[HBR] Re: W1KLK receiver

[N2EY at aol.com]

In a message dated 8/17/04 10:39:24 PM Eastern Daylight Time, 
Bill at LizCurtisHiggs.com writes:


> Doug's purpose with the elaborate RF stage is to eliminate the image; with 
> a 455KC IF, you generally need all the front end image rejection you can get.

Agreed but I read the article a bit differently.

I think you could get adequate image rejection without all those tuned 
circuits. What 'KLK wanted was not just image rejection but rejection of strong 
out-of-band signals - meaning anything more than a few dozen kHz away from the 
desired signals. If the signals don't reach the mixer, it doesn't have to deal 
with them. 

If I understand the process, we hams measure 3rd order IM by injecting two 
relatively close-spaced signals into the rx and raising the level until an 
inband distortion product shows up. Typically it's two signals 20 kHz on 20 meters. 


Such a test gives a good indication of the dynamic range of the front end 
with a few very strong, closely spaced signals. But "in real life", many 'modern' 
receivers inhale huge chunks of spectrum and run it through several stages 
before there's any significant selectivity. If there are two or more properly 
spaced big sigs, you've got chatter. What 'KLK did was to cut down the number of 
big signals that even reached the mixer. Not just images but any strong 
out-of-band signals. 

I don't know if the HF spectrum is more or less crowded than it was back 
then. From what I recall, HF used to be stuffed full of high power RTTY, CW and 
'phone stations for a variety of services. SWBC, military, maritime, etc. 

Note the story about how he loaned the rx to the Novice across the way. The 
Novice's 75 watter desensed his regular rx (probably a 75A-4 or 75S3) but the 
Novice had no idea 'KLK was even on the air. In those days the Novice band was 
3700-3750, just below where the DX 'phones hung out, but far enough away that 
the front end selectivity would knock down 'KLKs big signal before it got to 
the mixer. Not so with the 'regular' receiver!.

> 
> As far as the 455KC IF is concerned, Doug used what he had. The image 
> rejection he was seeking IS possible on 75 with a 455KC IF, but you have to jump 
> through a lot of filter hoops to do it - which is what he did. Anachronistic? 
> Possibly; HF crystal filters had been around a few years when he designed the 
> receiver. Still, the better Collins filters had excellent skirt selectivity 
> and good ultimate rejection. The major drawback is insertion loss; some of the 
> better filters also had 10-12 dB insertion losses. Hence, often an extra IF 
> stage was needed (Doug's design had three, plus the Q-Multiplier).
> 

The Q multiplier was primarily for a rejection notch.

The three IFs were there for a couple of reasons. First, the lowgain RF amp 
meant the mixer output was pretty low, so three stages more than made up for 
it. Second, it can be easier to stabilize an IF strip if you don't try to get 
every last dB of gain out of each stage. 

But I think the most important reason (to him) was AGC range. With three 
supercontrol IF tubes on the AGC line, it doesn't take a lot of AGC swing to have 
a flat characteristic. IIRC, he did not apply AGC to the front end, so he 
needed to make up for that. I recall something like 40 dB gain control range per 
stage?

The W1KLK receiver has influenced many of my receiver designs. Not just 
electrically but mechanically. 

73 de Jim, N2EY