I carried out a further
test once I'd repaired the sticky pointer on my HP
Wattmeter. Previously I'd measured total noise power from
the initial circuit as +1.4dBm but checking this measurement
with the 7.5 volt zener gave me areading of -1dBm**. I then inserted
my 30MHz low pass filter to see the level within the range seen
by a standard shortwave communications receiver and measured
-13.6dBm. What do these numbers mean in practical terms? This
is not straightforward because of the nature of the noise output
but let's say to give a reading on the power meter of -13.6dBm
at 7MHz would require a discrete CW signal of about 47mV or roughly
S9 +60dB. The total power of -1dBm is similarly equivalent to
200mV and the previous value of +1.4dBm about 260mV. Comparing
with the Nooelec gave readings of +12.8dBm total and -5dBm filtered.
All these figures are shown below for clarity. At first sight
(by comparing total power and filtered power), the Nooelec is
producing more power at frequencies beyond the capability of
my noise source, in the region 2 to 10GHz.
How do these numbers relate
to the spectrum analyser scans above? Looking
at the shortwave scan the signal strength of the noise at
7MHz is -62dBm and, taking account of the 20dB attenuator, this
represents about -42dBm which is about 1.8mV or roughly S9+30dB.
This represents a reduction of about five S-points. In terms
then of using the homebrew noise source for shortwave receiver
alignment the equivalent noise signal is about the same as that
from a signal generator producing 1.8mV. Therefore, to reduce
this level to that of a weak signal requires 40dB to 60dB of
attenuation.
** Note that this discrepancy
was probably due to the effects of instability and oscillation
at 1.2GHz |