Steve, N4LO, wrote, wondering:
> Jim. That was good! Now, define "conversion".
As Spock might have said, "Fascinating"!
Superheterodyne principles were "developed" or invented
by Mr. Armstrong as one way to increase selectivity.
Frequency conversions were done to other frequencies
where it would be more easy to narrow the bandwidth, reducing
the noise contributions. But, other problems are generated
as a result.
It was learned, real quick, that such conversions also would
introduce "image" and "alias" signals into the passbands if
such frequency conversions were not very well planned. These
added "signals", which could be mistaken for real ones, were also
called "spurious products". Whatever, they were/are a nuisance
as well as is the noise that comes along with the heterodying!
The needed engineering study for a "new" rcvr design, at least
this part of it, is called the "frequency plan"; I did many such decades
ago while employed and paid real money, hi (would help to have some
of that coming in now to buy a new Orion!).
The frequency plan must accomplish two prime objective goals:
maximize the probability of rcvg'ing desired signals in the needed
frequency bands, and eliminate ALL spurious (non really there signals)
from those same needed passbands. Maximizing the probability of
reception requires necessary signal to noise ratios, which requires
bandwidth control, and the elimination of any "false" signals from within
the rcvg apparatus itself as well as added noise from the added processes.
Every probable signal to be rcvd and every Local Oscillator added to
effect frequency conversion will caused an infinite series of alias/
spurious frequency signals at (f1 +/- n f2) and at (f2 +/- n f1) .
Third order products are at 3f1 and 3f2 and are ALWAYS the most
troublesome, because they can ALWAYS fit inside the passband, no
matter how narrow you might design it --- try a few numbers and you
will see this to be true. This is why the 3rd order intercept point
specification for a given design is SO important. Two strong signals
in the passband will generate third order products within the same pass
band! And, every added oscillator to the rcvr to effect these heterodynes
bring along another fn which creates more intermodulation products!
Therefore, a "ham" band only rig is more easily designed to be
"birdie" free (spurious signal free) than is a general coverage
rcvr. But it can be done in both. That is a prime goal reached by the
Ten Tec RX-340 general coverage rcvr-- you just have to look real
hard to find birdies.
We hope the Orion frequency plan will be as carefully done!
I can find birdies, and did, in both the Omni VI+ I owned, and
the Corsair II I still own. In fact, did so today while finding a good,
weak beacon to run the previously reported experiment. But
I have found essentially NO birdies in the RX-340. It is a Mil-spec
rig and could NOT be allowed to have them in the Gov's frequency
ranges of interest, hi. The couple that I have found are way down
near the noise level, not so in the others. I posted those test results
back in January on the TT reflector.
Another fact of heterodyne rigs is that the noise in the frequency
range being converted to some other range, goes right along
with the signals, hi! So noise is NOT being eliminated with
heterodyning.
So, the question: what is "conversion"?
Answer: It is another way to WAY complicate the receiver design,
and must be VERY carefully planned and implemented; otherwise,
more mischief will result than desired outcome! If the frequency
plan is well done/implemented, and excellent rcvr results!
Final point: just because, in a DSP core rig the A to D converter,
or the DSP processor itself is spec'd with more bits; that is
32 bit vs. 16 bit processor or 24 bit vs. 16 bit A/D converter,
does NOT mean better performance. What results in better/
worse performance is the accuracy to which the bits represent
real sample values of the analog signal amplitude!! Now that
IS a horse of an entirely different color and design problem!
What determines when a bit "flips" from 1 to 0?? That is the
question, hi. The REAL accuracy of such is somewhere
near to 16 to 18 bits.
Enough for tonight from here.
73, Jim KH7M
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