[BC] AM Stereo On A Mono Radio-FM Loudness

[Paul Christensen]

> It was once explained to me that the FCC van
used a receiver that they 'calibrated' for each
measurement. 

In the early '80s, either BE or BM/E provided a very informative write-up on
the FCC's field method of modulation measurements.  If I am not mistaken,
Dane Ericksen was used as a source for the article back at a time when he
was an FCC inspector. 

Here's the way I remember it:  an audio oscillator, FM modulator/exciter,
frequency counter, RF spectrum analyzer, precision tuner and oscilloscope is
used for FM modulation measurements.  

First, the audio oscillator is fed into a test modulator/exciter and
adjusted to one of the Bessel function audio frequencies that corresponds to
the FM carrier null at +/- 75 kHz deviation (100% FM modulation).  Next, the
oscillator amplitude is gradually increased until the FM carrier dips to a
minimum while observing the spectrum analyzer.  This sets the 100%
modulation point and is used to normalize the oscilloscope.  

Most scopes have a percentage graticule.  To calibrate the scope, the "Y"
input channel is briefly switched to ground and the scope's swept trace is
placed on the 0% graticule line.  The discriminator output of the FM tuner
is then routed to the scope's Y channel input, generally with DC coupling.
While leaving the oscillator frequency and level untouched, the scope's
variable-amplitude input is adjusted to read 100% as read on the scope's
graticule.  The scope input is briefly killed to ensure that any DC-restore
on the input does not affect the zero reference point.  This can be done
with several iterations to ensure that both the 0% and 100% modulation
reference points are perfectly accurate.

Next, place the scope in X-Y mode such that the scope is now reading
modulation as a straight vertical line.  Although a swept trace works, it
actually makes observation of brief peak occurrences difficult to see. 

Finally, the tuner can be swept up and down the band, one station at a time,
to observe modulation of the entire market.  It's important to ensure
centering of the discriminator for best results. 

Ultimate accuracy is primarily a function of received signal strength,
presence of multi-path distortion and audio oscillator level.
Interestingly, accuracy is not dominated by the audio oscillator's Bessel
frequency.  

Even at great distances, it's possible to accurately measure FM modulation
provided that no co-channel or alternate channel IX is present in the I.F.
passband.  Tuners with selectable I.F. bandwidth (e.g., Technics ST-9030)
and a highly directional antenna are indispensible for this purpose.   

Today, I would be surprised if the FCC still had regional enforcement staff
with the wherewithal, resources and time to make such detailed measurements.


Paul



====================================
Paul Christensen, CPBE CBNT
LAW OFFICE OF P. B. CHRISTENSEN, P.A.
3749 Southern Hills,  Jacksonville, Florida 32225
Office: (904) 379-7802    Facsimile: (904) 212-0050
pchristensen at ieee.org