Somehow I must have damaged the tracking generator output port on my DSA815. Attached are screen shots to show what I am seeing.
If I feed the tracking generator into the SA input, span set to 6-9 MHz, I see the following levels at these frequencies:
TG Level set to -10 dBm: 6.193333 MHz: -8.86 dBm 6.206666 MHz: -9.86 dBm
TG Level set to -20 dBm: 7.293333 MHz: -18.4 dBm 7.306666 MHz: -19.4 dBm
If the TG Level is set to 0 dBm, the output level is flat across the entire 6-9 MHz range. If I set the start frequency to 5 MHz and stop frequency to 30 MHz, the output level is flat across the entire range. If I set the start frequency to 1 MHz and stop frequency to 5.5 MHz, I see a lot of level breakup at about 4.48 MHz.
The change when set to -10 dBm or -20 dBm is an abrupt change. It looks like this on the screen:
6.193333 MHz
------------------------
| 6.206666 MHz
-------------------------
If I set the start frequency to 5 MHz and stop frequency to 30 MHz, at 6.208333 MHz I see -9.03 dBm. If I turn the Marker knob one more click, the frequency changes to 6.250000 MHz and the level drops to -10.03 dBm. RBW and VBW are set to 300 KHz for all of these tests which is why the frequency numbers look strange.
I have a Tektronix AFG3021B. If I feed that into the DSA815 and set it to sweep from 6 to 9 MHz, I do not see the 1 dBm drop at any point in the scan. Also, a sweep from 1-5.5 MHz does not show any abnormalities.
I had 4.3 volts applied to the TG output jack for about 10 minutes. I was testing the gain of an amplifier and didn't realize the input and output switching relays have wetting current flowing through them. The TG output is labeled Max 50 V DC so it should have been OK. However, I am fairly sure that is when I started seeing the 1 dBm drop.
I could open the case and remove the RF shield enclosure and check for signs of damage. However, the warranty seal is still in place even though the warranty ran out years ago. I have contacted Rigol about a return for repair but haven't heard from them in the last 2 days. Apparently their service support isn't that good in the U.S. My unit is too old to upgrade for 10 Hz resolution so I am stuck with 100 Hz which is good enough for my uses.
I watched Dave's teardown and it looks like there is no capacitor protecting the output of the TG. It appears there is either a resistor or a diode connected from the output terminal to ground.
Has anyone else experienced a similar problem? If so, were you able to fix it?