[Hammarlund] Sweep method of alignment - sync waveforms.

[Kenneth G. Gordon]

I am forwarding this post from the Glowbugs list, since this discussion 
started on the Hammarlund list.

Ron's information is very useful.

At this point, it appears that feeding a swept signal into the antenna 
jack will accomplish the same thing as doing the entire job at IF.

Fascinating...and useful!

Ken Gordon W7EKB

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I'll not dispute the ease of using a display produced by a sawtooth
horizontal drive, but the process I mentioned of using a service
monitor to sweep the bandpass used a sine wave. The service monitor
was set in FM mode, and a very low frequency modulation was applied 
- a sine wave - that caused the frequency to sweep back and forth 
across the RF passband. 

Yes, it appeared as a double trace on the CRT whose horizontal plates
were fed by the modulating audio so they were in sync. Any 
discrepancy in the shape of the traces - one from low to high and the 
other was from high to low as the frequency moved up and down - was 
caused by assymetery in the filters - tilt in the bandpass, etc. Usually 
one wanted the assymetery minimized so adjustments were made to 
make them superimpose as well as possible. The fact that the actual 
sweep rate varied sinusoidally didn't disturb the display on the scope 
because the horizontal sweep was linear in terms of frequency vs. 
position.

All that using a sine wave in the horizontal caused was the rate at
which the sweep was traced on the scope changed with position, not 
the shape of the bandpass described. 

Looking at the double trace I aligned the I.F. for the desired
bandwidth and symmetry. 

And it is true that the gain of the I.F. will drop when it's aligned
for a wider bandpass. Peaking everything for minimum bandpass 
produces highest gain, but that does NOT mean highest sensitivity. 
Sensitivity and gain are two totally different things and only related to 
each other in certain situations. Gain is, of course, how much
amplification of signals that occurs: how loud the receiver makes the
speaker or phones rattle. Most receivers have plenty of reserve gain.
If one doesn't, it's easy to add some with another audio stage.
Sensitivity is all about signal-to-noise ratio, not gain. That's a
whole different set of conditions that relate to the amount of signal
present at the antenna input and the amount on internal noise
generated within the receiver. The sensitivity is usually defined by
the first stages of a receiver while the bandpass is defined by the
I.F. stages and the gain is defined by the audio stages. 

Ron AC7AC

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