[Elecraft] XTAL AND DSP simply is the best design

[Bill Coleman]

On Mar 4, 2005, at 2:07 AM, WA2JJH at aol.com 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: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901