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Subject: Re: IC-706 Service Manuals are HERE!!!
From: (Gary Coffman)

Ok, here's the computer simulation info I promised. I modeled
the 60 MHz to 200 MHz bandpass response. To say the least, it
sucked rocks. There are two poles, one at about 136 MHz and the 
other at about 160.5 MHz. Response is nice and flat from 60 MHz 
to 120 MHz with less than 4 db of ripple, but it takes a dive from 
there. In the 130 MHz to 170 MHz range, the loss is at least 36 db,
rising back to only 30 db at 200 MHz. Image rejection at 280 MHz
is only 26 db.

Now the ideal fix is to change the inductors so that L49 is 27nH,
L40 is 22 nH, and change C152 to 30pF. That gives a lovely passband
across 60 MHz to 200 MHz, and gives strong image rejection at 280 MHz 
and up, with one of the nulls falling right on that frequency. The 
factory ought to do that.

Unfortunately, examination shows these coils aren't easy to modify. 
So there's another way that still gives a pretty good response. If 
you remove C53 and C153, you'll get full response, though with a dip 
of 4 db in the 160 MHz to 180 MHz range, and only 26 db of image 
rejection at 280 MHz. But if you increase the value of C152 to 32pF 
by tacking the removed C153 across it, response flattens and tightens 
in the upper VHF range, and image rejection improves to 31 db at 280 MHz. 
That's about as good as we're going to get without replacing L49 and L40 
and C152 with the recomended values. 

Here's the ARRL Radio Designer file for the filter.

*ARRL Radio Designer v1.0
*IC-706 60 MHz to 200 MHz bandpass filter
* Gary Coffman KE4ZV 1/18/96

**Component data inductors

L49:82nH	;ideal 27nH
L40:68nH	;ideal 22nH

**Component data capacitors

C153:0pF	;was 12pF	
C152:32pF	;was 20pF
C53:0pF		;was 20pF

**Node list

IND 1 0 L=L22
CAP 1 2 C=C51
CAP 2 0 C=C154
IND 2 3 L=L49
CAP 2 3 C=C153
CAP 3 0 C=C152
IND 3 4 L=L40
CAP 3 4 C=C53
CAP 4 0 C=C52
CAP 4 5 C=C151
CAP 5 6 C=C54
IND 6 0 L=L25
CAP 5 7 C=C55
CAP 7 8 C=C56
IND 8 0 L=L26
CAP 7 9 C=C57
IND 9 0 L=L27
BPF:2POR 1 9


**Frequency block

ESTP 50MHz 300MHz 511

Gary Coffman KE4ZV          |    You make it,     | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems |    we break it.     | emory!kd4nc!ke4zv!gary 
534 Shannon Way             |    Guaranteed!      |
Lawrenceville, GA 30244     |                     | 


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