I was looking at some linear (non-log) sweeps using a HP 8591E spectrum analyzer and I noticed an odd dip in amplitude at around 59MHz. Double checking on a 8595E and two different oscilloscopes, I concluded the dip was not in the source.
Here's what the 8595E shows (scale 1dB per div):
And here's the 8591E:
The approx +/-1dB variation is not terrible, but that's the max spec for the 8591E. It looks unusual enough that I thought I'd poke at it a bit. I've included the block diagram PDF from the service manual below if you want to follow along.
To get your first thoughts out of the way, the dip happens in log or linear mode, and it also happens at any attenuation setting.
Here's what I've discovered using the 8595E to look at various points:
- The source is flat coming out of the attenuator and into the A4 first converter. So the problem is not in the attenuator or anything previous to A4.
- The A4 J2 LO input is flat during 8591E sweeps. LO is ok.
- The A4 J4 counter output is flat during 8591E sweeps. LO passing through A4 is ok.
- The A4 J5 tracking gen output is flat during 8591E sweeps. (This unit does not have a tracking gen, so there was just a terminator on that port.) More verification that LO is ok.
- Using the 8595E in zero-span mode @ 2.1214GHz, I can see the dip looking at the J4 band-pass align port on the A5 second converter. So, is it coming out of the A4 first converter? Let's see...
- Disconnecting the A5 second converter and looking at the signal right at that connector coming from A4 & FL1, again using zero-span, I do NOT see the dip. Is it something with the A5 second converter?
- Inspected the innards of the A5 second converter. There's not much to it. Re-assembled it and re-aligned the band-pass filter. Dip unchanged.
- Inserted the 8595E with a "T" at the A5 second converter input. I could see the dip, although it's now at a different frequency. I had to replace the hard line with a 12" jumper to get the "T" in place. It's interesting the dip moved.
- Removed the "T" and left the jumper. The dip is still there but at a different frequency. So, now the only difference is the connection between the A4 output filter FL1 and the A5 input is a different length coax, and it's RG174 instead of hard line.
- Inserted an SMA elbow on the 12" jumper. The dip moved. Here's the difference with and without the elbow (yellow is without the elbow, green is with):
So this says there's sensitivity to the length of the signal path between the first and second converter. The elbow adds about an inch to the path.
My theory is that reflections are occurring on the input of the A5 second converter and they're making it back to the A4 first converter and causing interference. It could be happening as far back as the A4 mixer, but I'm not sure.
Any other ideas?
Or should I not worry about it and and just fix it in the calibration constants? Oddly, there's already a noticeable delta from the factory at 41MHz:
Freq(MHz) Error
4 -0.90
41 -1.40
78 -0.63
115 -0.73
152 -0.49
(...etc...)
Maybe it's always been like this since the day it was made?
I haven't opened the A4 first converter (yet), but I suspect it more than the A5 second converter at this point.
Anyone have a 8591x that can compare their lower frequency response?