kHz above 30 MHz. A
theorem of communications theory is that use of any other bandwidth
different from that of the signal being received degrades measurement
sensitivity.
>
> - Measurement time: If I don't know (a priori) the noise frequencies,
> how do I set the measurement time?
ANSWER: As a minimum, you have to dwell at the frequency for one filter time
constant, which is the multiplicative reciprocal of the filter bandwidth.
Issues related to the nature of EUT operation may require much longer scan
times.
>
> - Frequency Step: What is the step that I should use?
ANSWER: The maximum step size allowable is one-half filter bandwidth, in
order to not degrade accuracy by more than 3 dB (assuming 3 dB filter
bandwidths).
It follows directly from the above two answers that minimum sweep time
varies as the square of measurement bandwidth. That is, if you increase or
decrease the bandwidth by a factor "a", the sweep time is increased or
decreased a factor "a*a", respectively.
>
> Any comments, URLs, PDFs or tutorials will help.
>
> Thanks in advance for the help and Regards
>
> Muriel
>
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