[CQ-Contest] More on rig comparisons (IC746) and narrow bandwidth

[00tlzivney at bsuvc.bsu.edu]

Several have asked about the ICOM IC746.  Here are the numbers, same
format:

IC746 - March 1998

                 3k      5k     10k      15k     20k     30k
DR               73      77     88       94      101     99
Recip            81      85     90       92       95     98
TX Noise         75      81     87       91       93     96

All of the measurements made by Peter Hart use the rigs normal SSB
filtering, which is pretty much the same from rig to rig in terms of
noise bandwidth.  Two tone dynamic range is defined as the difference
between the input intercept of the receiver (the point where two
interfering tones spaced, say 3kHz or 5kHz or ..., create a spurious
response of equal strength in the passband of the receiver and the
noise floor of the receiver.  The noise floor is a function of the
noise figure of the receiver and the ultimate bandwidth.  Thus, using
a cw filter with a 500 Hz bandwidth will improve the noise floor by
the log of the ratio of the bandwidths, 20log(SSB/CW), or about 12-14dB
for a typical set up.  That is why others often report dynamaic ranges
of 100+dB.  The second reason, is that the wider spaced tones, such
as 30kHz or 50kHz, as used by ARRL, are so far apart that at least
one of them is OUTSIDE of the roofing filter in the receiver.  Thus,
those measurements will only show overload of the front end, while the
close in measurements made by Peter Hart, and summarized in this series
of notes, reveal the entire overload picture.  Since CONTESTERS have
to deal with big signals much closer than 30kHz to 50kHz away, I believe
the close in measurements better represent the limitations of the 
receiver for contesters.  

The reciprocal mixing figure shows how loud a single tone, spaced at
the measurement distance, must be relative to the noise floor of the
radio to raise the noise by 3dB.  In practice, this means you will hear
the other guy's cw as noise in your radio.  If the problem is caused
by your radio's noisy oscillator(s), it is termed reciprocal mixing.
Since most rigs use the same oscillators for transmitting (hence, the
term transceiver), the noisy oscillators, combined with broadband noise
from the transmitter stages, can also cause this effect in an otherwise
clean receiver.  This also can be heard as noise following the other
guys transmitter.  The IC701 was FAMOUS for this!  In general, the
TX noise and the Reciprocal mixing numbers will tend to "track" each
other, although some rigs, such as the TS940, have a great deal of
additional TX noise not directly caused by their noisy oscillators.

I hope these brief comments help explain the underlying numbers. 

My original reason for the post was to demonstrate that the super-duper
new radios were not necessarily better radios, just had better bells-and-
whistles.

Terry Zivney, N4TZ/9
n4tz at arrl.net


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