Playing with the Synchroniser
My main application for this unit as as a sythensizer and the sweep functionality isn't overly useful as I have the spectrum analyser with tracking generator. So now that the unit is generating an output I wanted to play with this.
Without the synchroniser enabled the output is miles off. 10MHz comes out as 7, 100MHz as 104MHz, 1GHz is nearly 15MHz off. My understanding is this is pretty normal as the YIG is hard to steer accurately.
With the synchroniser on the output is really close according my frequency counter. My counter can only count up to around 300MHz but at this frequency it is less than 20Hz off. No idea which of the two is more accurate at this point.
I noticed an anomaly however in that if I choose a frequency less than 20MHz the output was spot on. If I chose a frequency above this (like 20.001MHz for example) the output jumps off to 54MHz or so and wanders around. After experimenting a little I figured out that above 70MHz it comes back together again. So what is special about this frequency range?
Understanding the Synchroniser
The block diagram below shows the major functional components of the synchroniser. The bit highlighted in green is what I figured had to be in the signal path when it was going wrong.
The basic plan here is to take the output signal, divide it down enough that you can lock it to the reference frequency and then generate an phase error signal that can be fed back to the sweep board to adjust the frequency of the output. A big PLL in other words.
It's not that simple though as the frequency range is broad and because the unit can sweep rapidly across a range while maintaining lock on the reference. For now I'll neglect a lot of this detail and just focus on what was going wrong.
If the frequency is between zero and 20MHz the 97-122MHz VCO is locked with the reference and mixed with the 100MHz coming from the converter board to generate the output signal (doesn't use the YIG at all).
Over 20MHz it starts to look at the output signal and processes that down to something it can compare with the reference. There are three ways it can do this
- Take the output signal, filter it to cut-off anything above 70MHz and use this directly.
- Mix the output signal with a 1.2GHz signal produced by multiplying the 600MHz generated on the converter board. Then take the 0..700MHz output and divide this by 10 to give a 0..70MHz signal and process that
- Mix the output signal with a 1.8GHz signal produced as above and divided as above.
The path it chooses depends on the frequency range it is generating an output in. The first bit that is confusing is that the schematic shows the first of these paths as producing 0.1..70MHz where as in the 0...20MHz range a different path is used so this isn't true.
So in my case it had to be something in this 0.. 70MHz filter or the switch as everything else works in other bands.
Synchroniser RF Board
The synchorniser RF board is another fine work of RF voodoo. The multipliers and distributed element filters look pretty interesting although I have only the vaguest clue how they work.
Thankfully none of that mattered as I was looking at a relatively low-frequency path through this board. Here is a block diagram of just the RF section with the areas I was interested in high-lighted.
Now it did take some trial and error as well as head-scratching to narrow the schematics down to this. The other annoying thing is that two of the connectors on the RF board are the SMCs similar to the converter board but two are SMB connectors. I have patch leads for the SMCs but not SMBs so I also wasn't sure if what I was seeing was correct because things were disconnected.
As it turned out this wasn't a problem as the SMBs were generally outputs and even though the system might not have been working, the board I was looking at was running as it would if it were plugged in (and not up on riser boards).
The schematic below shows the low pass filter (shown as 100MHz but the other block diagram showed this as 70MHz).
The three amplifiers at the top marked D70 plus their associated components carried the signal in this 20..70MHz range. They use this ECL NOR gate chip (D90) as a sort of switch to control the signal path. One of the inputs is a control line and the other carries the RF signal.
Because this is ECL the logic levels are between -2.5V and ground which looks a bit odd at first but you get used to it.
At the top right are two gates (near R95 and R92) that feed into another one below (near R94). I was looking at the signals on the pins of D90 and it seemed like one of these gates wasn't switching it through. Eventually I figured out that pin 14, 7 and 4 should all have the same signal but they didn't.
I pulled the board out and checked continuity and sure enough there was no connection. Thankfully the chip was socketed so I pulled it out and checked continuity again.
It turned out that pins 14 and 3 are directly connected by a track and another track from 7 meets them at a via. The via was basically loose enough that it made intermittent contact. I soldered a jumper wire between the pins, re-tested and it basically worked.
Still more Problems
So at this point the unit is looking pretty good - the 600MHz problem seems to have gone and now the synchroniser problem is solved. All the self-tests pass and I can generate relatively accurate signals.
I did notice that if I used my spectrum analyzer to look at the output, as the frequency goes above 1GHz the accuracy gets steadily worse. At 1GHz it is less than 1MHz off but by 2GHz it is nearly 20MHz.
While looking at this and pondering if this is a fault or just a limitation of the device my 600MHz fault (self-test code 22 1C 00) came back again.
So I suppose I better look at that next!
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