Topband: Re: Minimal discernable signal

[Tracy, Michael, KC1SX]

W5PR:

> I read this with interest, but I would like to know what it means
> more in layman's terms.  What is "BDR" and "IMD DR"?

Chuck,
     That's a good point!  :-)   Since these things are quite significant to
160, I'll try to explain:

Dynamic range in general is the difference between the weakest signal that
can be perceived and the strongest signal that can be present without
adversely effecting the weakest signal.

In the case of ham rigs, the weakest signal is generally considered to be
one that is equal in power to the receiver's internal noise, although most
folks can pick out weaker signals using the "filter" between their ears.

BDR stands for Blocking Dynamic Range and it refers to a condition where the
weak signal is "blocked" or suppressed.  In layman's terms, you'll often
hear this described as "desense" because it reduces the effective
sensitivity of the receiver.

BDR as a lab measurement normally refers to the point where the weak
(presumed desired) signal is reduced by one dB by the presence of a strong
(presumed undesired) nearby signal.  The frequency difference between the
two is the "spacing".  The blocking dynamic range is a measure of the
difference between the receiver's noise floor and the level of the signal
that caused the blocking condition.

A "noise-limited" measurement means that the undesired "blocking" signal
caused an increase in receiver noise output before the "desense" effect was
observed.  Usually this is caused by interaction of the signal with the
phase noise of the internal oscillators (phase noise being energy that every
oscillator has close to its desired output frequency) - that's why rigs with
a high transmit composite noise (most of which is phase noise usually) often
also have noise-limited dynamic range measurements.
Some folks consider this to mean that a "real" BDR measurement can not be
made for that rig, but in the ARRL Lab we believe that the _effective_
blocking dynamic range on a noise-limited measurement is where the noise
increases by 1 dB, which is the same change in the signal to noise ratio
that you get when the signal decreases by 1 dB.

So if you see a rig where the sensitivity (aka MDS) is -140 dBm (which is dB
relative to a mW, in case anyone didn't know) and it has a 20-kHz spacing
blocking dynamic range of 125 dB, that tells you the level of signal that
caused blocking 20-kHz away was -15 dBm.  Now, all S-meters are different,
but it is convenient to use the Collins standard for discussion purposes.
In that standard, S9 = -73 dBm (or 50 microvolts, assuming a 50-ohm system).
Therefore, -15 dBm would be about S9+60.  Quite a strong signal, but
certainly not unheard of and many rigs don't have a BDR that high.

Intermodulation is when two or more signals mix, creating false signals that
aren't really there.  These false signals are referred to as intermodulation
distortion (IMD).  For lab testing purposes, it is most convenient to use
only two signals (requires the fewest expensive generators) for this test.  

IMD dynamic range is the difference between the receiver's noise floor and
the level of two nearby signals that caused a false signal to appear right
where we are trying to listen.  The process of mixing makes the highest such
false signals appear at a frequency spacing equal to the difference of the
two signals.  If you are listening on 1820 kHz and you have strong signals
at 1840 and 1860 kHz, you will hear a false signal, since the two are spaced
20 kHz apart and you are 20 kHz below the "lower" one.  Someone listening on
1880 will also hear the same false signals.

As with BDR, IMD DR can also be noise-limited.  In this case, the effect on
the frequency you are trying to listen to is being created entirely by the
interaction of the nearest strong signal and the receiver's phase noise and
oscillators.  Sometimes the result is just an increase in noise and
sometimes it can be a signal.  However, in both cases, the more "distant"
generator is turned off when the IMD is observed and you would find the IMD
is still there.  In the case of non-noise limited measurements, if you
remove either signal, the IMD goes away.

Now, the important thing to note is that these lab measurements don't
exactly duplicate real-world conditions because only unmodulated carriers
are used and you usually find more than two signals near the one you are
trying to listen to.  However, they are a good yardstick for comparing
real-world performance of different radios.

73, Michael, KC1SX

(I think maybe I should collect these posts together for a web page...)


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