[ARC5] ARC-5/SCR-274-N receiver bandwidth spec

[Kenneth G. Gordon]

On 23 Mar 2014 at 12:16, D. Platt wrote:

> Gentlemen,
> 
> Continuing with my ARC-5/SCR-274-N receiver IF coil mod investigation, I 
> just completed yet another 80 meter coil set, this one from an 
> ARC-5/R-26.  
>
>The question for today is what is the original 
> published (sic) receiver bandwidth of this receiver?

According to the ARC-5 manual on PDF I have here, the selectivity 
characteristics are as follows:

.19 - .55 MC - at .19 MC - 2X 1.1 kc: 1000X 4.5 kc  IF 85 KHz.
.52 - 1.5 MC - at .52 MC - 2X 2.1 kc: 1000X 8 kc     IF 239 KHz.
1.5 - 3 0 MC - at 1.5 MC - 2X 3.2 kc: 1000X 13 kc   IF 705 KHz.
3 - 6 MC       - at  3 MC   - 2X 7.3 kc: 1000X 26 kc   IF 1415 KHz.
6 - 9.1 MC    - at 6 MC    - 2X 13 kc:  1000X 56 kc   IF 2830 KHz.

My understanding is that 2X is the same as 6db down, while 1000X is the 
same as 60DB down.

In the manual, there are values for 10X and 100X also which I have not 
given here.

In any case the R-26's 1415KHz IF is originally quite broad: from what I read, 
it is 7.3 KHz wide at 6db down and 26 KHz wide at 60 db down...which IMHO 
is not very good for OUR uses, and from my experiences with this receiver, 
that feels "about right".

However, the R-27 is just plain "broad as a barn door", i.e. 13 KHz wide at 
6db down and 56 kHz wide at 60 db down, and in fact is probably actually 
worse than the ARC-2's IF bandpass that Dave published a day or two 
ago...even if that had been plotted logarithmically.

Also, I am reasonably certain these selectivity "curves" also apply to the BC- 
series receivers. In fact, given that the IF frequencies are identical, I see no 
reason why they would not be.

According to the data I have in an article in ER magazine which was 
published some time ago, ARC was able to achieve a selectivity of 3 KHz at 
the 6 db down point for the 2830 KHz IF receivers by separating the coils in 
the IF transformers. I have no data on the selectivity achieved at the 60 db 
down point, although Bruce reminded me that the selectivity at that point is 
determined by the IF frequency and not by the coupling coefficient: i.e. it 
becomes so many db per octave.

>From the ER article, I have no idea whatever what ARC did about gain 
changes, if any, made to compensate for any loss of signal by separating the 
IF coils, or even whether that was necessary or not.

YMMV.

Ken W7EKB