[BC] Accurian HD receiver test

[Mark Humphrey]

I ran a few digital sensitivity tests on the Accurian HD receiver last
night and came up with some interesting results on several local
stations.

For a signal source, I used a rotatable nine-element FM yagi
(Jerrold?) about 30 feet above ground level, feeding about 50 feet of
RG-6 CATV-type cable.  I ran this through a Kay Elemetrics 70 ohm
variable pad (which can insert up to 41 dB of attenuation in 1 dB
steps) then into the FM input of the Accurian receiver.  I positioned
the antenna for maximum signal, then added attenuation until the HD
indicator dropped out.  Then, I backed off the attenuation in 1 dB
steps until HD locked back in, and listened for at least a minute to
make sure the receiver was not "blending" back to analog.  As
expected, the threshold could be defined within 1 dB, due to the
digital "cliff effect".  The Accurian S-meter would either show 5 bars
or no bars.

After determining the digital threshold, the output of the pad was
connected to a calibrated spectrum analyzer (R&S FSH-3) set for 300
kHz RBW and the FM carrier level was measured.  This procedure was
repeated for each station.

Here are the threshold numbers I obtained from six Philadelphia
stations.  The required attenuation is shown in parentheses:

88.5 WXPN   -84 dBm  (28 dB)

90.1 WRTI    -71 dBm   (2 dB)

93.3 WMMR -77 dBm   (28 dB)

98.1 WOGL  -69 dBm   (18 dB)

102.1 WIOQ -80 dBm   (30 dB)

106.1 WISX  -76 dBm   (24 dB)

Also, I checked WAWZ "Star 99.1" in north-central New Jersey.  WAWZ is
first-adjacent to WUSL 98.9 in Philadelphia; therefore, its lower
group of digital sidebands are obliterated by strong interference.
On the analyzer, WUSL's FM carrier was about 10 dB higher than WAWZ's,
even after orienting the antenna for the best tradeoff between the two
signals.   Surprisingly, WAWZ's digital threshold measured near
average at -77 dBm, after inserting 7 dB of attenuation.

As shown above, the worst performance was obtained while tuned to 98.1
WOGL, and the best from 88.5 WXPN.   A probable explanation for WOGL's
poor performance is that they use a side-mounted aux antenna to
transmit the digital signal, while the analog transmitter feeds the
main ERI "cogwheel" antenna at a higher elevation.  I'm on the back
side of WOGL's aux antenna and behind a ridge, so the digital signal
is not very good in this direction.  WXPN may have done exceptionally
well because they don't have any secondary HD channels, while all of
the other stations transmit an HD-2.  Although this shouldn't have any
effect, I can't offer a better explanation for WXPN's superior
performance.

Bottom line:  The average "digital threshold" (not including WOGL,
which clearly has an antenna problem) measured 77.5 dBm = 42.5 dBf,
approximately 37 microvolts across 75 ohms.

This is about 6 dB worse than the "50 dB stereo quieting" spec on most
high end tuners, (which generally falls around 36 dBf) and about 24 dB
worse than the typical "50 dB mono quieting" sensitivity of modern-day
receivers.

I'll let you draw your own conclusions!

Mark