On Mar 4, 2005, at 2:07 AM, [EMAIL PROTECTED] wrote:
You look at the TS-870, 746pro, and many other DSP rigs all make the
same
stupid design flaw.
Dual conversion DSP! No multiple pole xtal filter.
No computer algorythm can emulate a multipe pole IF filter.
That's not true. In theory, it is possible for DSP filters to produce
filters with far, far better characteristics than multiple-pole crystal
or mechanical filters.
The real problem is reducing it to practice. You'd need DSP functioning
at RF frequencies -- not AF as many rigs today. But, as the sample rate
goes up, so does the required processing power.
An audio-level DSP is a compromise.
The TS-850 had dual xtal IFs. However there was much junk from all
the
CPU's and yes up conversion, then downconversion and demod after the
2nd IF
filter adds to the noise floor.
The 850 is still a great radio.
How the R-7A did it was interesting. A 4 pole Fixed Xtal filter
followed
the upconverter.
It has a pass band of 12kcs. That's right a 12Khz wide IF.
The 2nd IF used those 8 pole can filters.
This is typical for modern transceivers with general-coverage receive.
Most of them have a first filter of 15 kHz, typically to support NBFM.
They usually have good dynamic range over 20 kHz splits, but this
degrades markedly at 5 kHz. That's why the Orion and the IC-7800 have
multiple roofing filters.
The K2 wisely put much non xtal filtering right after the antenna
jack.
The band pass filters are wide enough for the Ham bands and very
little general
coverage. You lose general coverage ability of an R-7A. However do you
really need a short wave rcvr in a ham rig?
It's certainly a specific design decision. Most solid-state
japanese-made rigs from the mid-80s touted general-coverage receive as
a feature. Other manufacturers have seen this benefit as well. Ten-Tec
had a micro-processor controlled rig with general coverage receive. It
had some problems in the receiver chain because of all the junk put out
by the heterodyne PLL.
Suddenly, it occurred to someone to go back to a crystal-controlled
heterodyne oscillator. This meant losing general-coverage receive and
using a bunch of crystals. But, for a brief time, the Omni-V was one of
the best receivers on the market.
The K2 is a much more clever design -- still no general-coverage
receive, but it covers all 10 MF/HF ham bands quite well.
Bill Coleman, AA4LR, PP-ASEL Mail: [EMAIL PROTECTED]
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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