W2VJN Explains the Inrad FT-1000MP 70Mhz IF Mod

 
In the MP and Mark V the second mixer (at 70.455 MHz) has a high noise
floor.  The RF section of the radio has a very good noise floor, but it also
has low gain.  Because of this, the second mixer and following IF stages
contribute substantially to the overall receiver noise floor.  The ARRL
tests (and Inrads) have shown the noise floor in a 500 Hz filter to be -128
dBm or so depending on band and whether the Tuned or Flat mode is selected.
(This is a noise figure of 19 dB, for those that prefer this type of
measure.)  Tuned is the preferred mode for most kinds of operation.  Also,
it has been widely stated that the MP in its unmodified state does not hear
as well as other radios.  The worst band is 21 MHz (and below) as there is a
separate preamp in line on 28 MHz which helps greatly.  (On 21 MHz pulling
the 8 MHz filter out produces no change in the measured audio output noise.
i.e. The front end is not overriding the IF noise.)

Spurious free dynamic range is defined as the "difference between the
maximum allowable input signal and the systems minimum detectable signal".
Note that it includes two factors.  Minimum detectable signal is usually
taken as the noise floor, in dBm.  Maximum allowable input signal is that
level which produces IMD at the noise floor.

The 1000MP and Mark V have a 70.455 MHz crystal filter which has a bandwidth
of 12 kHz.  The SFDR for signals outside this filter bandwidth is determined
by the "strength" of the first mixer and RF stages.  The Inrad mod is
inserted after this filter and just prior to the second mixer. The mod gain
is adjusted to 8 +/- 0.5 dB during final test.  So, the maximum allowable
signal within the 12 kHz bandwidth is reduced by 8 dB.  However, the
improvement in noise floor as measured in several MPs has been 4 dB.  Thus,
the SFDR outside 12 kHz in not changed and inside 12 kHz it is reduced by 4
dB.  My one measurement on a Mark V showed a noise floor improvement of 6 dB
with the Inrad mod installed.

What does this all mean?  There is a small degradation in strong signal
handling within the 12 kHz filter.  If this is a problem, use the IPO or the
ATT to restore the strong signal handling and poor noise floor.  The IF hiss
will not return when this is done as the gain redistribution in the radio
doesn't change.  The actual result is that the improvement in noise floor is
very noticeable whereas the small degradation in IMD dynamic range is not.
Most of the time, the benefits of the mod will outweigh its drawbacks.

Aside from all the lab tests, I installed a mod in my radio which had an
IN/OUT relay controllable from the front panel.  For a year I switched the
mod in and out hundreds of times.  I operated two DX contests for over 40
hours out of 48.  I did a CW SS contest and a couple of 160 meter tests.  I
heard IMD many times, but there was never a time when switching the mod out
made any noticeable difference.  The position of the NB control has a great
effect on IMD.  If it's turned even part way up the IMD is substantially
increased.  This is easily noticed in a crowded band.

Much of the time a noise floor of -128 dBm is adequate.  However, if band
conditions are quiet and you are running an antenna which has good
directivity, you will miss weak signals.  We learned this years ago when the
first solid state radios came and had high noise figures.  The Collins S
line with its 4 dB noise figure would hear signals which simply were not
there in the TR-7 and other radios.  The TR-7 had a noise floor a dB or 2
better than the MP.

This all started because I could not stand the IF hiss in the MP when
operating for long periods of time.  The improved noise floor was a bonus
which, frankly, I wasn't thinking about at that time.  The combination of
the Inrad mod and the K9AN capacitor mod have been installed in thousands of
radios.  This is not very scientific, but it is widely accepted as an
improvement to the radio.

George, W2VJN