Marty Ray, N9SE, asked:
"Anyway, the reason I responded is to ask you to elaborate on
the IMD. Can you explain IMD in terms of dBm? If I have a
receiver spec of, say, +20dB IP3, and an IMD3 of 100dB, what
does that mean in terms of actual signals? Can you explain using
my real world 160m example. Let's say two S9+60dB signals
mixing into my passband. How strong would the resulting spurious
signal be?"
Wow, two 9+60 dB signals quite near by; where do you live? Hi.
Here is how to determine the IMD level possible from those two
signals.
Step 1. The 3rd Order IP you specify is +20 dBm. Your two +60
dB over signals are at a level of -13 dBm as you state correcly.
Step 2. Third order intercept point theory then says that the
resulting IMD products from those two gigantic signals will
be "exactly" twice as far below the power level of the two
generating signals as those two signals lie below the intercept
point power level. Or, in this case, the two powerful signals
lie 33 dB below the IP3 level (the delta in dBm between plus
20dBm, the 3rdIP level and -13dBm the power level of the
two interacting signals. Therefore, the IM products generated
will have an "in-band" power level 66 dB below -13 dBm, or
they will be at a power level of only -79 dBm (25 microvolts)!
That is an S8 level spurious signal!! Not good.
So that would NOT be a good situation in which to be operating, hi.
Now, more realistically, we can determine what is reasonable
from the IMD3 dynamic range spec. And, just for fun, let's use
the Ten Tec published typical specs on the Orion as now listed
on their web site. In addition to the IP3 and IMD3 numbers, we
will also need the MDS, minimum discernable signal spec.
For the Orion, TT gives three bits of pertinent info about the
MAIN receiver:
IP3rd: +24 dBm typical for two signals using an IF bandwidth of
500 Hz, and the Pre amp OFF.
For the IMD3: 101 dB typical also preamp OFF and and for both
20 and 5 kHz intermodulating signal spacing.
But, no MDS signal level is given for a CW signal in the 500 Hz
bandwidth with the preamp ON, which is what we want to reach
the MDS level potential of the main rcvr!
TT gives the following sensitivity info for SSB signals:
<0.18 µV for 10 dB SINAD at 2.4 kHz BW, pre-amp on typical;
<0.5 µV for 10 dB SINAD at 2.4 kHz BW, pre-amp off typical.
OK, so we are going to have to do a bit of jiggering and estimating,
will try to be conservative, hi. So here goes given the info we
have to work with.
We can go straight to a probable MDS number from using
the IP3 and IMD3 specs given. However, we must assume
that the preamp is turned on to reach the MDS level, and
since no gain is listed for the preamp, I will assume it to be
+10dB. (Note, however, that even a 10 dB gain preamp
only will lower the MDS level 7 or 8 dB because of the
randomness of the noise level as that minimum signal
discernment level.)
Also, when the preamp in turned ON, the IP3 level, for a
10 db gain preamp, will drop exactly 10 dB, or from the
typical +24 dBm level given, dropping to "only" +14dBm typ.
(Note: you can check the impact of preamp on or off on both
the MDS and IP3 levels by seeing the numbers the ARRL
lab collected on several rigs in the comparison chart that
Elecraft has posted, you will see that my above assumptions
are about correct. Again, check out:
http://www.elecraft.com/K2_perf.htm )
Now, with the 10 dB gain preamp ON, our Orion operating IP3
will be typically around 14dBm.
Also, note that the 101 dB typical IMD3 level is given also with
the preamp OFF. Looking at the Elecraft chart of typical rig
numbers, we see that turning a 10 dB gain preamp ON, will
decrease the IMD3 range from 1 to 2 dB or so depending upon
which rig the ARRL has measured. Lets say the Orion IMD3
given number will drop to 99 dB IMD3 with the preamp ON.
Now we can calculate a "probable" and typical MDS for the
Orion looking at a CW signal in a 500 Hz bandwidth IF set up.
We can first estimate the MDS level by fooling around with
the given info for SSB signals operating using the spec'd
2.4 kHz bandwidth above.
With the preamp ON, TT gives a sensitivity level of less
than 0.18 microvolts (-122 dBm) using 2.4 kHz bandwidth IF
with the SSB signal level +10 dB over the noise level (that
SINAD stuff). So, in the 2.4 kHz bandwidth a CW signal
could be "just" discerned down at -132 dBm, probably.
Now, we can lower the MDS some by narrowing the
IF bandwidth; the noise level will drop 3 dB each time
we cut the bandwidth in half from the given 2.4 kHz. Or,
for 1.2 kHz BW, the MDS will drop to -135 dBm; and
for an IF BW of 600 Hz, it will drop again to -138 dBm,
and of course a bit lower for the 500 Hz BW of interest,
maybe to -139 dbm (.03 microvolts). Measurments down at
these levels are tough and require pretty fancy test gear as
does the ARRL lab). Note that the ARRL labs lists several
of the rigs tested with MDS levels of -139 dbm or so
operating with 500 Hz IF bandwidth and the preamp ON..
With this estimated MDS level for the Orion in the given conditions,
we can now check for the levels of signals which would cause
spurious response signals at the MDS level. This defines the
IMD3 number. TT says the IMD3 level is 101 dB with the preamp
OFF, but we turned it ON, so we guess the IMD3 will degrade
just a bit to, say, 99 dB (seems fair given the data on the other
rigs tested). Or the strongest spurious free signal pairs the
Orion can handle will be at a level of about -139 dBm + 99 dB;
or at -40 dBm (2236 microvolts) or about S9 + 33 dBm.
We can check now using the IP3 number to see if all of these
numbers and guesstimates "compute", hi.
So, how far below the IP3 number (+14 dBm with the preamp ON)
is our -40 dBm signal? Answer: 54 dB. Therefore, spurious
signals generated with this operating IP3 number should lie
2 x 54 dB below -40 dBm. Or, the spurs should be down at
-148 dBm..........?
Something does not compute! Why, our use of the given and
guessed at numbers gave us an MDS level of -139 dBm or so;
and the IP3 number, with the preamp ON says the spurs are
9 dB further on down. So to bring the spurs up to the MDS
level, we should be able to increase the strength of the two
intermodulating signals up 4 or 5 more dB, or to a level of about
-35 dBm. Or the Orion, per this calculation can handle signals,
in the 500 Hz BW, CW mode, up to S9 + 38 dB based upon
the Intercept Point numbers and all else we have calculated/
estimated before intermodulation occurs to clutter the passband.
Perhpas the Orion IMD3 is even better than the 101 typical
number under the conditions just calculated here. It would
seem the IMD3 number becomes the delta between -139
dBm and -35 dBm, or 104 dB..... Recall that using our
"assumed" degraded IMD3 number with the preamp ON, we
used only 99 dB, which brought the strong signal level up to
-40 dBm from -139 dBm.
So, we find a 5 dB or so delta between our presumed IMD3
number with the preamp ON, and what we seem to calculate
using the MDS we infer and the given typical IP3 number
with the preamp ON. That is pretty good agreement using
our various assumptions and typical only given numbers.
We will just have to wait to see what the actuals will be
spec'd by Ten Tec, and what the ARRL labs measure off
in the future some time.
Hope this long piece helps Marty and anyone else interested
in what is done with such dynamic range spec numbers, hi.
Maybe someone should check my assumptions and number
crunching also, hi.
73, Jim KH7M
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