Help me understand demodulating with the DSA815

[josephny]

Can someone explain why the following happens:

I generator a 400mhz carrier FM modulated with a 1Khz tone and feed it into the DSA715 SA

I see the signal displayed.

I turn on DEMODULATE, plug in a headset and I hear the tone go on and then off and repeater (a pulse).

I then tuned the SA to a local FM broadcast, did the same thing and heard the broadcast station in the same way (1/2 second on, 1/2 second off).

I played with "demod time" but it didn't get rid of the on/off pattern.

What am I doing wrong?

Thank you!

[TheSteve]

You're doing nothing wrong, the spectrum analyzer is "sweeping" a single frequency. Even if the frequency isn't changing it is effectively restarting the sweep each time at an interval determined by the "demod time".
Moral of the story - a spectrum analyzer isn't a standard receiver.

[vk6zgo]

You're doing nothing wrong, the spectrum analyzer is "sweeping" a single frequency. Even if the frequency isn't changing it is effectively restarting the sweep each time at an interval determined by the "demod time".
Moral of the story - a spectrum analyzer isn't a standard receiver.

Unless it is an old analog HP,which will happily act as a receiver.

[josephny]

I see!

So even at zero span, it's restarting it's measurement at the interval that "demod time" is set to.

Thank you so much!

[Electro Fan]

You're doing nothing wrong, the spectrum analyzer is "sweeping" a single frequency. Even if the frequency isn't changing it is effectively restarting the sweep each time at an interval determined by the "demod time".
Moral of the story - a spectrum analyzer isn't a standard receiver.

Unless it is an old analog HP,which will happily act as a receiver.

What models would constitute an old analog HP (where is the demarcation in their models between old and new?)

[George_Race]

Any of the HP 8590 series will give you excellent audio either AM or FM.  Just make sure it has option 102, which is the AM/FM demodulator board.  No matter what the scan you will hear a constant good signal on AM or FM, and you can do Zero Scan as well with the same results.
I had a new DSA815 and sent it back for that exact reason.  Also the Rigol front end sensitivity was not anywhere near as good as the HP 8595E that I purchased to replace it.  You can purchase one of the 8590 series HP SA's for about the same price, or less than the DSA815.
Hope this helps..............