[TMC] GPR-90 RXD Tuning

[Richard Knoppow]

----- Original Message ----- 
From: <jvendely at cfl.rr.com>
To: <tmc at mailman.qth.net>; "W2HX" <w2hx at w2hx.com>
Sent: Tuesday, March 18, 2014 12:48 PM
Subject: Re: [TMC] GPR-90 RXD Tuning


> Hi Eugene,
>
     The history is interesting and I am not sure I know all 
of it. 455 khz was adopted because it was possible to obtain 
reasonable selectivity at that frequency and image rejection 
was reasonable up to perhaps 10 mhz although many receivers 
were built that pushed the high end to perhaps 40 mhz.  At 
some point the images were just about as strong as the 
desired signal. The use of higher IFs was certainly known 
but the problem was the bandwidth of the filters. First of 
all the percentage bandwidth must be reduced and the Q of 
the tuned circuits must then be quite high. I think such 
high Q was just not practical. This is also the reason for 
using very low frequencies for filters such as TMC does in 
the MSR series.   I don't know for certain what the first 
commercial double conversion receiver was but I think the 
Collins 75A-1 must have been one of the first if not the 
actual first. Its performance revolutionized ham receivers 
and probably also commercial receivers.  Quite a few double 
conversion sets made their appearance after the 75A-1, some 
quite good, some not so good. In a few cases the requirement 
that the broadcast band be covered made the designers jump 
through hoops, the 51J series is an example.
     One problem with multiple conversion receivers is the 
number of spurious responses they have. The more oscillators 
and mixers the worse it gets.  The ability to obtain 
sufficient selectivity with high frequency filters allowed a 
return to single conversion receivers but as John points out 
the IF was often in the covered bands.  Actually 
Hallicrafters got around this in a couple of receivers by 
just leaving out the band around the IF.


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