[Hammarlund] Re: Sweep Method for Alignment (explain?)

[James A. (Andy) Moorer]

I don't know anything about the HQ-110. I work mostly with the SP-600.
But, in general, the selectivity of a receiver is determined by the 
filtering in the IF strip. The width of the peak you are measuring on the 
'scope is exactly the "bandwidth" of the IF strip. The bandwidth of the IF 
strip of any communications receiver has to be more narrow than 20 kHz when 
it is properly aligned.

I have never heard of a communications receiver that had a 20 kHz IF 
bandwidth. The widest setting on the SP-600 is 13 kHz. That means that the 
response is down 3 dB from the maximum at frequencies that are separated by 
13 kHz. The SP-600 has a number of other selectable bandwidths (4 kHz, 1.4 
kHz) before it switches in a series crystal. When I measure the frequency 
response of a properly aligned IF section, I get a peak that has roughly the 
frequency on the bandwidth selector between the 3dB points, and falls off 
very rapidly outside of that range. So, when I set the selectivity to the 
minimum (200 Hz), I see a peak with 200 Hz between the 3dB points. See 
Figure 5 in the Hammarlund manuals (well, they are in "rejection" rather 
than "attenuation", so the curves you see on the 'scope will be upside-down 
from what is in Figure 5).

I'm not sure why you are seeing wider peaks. The HQ-110 has to have a lot 
more selectivity than 20 kHz, and it has to happen somewhere. I'm surprised 
it is not happening in the IF strip. We have a mystery. I could believe it 
might have a 20 kHz bandwidth (or worse) before alignment. It better have a 
LOT more narrow peak after alignment. It is when that peak gets really, 
really narrow that you start having trouble with a 1 ms sweep rate. At 20 
kHz, 1 ms is fine. At 200 Hz, it is not.


James A. (Andy) Moorer
www.jamminpower.com

----- Original Message ----- 

> By the bandwidth, do you mean the width of the IF tuning peak? If so then 
> this width is
> roughly 1/5 of my sweep frequency range i.e. about 20kHz. It varies 
> (narrows) as the IF tuning is done.
>
> (My goal when tuning the IF is to maximise the height of the peak, which 
> is achieved when
> the peak is at its narrowest, and so, I suppose, when its bandwidth is at 
> a minimum?)
>
> I have a feeling I may be missing understanding something important here 
> ... if the
> width of the peak isn't the bandwidth, then what is it, and how do I 
> change it to
> observe the ringing effect you and Ken are talking about?
>