[ARC5] RAX If Frequency

[Richard Knoppow]

     The question of how IF frequencies were chosen and became 
standardized is an interesting one. For instance 455Khz was a 
fairly late development as a standard, many earlier receivers 
used 456Khz. I think in commercial operation 455 became a 
protected frequency with no stations operating on it.
     Many earlier broadcast receivers used lower IF frequencies 
down to around 175Khz. There are two reasons for the choice of 
frequency from the receiver design standpoint; one is the 
selectivity, which is easier to obtain at lower frequencies due 
to the higher Q of the coils in the transformer, the other factor 
is image suppression which gets better as the frequency is 
raised. I think putting the IF above the broadcast band was not 
much done both because the selectivity was not good enough and 
because there was too much chance of interference from strong 
signals at frequencies above about 1500Khz. Another reason was 
because a receiver doesn't work well around the IF frequency. 
Hallicrafters chose to simply leave a hole in the coverage of 
some of their early double-conversion receivers. These had a 
first IF somewhat above the broadcast band so they just left a 
gap in the coverage between that the beginning of the short wave 
coverage.
     I am not sure about the regulations about banned frequencies 
in the MW ship band, that is, whether it was world wide or only 
the region where the U.S. is. In any case, the use of a higher 
IF, which was common in receivers covering medium wave and not 
having the broadcast band. These sets often had an IF in the 
vicinity of around 800Khz. It is not too difficult to shield a 
receiver from external signals at the IF frequency and the higher 
IF would have given these sets very good image rejection. Many 
had either crystal filters, which are practical at those 
frequencies, or a regenerative detector which would provide good 
selectivity for CW.
On 2/15/2022 3:25 AM, releazer at earthlink.net wrote:
> Well, here is an observation of mine.  The AM BCB is a logical place to put IF's, since it is the only suitable frequency range that is NOT of interest to the military except for navigational purposes, and for that you use the ADF gear.  And 915 KHZ is not one of the allowable AM broadcast frequencies.  We have a 920 Khz station here locally and I hear it whenever I crank up a BC-348, but actual signals in the received band override it easily.  And the BFO in the BC-348 is not going to jam any BCB stations in the ADF.
>   
> A similar question would be why would they choose 456 KHZ as the IF in the GRC-9.  I assume that it was because they used a BFO operating at 1/2 the IF frequency and they did not want to have to deal with the 0.5 KHZ difference.  Now as to why they chose 1/2 the IF frequency for the BFO, you got me here.
> 
> Wayne
> WB5WSV
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: https://www.qsl.net/donate.html

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL