[ARC5] Re-reading Gordon White article...

[Richard Knoppow]

----- Original Message ----- 
From: "Bruce Long" <coolbrucelong at yahoo.com>
To: <gewhite at crosslink.net>; "Ken Gordon" 
<kgordon2006 at frontier.com>
Cc: <Arc5 at mailman.qth.net>
Sent: Friday, June 07, 2013 12:04 PM
Subject: Re: [ARC5] Re-reading Gordon White article...



I have a vauge memory of reading somewhere the gain 
adjustment with frequency is needed because of the change of 
the tuned resonant circuit L/C ratio changes its effective 
impedance and there for the gain of the associated amplifier 
stage. Can't remember any details----but just found this in 
section 23.3 of The Radio Designers handbook Langford-Smith
Section 3 RF Amplifiers
Parallel Tuned Circuits
" A simple parallel tuned circuit is sometimes used in the 
RF stage and can be made to give higher gains than the 
transformer coupled arrangement. The difficulties 
encountered are that:
1 large gain variations occur across a band of frequencies.
2 the skirt selectivity is rather poor
3 tracking with conventional aerial circuits is difficult
etc"

So i think the previous comment- sorry i forgot who made it 
that the fixed tuned resonance below the operational tuning 
range sets up a decreasing gain slope that compensates a 
companion increasing gain slope is probably right on the 
money.

     FWIW, the RF circuits in the RCA AR-88 are like this. 
The antenna to grid circuit is a transformer on all 6 bands 
and the RF and mixer interstange coupling on the lower 3 
bands but the interstage coupling between the two RF stages 
and the second RF and mixer stage are parallel tuned single 
circuits on the top three bands. I am not sure why this 
method was chosen, perhaps to maximize the Q of the circuits 
and thus the selectivity.  I am certain I've seen the same 
arrangement in other receivers.  I don't know how the gain 
is equalized in the AR-88 but its gain is very uniform 
across each band and from band to band. I've had my nose in 
the AR-88 circuitry a lot recently so I thought of it when 
you mentioned this.
    The Radiotron 4th edition is usually a very good source 
for this sort of information.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com