[1000mp] Keep the NB Control Fully CCW

[1000mp@mailman.qth.net]

Bob, K6XX wrote:

:Have you ever noticed the noise blanker thing? (Where it has to be fully
CCW to avoid distortion). W8JI keeps jabbering about it, but I do NOT see
it here; others have publicly disagreed with him, but he persists. It
does not occur in my (1996) MP."

and Steve, K8WK wrote:

"Nor mine...not even close...only does the distortion bit when it's on. 
When off it has zero effect..."
==========
The NB thing (with the NB turned off) is not distortion in the received
signal, but rather an increase in IMD when the NB control is not fully
CCW.  Following are the messages posted by George, W2VJN about the
problem:

On 02 Feb 1999 in a reply to Clive, GM3POI, W2VJN wrote:

"The noise blanker problem is not as bad in the MP.  In the 1000D the
noise blanker amplifier is run at full gain even when the NB is off  and
the thresholding is done at it's output.  In the MP, the NB control
adjusts the gain of the amplifier.  So, the simple solution to avoid any
bad effects from the NB amplifier is to
keep the NB gain control fully CCW when it's not in use.  This has the
same effect as the W8JI mod for the 1000D."

Also, on 01 Jan 2001 George, W2VJN wrote:

"The increase in IMD when the NB pot is turned CW can only be heard under
certain conditions and the change is quite subtle.  Since it's a third
order effect, the IMD will appear at 2F1 - F2 and 2F2 - F1.  Thus, if
there are signals at 1830 and 1832 which are both strong enough to cause
IMD above the noise floor of the radio, the IMD products will occur at
1834 and 1828 kHz.  Both signals need to be within the bandwidth of the
70.455 MHz roofing filter, which means they can be no further than about
10 or 12 kHz apart.
 
My lab instruments showed the third order intercept degraded from +14.5
to -7 dBm from having the NB knob fully CCW to fully CW with tone spacing
of 10 kHz.  In order for the IMD to reach the noise floor in the first
case, the interfering tone levels would need to be about -34 dBm and in
the second case they would be about -48 dBm.  If the S meter is
calibrated for S9 = 50 uV or -73 dBm, these tone or signal levels would
be S9 + 39 dB for the CCW case and S9 + 25 dB for the CW case.
 
The two radios tested here were vintage 1995 and 1999 and both exhibited
this performance.  There is no question that the MP will show third order
IMD at some signal levels, all radios do.  It's easiest to note the NB 
problem if the  knob is set fully CW and the radio is tuned to the
correct frequency to observe the
IMD as noted above.  If you find two signals strong enough to hear the
IMD, turn the knob CCW and most likely the IMD will go away.  I've
observed it in real on the air situations many times.
 
The MP has a very strong front end and IMD is not heard very often."
==========
As George points out, with the NB control fully CW, two signals spaced 10
kHz would have to be (where S9 = 50 uV) S9 + 39 dB on SSB and S9 + 25 dB
on CW to produce third order IMD in the MP.  With the NB control fully
CCW, the same two signals would have to be 21.5 dB stronger (if I've
figured correctly) to produce third order IMD.

I once briefly tried looking for IMD products with the NB control fully
CW on a crowded band during a contest, but heard none -- however I will
always opt for the better IMD figure and keep the NB control fully CCW
when not using the NB.

73, de Earl, K6SE