Gary wrote:
> I agree George with all your comments about the effects of
> linear vs "less than linear" circuits.
After George wrote:
> > Location of the filter(s) *should* have no effect on the overall
> > performance of the IF chain, in fact of the entire receiver,
> > *provided* that the entire system is operating linearly. In
> > a cascade of linear systems, the overall transfer function
> > is the product of the individual functions, etc. The trick, of
> > course, is to maintain a cascade of truly linear sub-systems....
> > This suggests that the filters are located "up front" in a deliberate
> > effort to allow the use of subsequent stages with a lower
> > overall dynamic range than would otherwise be required.
So...what limits linear operation these days??? What is the
transfer characteristic of the front end components?? A diode
in a switching matrix... a JFET or diode array in some sort of
mixer driven by an LO delivering + 10dBm of power....
Let's have a look at what Ten Tec might be using in the new and
coming Orion rig. Let's suppose they have no cost reason not
to use components similar to those in their RX-340, which George
has mentioned.
The first semiconductor junction to be encountered is in the front
end switching matrix: In the 340 these are MCOM PIN diodes,
MA4P7001F-1072T; do better front end diodes exist which
could have been used instead, and at what added cost?? The
wide dynamic range input signals must traverse two of these
diodes into and from the bandpass preselector filters.
To compensate for losses in the input bandpass filters and any
switching circuit losses, the signals next are applied to a high
level, push-pull FET amplifier made up of six MMBFJ310LT1, each
an N-ch JFET device. Do not know if the Orion will have such
a loss compensating amplifier system. These devices are spec'd
to 450 MHz, by the way.
In the Orion, to which George seems to be referring his comments
(the use of narrow crystal lattice filters near the front end) the first
active devices following the front end switching arrays seems to be
an RF preamplifier (see: http://www.doug-smith.net/orion.htm ).
Why would it not be the same as the one in the RX-340? Which
is a Philips Semiconductors - PIP - BFG16A; NPN 2 GHz wideband
transistor. This provides + 10 dB of gain when selected, or may
be bypassed, or the signal may be switched through an attenuator
using two more of the same diode switches as that in the front
end preselector, above described.
So far, we seem to have "state of the art" linear dynamic range
front end components. Lets go on to the next element which
could introduce non-linearity....the first signal mixer. In the
RX-340, this mixer is "high level" balanced mixer of four diodes:
BAT68_04 which are Infineon Tech Silicon RF Schottky
diodes designed specifically for mixer applications.
Now, at this element, the Orion departs from the RX-340. This
first mixer is described in the piece referenced above:
" A high level JFET mixer is employed. An amplifier is used at
the local oscillator (LO) port to reduce the LO level coming
from the synthesizer itself." Why a "high level JFET" is used
instead of the Schottky diodes as used in the 340, I don't
have any idea.....cost, even better performance/linearity???
Now, immediately after this first mixer in the Orion, as in the
RX-340 appears the "roofing" filter. But, in the RX-340 there
is first, right out of the first mixer, another set of six high level,
push-pull JFET amplifiers, identical to that described above
following the input switching matrix network. Also, the roofing
filter in the 340 is a pair of cascaded two-pole filters which
are 20 kHz wide; their purpose is to filter out and reject the
unwanted mixer image plus spurious mixing products and the
2 nd mixer's image frequency---this filtering MUST be done
in any superheterodyne system!! Why?? Because "mixing"
by its' very nature is a NON-linear operation. If you use a
mixer for frequency heterodyning, you are using a non-linear
process! The question is: what is the transfer function
"linearity" of amplitude response between the transposed
frequencies? And by products arise from the mixer heterodyne
process -- they are called mixer images and spurious products
made up of integer values (of sums, differences and multiples)
of both the signal and local oscillator frequencies.
The issue here: every rcvr must at this point have 1st mixer image/
spurious mixing products reject and 2nd mixer image frequency
filters which can be wide or narrow: why should not the Orion
use narrow filters, selectable by the operator to suit the band
operating conditions???
My point: the bandwidth of the filter at this point in a heterodyning
receiver frequency plan is NOT an issue, in and of itself to
criticize, as George seems to be implying. A filter here at this
point is just plain needed! And needed even if every single
RF front end component to this point is operating absolutely
linear (which of course, the mixer cannot be, or it could
not do it's job of frequency heterodyning!).
The argument that filters in the front end of a heterodyning
receiver are not needed if "everything" is absolutely linear
is just not so. The mixer is not linear, so an image reject
filter is NEEDED. And it might as well contribute to the
narrowing of the spectrum of energy to be handled by
the entire following system as well as doing the job of image
and spurious signal product rejection. To imply that the use
of narrow, muti-pole crystal lattice filters following the first
mixer is a "cover up" for poor linear design characteristics
of the front end or following stages, is just not being technically
honest.
Lots of words, hope this is understandable.
73, Jim KH7M
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