[Hammarlund] Sweep Method for Alignment (explain?)

[Kenneth G. Gordon]

On 7 Apr 2006 at 12:50, K3PID wrote:

> I am not looking for a doctoral disertation but can someone explain
> the "sweep method" for alignment as referenced in the HQ-145 manual?
> It's not quite clear to me what is going on.

Ron:

That is just what we have been discussing in relation to Julian's 
method of aligning his HQ-110.

It requires an oscilloscope equipped with a means of using an 
external sawtooth to control the horizontal trace, and a sweepable 
signal generator, sometimes called a Sweep/Function Generator, 
that will cover the IF frequencies involved, and which has a 
sawtooth wave form output, sometimes labeled "Sync Output". 
However, the Sync Output MUST be a sawtooth, not a triangle 
wave or a sine wave. Although a triangle wave will work, it gives 
you a double-hump trace on the screen which is somewhat difficult 
to interpret.

You connect the SG's sawtooth (sync) output to the scope's 
external horizontal sweep input, and set the SG to sweep from 
below the IF frequency, to above it. 

Usually the upper and lower limits are outside what you would 
expect the IF bandwidth to be, plus a bit more. I.e., if your IF is 455 
Khz, and the 60db bandwidth is about 20 Khz, set the SG to sweep 
from at least 445 to 465 Khz. I would use from 400 to 500 Khz just 
to be safe.

Connect the sweep output (i.e. the swept IF frequency output) of 
the SG to the grid of the 1st mixer, or (less desireable) to the mixer 
plate, through a fairly large value coupling capacitor, and connect 
your scope's vertical probe to the output of the detector stage.

Turn the BFO off. BTW, the detector should be a simple diode 
detector, or if you have a choice, set the receiver to AM mode.

Set the SG to sweep at a fairly low rate. I use about one 
sweep/second or slower. You do this so that you get a true 
representation of the IF bandpass shape, not effected by "ringing" 
in the IF stage caused by "shock exciting" it by using too fast a 
sweep.

If everything is set up right, and your SG amplitudes are correct, 
you should see an exact representation on the scope screen of 
what your IF bandpass shape looks like. It should look exactly like 
those pictures you see in the handbooks of the "ideal" IF bandpass.

If you set the SG output amplitude too high, you can get ringing in 
the IF chain or other overload artifacts.

Now, you can visually see the effect of every adjustment you make 
in real time. It is usually a real eye-opener!

Since you have to use such a slow sweep rate, MOST 
oscilloscopes will show a very weak trace. The trace is not being 
refreshed often enough to keep it clearly on the screen.

Choosing the correct sweep rate is a compromise between keeping 
the IF chain from ringing, and being able to see it clearly.

SOME scopes have a "long persistence" type trace and those stay 
bright for quite some time after the trace has finished at the right 
side of the screen, but these are usually quite low frequency 
scopes and most are quite old, unless you have a modern 
StorageScope.

Otherwise, there are two ways to deal with this problem: 1) keep 
the room lights dark, and turn up the scope brightness as far as you 
can and still keep it sharp. Then use a spot light to light the various 
adjustments in the receiver.

Or, 2) you can get an instrument like the HP-8750 
Storage/Normalizer that can accept your slow trace, and output a 
much faster repeating trace that keeps your scope trace brightly lit 
up. What this instrument does, essentially, is to make ANY scope 
into a storage scope. I bought mine off Ebay for a pittance.

Believe me: if you ONCE use this method to align a receiver IF-
chain you will never, NEVER go back to the simpler method unless 
you absolutely have to.

I even use it on receivers with a single IF stage. It REALLY makes 
a difference. It is truly amazing.

The funny part of this is that I first heard of this method from an 
OLD handbook, and it was simply mentioned in passing, sort of. 
Then I found something in a service manual for a National receiver 
that covered it in greater detail. Later, I found a much more 
thorough discussion of it in an ARRL Handbook from about 1945, 
and an RSGB handbook from about 1960 or so.

Lastly, I then got a military manual on it entitled something like 
"Using the Visual Alignment Method with the R-390/URR Receiver" 
which covers the method exhaustively.

I hope this helps.

Ken Gordon W7EKB

Ken Gordon W7EKB