[Premium-Rx] WJ8718A i.f. filters

[Michael O'Beirne]

Dear Massimo

The question of appropriate IF filters is very much, as we say in England, 
"Horses for Courses".

Until fairly recently there was a bewildering choice of IF filters both 
professional and amateur, and at substantially different prices.

The majority of radio hams tend to favour fairly narrow SSB filters to cut 
through potentially acute man-made interference from other stations close 
by.  The same filter used to listen to a strong commercial signal or a 
military SSB channel would sound unpleasant to me, more like a cat being 
strangled.

I like plenty of bass and a good treble audio response.  Such a filter would 
require a -6dB bandwidth of about 250Hz to 3.2kHz.  You will find such 
filters used by W-J, Racal, R&S and others and they tend to be expensive.

Such filters also tend to have less phase distortion than the cheaper 
narrower amateur filters.  This is the distortion that is caused because the 
delay that is caused to the signals passing through the filter is not 
constant across the bandwidth.  There is inevitably a greater delay for 
signals at the edge of the passband than at the centre.  This leads to a 
"muddy" and confused sound.  It is similar when listening to music at a 
concert hall or church which has a big dome (such as London's Albert Hall 
some years ago or St Paul's Cathedral) because the dome creates a delay of 
several seconds depending where you are sitting.

Narrow mechanical filters tended to be the worse for this.

There are special (and very expensive) filters called "phase-compensated 
filters") that are so designed as to minimise this distortion, usually so 
that fast FSK may be received.

I have compared ordinary crystal filters with phase compensated crystal 
filters on my big Marconi H2540 receiver.  The phase compensated ones have a 
wider -4dB bandwidth (300 to 3,300Hz) but a narrower -60dB bandwidth (+300 
to -4000 Hz) and sound incomparably better.  Unfortunately they were very 
expensive.  I have a quote from Marconi for about 350 pounds sterling for 
the USB one and 1,200 pounds for the LSB one. (I did not pay those prices I 
assure you).

Current DSP techniques have virtually made conventional IF filters redundant 
because they are cheaper and offer far superior ultimate selectivity and 
phase distortion responses.  As an example the 2.7kHz SSB bandwidth on the 
STC STR8212 has a -3dB bandwidth of 2.7 kHz minimum and a -100dB bandwidth 
of 4.5kHz maximum and has a true linear phase response.  No conventional IF 
filter I know of can match that.

73s
Michael
G8MOB




----- Original Message ----- 
From: "Massimo Corinaldesi" <m.corinaldesi at mclink.it>
To: <Premium-Rx at mailman.qth.net>
Sent: Thursday, June 17, 2010 3:26 PM
Subject: [Premium-Rx] WJ8718A i.f. filters


>I read the data sheet of the WJ-8718A hf receiver, and I was surprised
> to see the shape factors of the i.f. filters are very high.
> I think in the years the 8718A was conceived - I suppose first half of
> '80 -, the technology allowed i.f. filters having better shape factors.
> Why they did so ?
>
> That's just a technical curiosity.
>
> Thanks.
>
>   Massimo Corinaldesi  I0MCF
>   Roma
>   Italy