[R-390] Okay, don't laugh at me but...

[Bill Hawkins]

Barry,

That last transformer that goes through zero is the discriminator.
It demodulates FM to audio.

Maybe this is too basic, but FM modulates the frequency with audio
like AM modulates the amplitude of a fixed frequency. So an FM
transmitter broadcasts a band of frequencies at a more-or-less
constant level. Broadcast FM uses about 100 KC which is 1% of a
10.7 MC carrier, not much of a strain on the bandwidth of the IF
strip in the receiver. Then there's narrow band FM, but never mind.

The FM IF strip has too much amplification for AM. It deliberately
limits (clips) the amplitude to get a uniform level across the 100
KC band. This top must be flat.

If you can get a DC connection to the discriminator output, and
have a high impedance voltmeter or DC scope, it doesn't matter how
fast you sweep it. A stable signal generator capable of fine
adjustment (.01%) is all you need to slowly sweep across the 100 KC
band and verify that the signal level to the discriminator is flat.
If it isn't, the discriminator output will vary with the IF
amplitude, which causes distortion.

The discriminator is designed to linearly change output across the
frequency band. The adjustment is to center it (zero VDC) on the IF
center frequency. If you can't get it to zero then something in the
IF strip is oscillating at a spurious frequency.

At least, that's how I understand FM.

Bill Hawkins



-----Original Message-----
From: Barry
Sent: Wednesday, March 09, 2011 3:46 PM

I am aligning an old FM radio and I want to sweep the IF transformers but I
don't have a sweep generator.  I know that my function generators with VCO
input can function as a sweep generator, but they don't cover the 10.7MHz
range.

Here's the dumb question:  Can I use my HP-8640B (in FM mode) and feed the
modulation input with a triangle waveform from a function generator to
essentially cause the 8640 to sweep the center frequency?

BTW, the reason I want to use the scope and sweep it is something doesn't
seem quite right using the VTVM method.  It's adjusting the secondary of the
last IF transformer is supposed to cause the measured point to go through a
zero point (e.g. +V on one side, through zero, and then -V on the other).
The procedure is to set it zero volts but I'm only able to get a non-zero
null point so I suspect something else is not right and a picture might help
me confirm what I'm seeing on the VTVM.

Again, please don't laugh at me.  FM is just weird to me...

Thanks,
Barry - N4BUQ