[NJARC] Re: [Boatanchors] Why 455?

[Al Klase]

Hi Nick,

NICHOLAS SENKER wrote:
> ......While we're on the subject of different IF frequencies, can someone comment on what a 'double conversion' receiver is for us non-engineers?.......... 
I hope this doesn't get too verbose. Here goes:

First, why would we want to do this?  Consider a receiver with a 455KHz 
IF tuned to WSM, Nashville, on 650.  The local oscillator is running at 
1105Khz.  So 1105 - 650 = 455.  If the signal from Radio Disney, 1560,  
gets to the mixer (first detector), it will also be converted to 455 
(1560 - 1105 = 455).  This unwanted signal is called an "image."  
Because the image is separated from the desired signal by more than 100% 
for that frequency,  a single tuned circuit on the radio input can 
provide acceptable image rejection.

Now, consider a similar receiver with a 455 IF tuned to WWV on 10MHz.  
The local oscillator is tuned to 10,455KHz, and the image frequency is 
10,910KHz.  The image is separated from the desired signal by less that 
10%, and achieving acceptable rejection is considerably more difficult.  
You'll want at least one tuned-RF stage for a total of two tuned 
circuits in front of the mixer.  Things get worse with increasing 
frequency.  Many of the classic communication receivers have two tuned 
RF stages.  Their image rejection on the 28MHz ham band still left a lot 
to be desired.

So, a higher IF frequency can solve the image rejection problem at 
higher frequencies, but it's difficult to achieve the ultimate 
selectivity we desire.  With all other thing being equal, a 4.55MHz IF 
filter will be ten times wider that a 455KHz device.

Lets look at a double-conversion set, the Hammarlund SP-600.  For tuning 
ranges above 7.4MHzm a second mixer is switched into the circuit.  The 
first mixer and tunable local oscillator convert the incoming signal to 
an IF of 3955KHz in the interest of improved image rejection.  Then, a 
second mixer and 3.5MHz crystal controlled local oscillator convert the 
signal to a conventional 455KHz if where the ultimate selectivity and 
the bulk of the amplification are achieved.

The other reason for double conversion is to achieve greater frequency 
stability.  In this case, the first local oscillator is crystal 
controlled, and the second LO is tunable.  Check out ground-breaking 
Collins 75A and 51J receivers of the late 1940's.

Hope this helps,
Al

-- 
Al Klase - N3FRQ 
Flemington, NJ 
http://www.skywaves.ar88.net/