[R-390] IF sweep alignment

[2002tii]

Brian wrote

 > So I feed the FM-modulated RF into the antenna input of the 390A, feed the
 > scope's vertical plates from the IF output on the rear of the 390, and feed
 > the scope's horizontal plates from the 400Hz sinewave output of the 8640B.
 > Can't get her to sync up with the scope in x-y mode.

That should work, with some caveats.  Because you are using a sine 
wave to FM the carrier, (i) the instantaneous frequency will not 
change linearly over time, and (ii) the retrace will be overlaid on 
the main trace.  (i) means that the brightness of the trace toward 
the left and right edges of the screen will be higher than in the 
center, because the trace dwells there longer.  The display will 
still be linear in frequency, as long as the FM modulator is 
linear.  (ii) means that there will be two versions of the display 
overlaid on each other, which will probably be slightly displaced 
horizontally from each other.  This is due to the phase delay between 
the 400 Hz driving the sweep and the 400 Hz modulating the 
oscillator.  To adjust this, you adjust the phase of the 400 Hz 
sweep.  This is exactly how the famed Sound Technology 1000A FM 
alignment generator worked.

I'm not exactly sure what you mean by you "can't get it to synch 
up."  You could either mean that the main trace and retrace envelopes 
are not overlaid accurately (see above re: adjusting the sweep 
phase).  Or, you could mean that the IF waveform is not stationary 
with respect to the sweep.  This you should not expect, because there 
is no constant integer relationship between the instantaneous IF 
frequency and the 400 Hz sweep -- you are interested in the envelope only.

If you use the "diode load" output instead of the IF output, you can 
drive the scope vertical with the detected signal and, instead of 
looking at two superimposed IF envelopes, you can look at two 
superimposed line traces that follow the IF envelope.  If you use a 
sawtooth to modulate the signal generator, the quickness of the 
retrace will make it less distracting (real spectrum analyzers blank 
the display during the retrace, which you could do relatively easily 
if the scope has a Z-axis or blanking input).

Best regards,

Don


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