Topband: Topband Receivers

[John Harden, W4NU]

Dear All,

The recent discussion about topband receiving has been very
interesting.....

Noise is a subject of its own as we well know. Composite noise (we tend
to call it
phase noise) is a combination of synthesizer phase noise, broadband
amplifier noise
and close-in spurs (see the OMNI VI-Plus review in November 1997 QST).

The ARRL lab has tested both the FT-1000MP, and the OMNI VI-Plus. The
reviews are
on the ARRL Members web-site. The noise floor of a receiver is the level
of input signal that gives a desired audio output level that is equal to
the noise output level. The more negative the sensitivity number
expressed in dBm the better the receiver.

The "minimum discernable signal" (MDS) is the parameter measured here.
At 1.82 MHz the MDS for the Omni VI-PLus was measured to be -131 dBm.
The FT-1000MP was -127.5 dBm with the Preamp OFF. 
The OMNI VI-Plus utilizes a voltage-controlled crystal oscillator. The
5.0 to 5.5 MHz synthesizer in the OMNI VI-Plus gets its 10 Hz tuning
resolution from the VCXO. THe phase noise characteristics depend on the
design of the synthesizer. Its output is divided by 40 to get the 5.0 to
5.5 MHz range. This division by 40 reduces the output frequency and also
reduces the phase noise and spurious levels by a substantial amount.
Ten-Tec specifies the OMNI VI noise floor as -133dBm @ 2.4 KHz
andwidth.

There have been many studies to determine how deeply we can hear a
sinusoidal tone that is buried in noise. I won't go into this here, but
it (like the above) is "all in the books".

73,

John, W4NU



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