Inline
On Jun 1, 2007, at 10:56 AM, [EMAIL PROTECTED] wrote:
I assume that receiver circuit characteristics before the roofing
filters (or IF bandpass filter) essentially determine the dynamic
range. However, dynamic range figures "always" seem to degrade for
small separations of fc and fc+x. Is this because fc and/or fc+x are
falling within the IF passband and are producing spurious signals in
the stages following the roofing filters?
You have to consider which stages set the dynamic range of the
receiver. In a well designed receiver it should be the first mixer
that sets the dynamic range (not the RF amp, if one is used, the
filter itself, the post-mixer amp or any of the IF or AF stages.
Yes (it's fc) that does the "damage" if it gets by a wide roofing
filter to the post-filter amp.
An in-band measurement will give you some info on the first mixer and
the crystal filter (the later starts to become an issue for mixers
with a very high intercept). See Sherwoods measurements going down to
2kHz seperation (though for CW signals this should still be outside
the first filter).
It's a good idea to do the measurement at multiple seperation and to
ALWAYS quote the separation with the dynamic range.
For the case of strong signals in the IF passband, are the dynamic
range tests run with the AGC disabled? In actual operation, wouldn't
the AGC reduce signal levels below the point where distortion products
were being generated (in the stages following the roofing filters)?
You normally have no AGC elements before the first mixer. It's
usually all after the crystal filter as the filter a delay in system
that will cause problems in getting the AGC loop well controlled.
Usually the AGC if off to make the measurement though.
The two tone test is only a proxy for the "real world". In actual
operation, if any signal (or signals) in the RF passband or mixer
passband exceeds the receiver dynamic range, will ALL of the signals
in the passband begin contributing third order products?
Yes, but only the strongest signals really make an impact. The
spurious signals are third order products so they vary as the cube of
the input power. Or their slope when you plot a graph in dB is three
times steeper than the input power. The spurious signals increase
twice as fast (on a logarithmic plot) as the input power increases.
The intercept point is the power when the spurious signal is the same
power as a single test tone. You can't actually get to that point --
gain compression takes over 10 or 15dB before you hit that point.
This is a more serious issue for CW with discrete, narrow (50Hz),
tones. The spurious signals often sound like "bad" morse code when
two CW signals intermod together -- they're the product (the AND in a
logical sense) of one or more CW transmissions. In other cases they
can sound like a good CW signal (say a strong CW signal
intermodulatinging with a SW broadcast carrier) on the wrong frequency.
For SSB there is less of an issue (in general). The power is spread
over a 3kHz so the components are typically -18dB down on the same
power CW signal (3kHz/50Hz) so you are less likely to hear the
problem in the same RF environment. When it does manifest it's more
like the noise floor increasing in stregth than discrete signals
though in the case of an SSB signal intermodulating with a strong
carrier you would hear a distorted SSB-like signal.
You can do the calculation to determine what a passband looks
(sounds) like when using "non-equal power" tones for the DR test (or
as you know them, the ham bands). Wes Hayward does it in his
"Introduction to RF Design" book.
Is the magnitude of the third order distortion products a function of
the "degree" of the nonlinearity?
Yes. This non-linearity is what controls the intercept points -- the
measure usually quoted for amplifiers, mixers and other devices (the
3rd order intercept point is the default but of course there is a
second order intercept point that is important too :-)
Can different receiver models with
the same dynamic range numbers (operating under the same conditions)
differ considerably in the "signal strength" of junk signals?
By "junk signals" you mean spurious signals from (2nd or) 3rd order
IMD so the answer is no. Assuming they measure the same at the
But of course there are other differences (phase noise which might
mask these effects) and other sources of spurious signals.
I'd recommend reading Peter E. Chadwick, G3RZP, QEX article "HF
Receiver dynamic Range: How Much Do We Need?" (QEX May/June 2002,
p36-41).
A good receiver needs a dynamic range of about 100dB according to a
study by Peter E. Chadwick, G3RZP HF Receiver dynamic Range: How
Much Do We Need? which appeared in the May/June 2002 issue of QEX.
The number 100dB for phase noise limited dynamic range given by
G3RZP refers to "SSB bandwidth" which means that the noise floor of
a good receiver has to be below -132dBc/Hz (3dB S/N loss, 3kHz
bandwidth).
When you have high dynamic range receivers then you get more and more
interesting in phase noise!
In Europe you can and do see -10dBm signals at the antenna port
around the 40m band (they're SW broadcasters). You might see similar
levels in an urban environment from local hams (or other sources) too.
<http://www.nitehawk.com/sm5bsz/linuxdsp/optrx.htm>
<http://www.nitehawk.com/sm5bsz/digdynam/practical.htm>
--
73 DE N7WIM / G8UDP
Kevin Purcell
[EMAIL PROTECTED]
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