Author Topic: Advantest R3131A Spectrum Analyzer 9KHz-3GHz  (Read 87405 times)

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Offline RadioAct

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #175 on: April 09, 2016, 07:15:14 am »
Thanks for the info Fraser.

I am expecting a fussy alignment process with everything and anything affecting the oscillator while the lid is open. Just adding the wire made a noticeable change to the oscillator, it's now closer to 2MHz off then 3MHz like when I started.

As for the VCO feedback voltage, I was thinking the same thing about holding it to 0v during the alignment. I wish I had looked closer at the chip driving the feedback signal to determine if I can simply tie the test point to ground since that part of the circuit will be unpowered. I will pop the board out again and prep it for the alignment. Try powering the VCO out side the chassis and determine the state of the feedback control and see how to hold it to 0V safely. My friends time is limited so I was going to figure this out before I head over there anyway.

I am having a bit of trouble attaching my photos, I will try and get them posted tomorrow.

Ty
 

Offline Fraser

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #176 on: April 09, 2016, 12:21:27 pm »
If you find the time you could complete my outstanding task and trace the route of the Varicap diode control line. you can then easily disconnect that line and isolate the Varicaps from the PLL drive. On my units it was obvious and easy, but on your units it takes a different path. It is much harder to me to trace the route without the module in front of me as continuity tests should quickly locate it. there will be a wire connection between the metal module chassis and the PLL board that sits on top of the module. It is direct and without buffering on the metal chassis side of the path. If you trace from the Varicap control point (IIRC the outer connections of the Varicaps on your module) to the PCB sitting 'North' of the VCO you will find that it terminates in the top tight of that PCB and then heads off to ??????

If you then use that point on the PCB for one continuity check probe, you should be able to find continuity of zero ohms between it and the connection on one of the PLL connectors and that is the PLL control line which needs to be isolated before tying the Varicap diodes control point to 0V

Some side comments......

1. The Varicap diode layout is different on your modules to mine. They are both reverses polarity and the control voltage is applied to the outer leads as opposed to the mid point in my module.
2. The mid point of the two Varicap diodes is biased by a voltage that must be present when setting up the VCO. In my module the bias was derived from the voltage supplied to the VCO power pin. Your module may apply a different bias voltage so the available swing of the VCO control may be less in your modules. This will mean more accurate setting of the VCO center frequency is advisable.
3. In my module there is a power resistor present that drops the voltage supplied to the VCO components (MMIC's) In your module this dropper resistor is not present, hence the lower VCO supply voltage at the power input pin.
4. Your module is an older version of the one for which I wrote the alignment notes. As such, please consider all guidance in that document suspect in relation to the alignment of your module. I suspect you have already discovered that !
5. The VCO center frequency remains the same on both the older module and the newer variant. You should aim for that voltage with the PLL control voltage at 0V as that will provide the most symetrical positive and negative frequency swing capability for the PLL.
6. Just in case it helps you ..... from memory, the PLL uses an LM324 to drive the Varicap control line.

Sorry I cannot help more at the moment. I am surrounded by work that I must get on with at the moment. My hobby work is effectively on hold  :(

Best Wishes and Good Luck

Fraser
 

Offline RadioAct

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #177 on: April 15, 2016, 07:06:14 am »
Hi Fraser,

The alignment was a success :-+. Went much easier then I expected. I found the key is to make an small adjustment, cover the VCO area with a chunk of aluminum, measure, repeat. Sweeping the tuning "fork" gives you a rough idea where to target then simple from there. The Lo drive level at the test port was about -6dbm so I think the MMICs are in good shape.

A colleague and I traced out the PLL circuit and I have pictures I will post once I figure out why I can't get them to upload. As I identified earlier the DC voltage is 6.5V to the VCO MMICs and the same rail is also supplying a buffer MMIC, RF prescaler (MC12080), and a couple logic ICs (74AC74 x2, 74AC00) in the PLL circuit. A OPA27 is providing the drive to the varicaps.

I disconnected power from the passthrough to the VCO MMICs and drive only the VCO circuit with my bench supply. Since the rest of the PLL was not active, I simply tied the varicaps to ground via TP501. I will get some notes together and update the missing bits of your schematic in a few days time.

Thanks for the info,
Ty

 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #178 on: April 15, 2016, 11:21:51 am »
RadioAct,

It seems that you an I have the same unit.

Was there anything different that you did differently to get your units to work, that wasn't in Fraser's original alignment procedure?

If i read your post correctly you applied 6.5V to circuit while adjusting, is this correct? I can't remember exactly but I think that is right around what I was supplying when I was able to dial int eh Freq. But still no luck for me.

Thanks
 

Offline RadioAct

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #179 on: April 16, 2016, 04:03:13 am »
Yes I do think we have the same boards. Mine matches your pictures, notably the different arrangement of the varicaps and the different insulated pass-throughs.

The steps that I did were :
-I desoldered the little blue wire by U303 and attached my power supply directly to the PCB. I wanted to make sure that I was only powering the VCO as I didn't check out what else the 6.5V supply was powering.
-I soldered a ground wire to TP501 as that is the varicap control voltage line.
-I connected the test port to a spectrum analyzer and powered on the bench supply.
-I went looking for the Lo and I found that it was some 40Mhz lower. Uncovering the VCO had a major effect on the frequency as expected. I got a chunk of aluminum to cover it between adjustments.
-I tweaked the tuning "fork" a bit, measured, repeat until I was under 500kHz from 3840MHz
-Found out that tightening the adjustment screw caused the frequency to jump so I aimed a but under 3840 then when tightened it was nearly on target.
-Disconnect everything, reconnect the blue wire to the MMIC power supply near U303, pop on the cover and put in ~50 screws. Slide the RF deck back in the analyzer and reconnect.
-Run self test and now RF passes.  :-+

The only thing I can think of is that there was some component damaged when you initially tried 11.2V before finding out that this version of the board uses a 6.5V supply to the MMIC. Did you disconnect the VCO MMICs from the power rail as I did? I have confirmed that the 6.5V that powers the MMICs is common to the PLL circuit and powers the 74AC47 and 74AC00. Those guys are cmos and I don't think they would have liked the 11.2V supply for long.

Let me know if there is anything you would like me to test/measure/poke at at the RF board is currently apart on my bench. Next time it goes back together its for good.

Ty
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #180 on: April 16, 2016, 01:03:08 pm »
Thanks for the feedback.

I did not de-solder/isolate the circuit when I was applying the voltage, nor did I ground to the varicaps (I grounded to the chassis).

I also noticed the change when tightening down the screw, but did not use an aluminum to shield the circuit while measuring.

Unfortunately I do not have another Spec A to check the output with, only a Freq Counter so I may be at a bit of a disadvantage there.

I have my kids soccer this morning and some spring cleaning in the afternoon but I will try your "tweeks" later today and finally put this thing back together.

Thanks again.
 

Offline RadioAct

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #181 on: April 18, 2016, 06:04:00 am »
If possible I suggest you give isolating the power to the VCO and cover it with some sort of shield between adjustments. I don't think there is anything wrong with using a frequency counter, if anything your frequency measurements will be more accurate than the way I did it with a Spec A. The spec A has the benefit of testing output level at the same time but it's not necessary. Tie the TP501 to ground.

Aim your adjustment for 3840MHz since the PLL will not be powered and the feedback voltage is tied to 0V. Absolute perfection is not necessary, I settled with 250kHz off of 3840MHz and when the PLL pulled it in it only took 0.3V of feedback voltage.


Now for some self promotion.
I have done up a short write up on the process I did on my website, best part is I posted all the photos.
http://www.tystech.com/repairs/aligning-the-advantest-r3132-spectrum-analyzer-2nd-rf-lo/

Ty
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #182 on: April 19, 2016, 01:29:09 am »
ok guys (and or girls).... I feel like I am actualy making some progress here with 1 last hurdle.

I just re-attempted the adjustment procedure with the "tweaks" posted by RadioAct.

During the adjustment I was only able to get it as close as 3810 (up to 3815 when I put the aluminum cover back on). I figured it was as close as I would get so I put it back together and fired the unit up.

(as a side note i still have the internal bracing removed so I am able to still monitor the "monitor port" while the unit RF Board is inside the chassis)

As soon as I turned it on I was getting 3480 on my Freq Counter (HOLLY CRAP!!!!)

So I ran the self test using Shift 0 and ALL TESTS PASS!!!! WHAAAAAT!!!!!

But then i tried to run the Self Cal (shift 7) and it still has the 807 Error "no cal signal detected"  |O

Anyone have any other suggestions? PLEASE!!!!!
 

Offline Fraser

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #183 on: April 19, 2016, 01:51:39 am »
I have to ask this so hope you do not take offence.... Did you connect a known good coax cable between the Cal output and SA input ?

Also, does your RF input N connector centre pin look to be in good condition. The pins leaves can break off due to abuse and cause a poor connection.

Sadly no immediate suggestions on other causes and I would want a Spectrum Analyser available to carry out checks on the signal oath. Your frequency counter may be of some use though.

I can say that 3815MHz is too low a frequency for the VCO centre point. I am surprised that the PLL is not struggling to achieve correct frequency lock at 3840MHz. It may be on the edge though so the VCO does need some attention at some point as it needs modification or repair to get it to tune around a centre of 3840MHz.

Fraser
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #184 on: April 19, 2016, 02:13:19 am »
Fraiser,

No offence taken...

In fact after I posted my post i sat for 15 min thinking about it and I had the same question (for myself).

So I went down stairs to try to swap around the cables. I only have 2 that I grabbed from work when i first started on this project. I assume they would work fine.

For a lack of better description one is long and black (maybe 5 ft or so) and one is short and grey (maybe 1 foot). They both have the same "BNC" style connectors.

Anyways, back to my results....

The "grey" 1 foot wire is the one that I always used to go form the cal out to the RF in because of its size. So i removed the longer one that was connected to my monitoring port and tried that.

Keep in mind that they both still failed due to "no signal" however one of them actually shows some sort of a signal (see pic).

I also confirmed that my output form the "cal out" is 30 MHz (30,000,008 Hz).

Do you think it is my cables?
 

Offline RadioAct

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #185 on: April 19, 2016, 02:21:18 am »
Can you measure the level of the cal signal? It should be damn close to -20dbm. If it's not then there could be a cal signal issue or the drive level from the Lo to the mixer may be low.

-Ty
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #186 on: April 19, 2016, 02:25:22 am »
Not form my house. I will need to take it into work to do that.

I was only able to confirm the Freq out was 30Mhz.

I do see that it puts the unit into -20db when the self cal starts. I assume you want me to hook that output to another Spec A?

Is that output a constant or does it only show up when you run the "self cal" program?
 

Offline RadioAct

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #187 on: April 19, 2016, 03:02:26 am »
Sorry,
I meant put the cal out into the R3231 and see how it measures, I believe it is always on. Set the center to 30MHz and span to 20Mhz'ish. You should see a tone. Peak search it and you will get a marker and an amplitude. Without cal it can't be trusted, but should be in the ball park of -20dbm. Of course a third party spec A or power meter would be great but we will work with what you have.

-Ty
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #188 on: April 19, 2016, 03:22:22 am »
This is what I get (when i use the black cable... i get nothing when i use the grey one)
 

Offline RadioAct

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Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #189 on: April 19, 2016, 04:04:49 am »
Ok I wrote up about four questions before I hit post. I thought about each then rewrote this lol.

I make the assumption that your black cable is serviceable.
During cal the first thing it does is a user test, look for the cal signal at about the right level and frequency, make sure the user actually plugged in the cal signal :) It will look for a tone at exactly 30MHz and within a power range about -20dBm. I don't know the exact ranges but -74 is not close enough.

-First I would like to confirm the measured frequency. Does it dance around at all?
-Can you run the frequency counter on the unit and measure the frequency of the tone. I want to see it come back as exactly 30.000000MHz. Since it generates its own cal signal from the same low frequency reference there should not be any variance if all the PLLs are locked and operating correctly.
-Is the RF still passing self test? I would bet $$$$ that the self test does not do much more than check PLL lock detect signals and some other simple checks, that is why it happens so fast. If you have all the lock detects and pass self test then I would say you do have a 3840MHz Lo.


Assuming that you do indeed have a stable exactly 30MHz cal signal then I am inclined to suspect the MMICs. My thoughts here need confirmation from an expert as I am only familiar in passing with RF. Here goes:

There are three MMICs in the 2nd Lo, one that drives the oscillator, one that amplifies a take off from the VCO and drives out to the mixer (also drives the test port), and the third that drives the frequency divider part of the PLL from the output of the 2nd MMIC. If for example the middle guy is damaged, the Lo drive to the mixer would be low which would show up as a low or missing amplitude on the trace, however the 3rd MMIC could still drag up the signal enough from the noise for the divider circuit to do its job and bang you get a functioning Lo with insufficient output drive into the mixer.

I do recall reading that Fraser had replaced the MMICs in one of the units he worked on, perhaps this may be needed.

-Ty
« Last Edit: April 19, 2016, 04:18:38 am by RadioAct »
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #190 on: April 19, 2016, 11:24:19 am »
To answer your questions...

1/2. When I hook the cal out to my Freq Counter (with the black cable) I get 30,000,008. I thought I posted a pic earlier of that reading... Not exactly 30, but I would think that is acceptable, no?

3. Yes the RF Self Test is still passing. Before last night that would always fail.

I will wait to see if anyone else, especially Fraser has any input.

I will also try to grab another set of cables from work today as i am still a bit suspicious of the huge difference in readings using the 2 that I have.
 

Offline Fraser

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #191 on: April 19, 2016, 11:48:35 am »
Had a thought overnight. You must be using an N to BNC adapter on you SA if using BNC patch leads. You need to check that adapter for good connection. I have had issues with even simple straight through Connector adapters not making good contact on the centre pin.

Fraser
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #192 on: April 19, 2016, 11:51:27 am »
Fraser,

I actually have 2 different N to BNC adapters, I get the same results from both.
 

Offline CJay

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #193 on: April 19, 2016, 12:00:15 pm »
Seeing as your marker suggests -74dBm (even uncalibrated I'd still expect somewhere near, seeing ~50dBm difference is too much)  I'd suggest it's a dodgy cable or extremely low output from the CAL signal, in lieu of another SA perhaps verifying it with something like an AD8307 based power meter might be an idea?

30MHz isn't a particularly high frequency and if you can meter out the cable from end to end as well as verifying no shorts I'd probably accept it is OK, especially as I think you have the same results with both cables.

oops, my bad, you get no signal with the grey one. Possibly two dodgy cables then, I'd be tempted to try and find another.
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #194 on: April 19, 2016, 12:03:30 pm »
CJay,

I currently don't get the same results form both cables. I am suspicious that one is not working at all.

I will check to see if there is a power meter here at work that I can grab.

Are there any other suggestions for test devices that might help make this easier to troubleshoot?
 

Offline CJay

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #195 on: April 19, 2016, 12:19:05 pm »
CJay,

I currently don't get the same results form both cables. I am suspicious that one is not working at all.

I will check to see if there is a power meter here at work that I can grab.

Are there any other suggestions for test devices that might help make this easier to troubleshoot?

Yeah, I realised after I'd posted that one cable was definitely faulty and edited but I guess you saw before I saved the changes.

I'd definitely try to find another cable and if possible ditch the BNC to N adapters if possible.

-20dBm is pretty low, in the region of 60mV P-P so you're going to struggle to measure it with something like a simple diode probe and you'll need to make sure that any power meter you borrow is sensitive enough to measure that sort of signal level too.

A 'scope would be useful if you can borrow one. 

The Analog Devices AD8307 (or similar) chip could be used to 'knock up' a simple power meter that'd give you a reasonably close measurement.

If you search for Wes Hayward AD8307 you'll get a ton of hits and schematics.
 

Offline RadioAct

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #196 on: April 19, 2016, 03:08:06 pm »
For what its worth, here is a half assed measurement of the Lo from the test port on my sepc A. Hopefully your issue is just a bad cable/connector.


-Ty
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #197 on: April 19, 2016, 06:52:10 pm »
So i tried a 3rd cable and i get the same results as i currently get from the black cable.

I'm trying to see if i can take a power meter home, and if you i will get all the screws put back into the unit and take it to work tomorrow and hook it up here.
 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #198 on: April 20, 2016, 11:48:36 am »
quick update...

I have the unit with me at work and just hooked it up to a power meter and I am getting exactly -20 db out of the "cal out" and confirmed that it is putting out a freq of 30 Mhz.

However when feeding directly into the RF in it is reading -70 db, I assume this is the issue with it not seeing the cal signal.

It is with one of out RF Technicians now and h said he also checked the attenuation and that seems to be working fine.

Also, we noticed that when you go into the Self Cal menu there is a options for input reference (or something like that) do you think that might be something that needs to be set or changed?

Any ideas?


 

Offline jc81382

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Re: Advantest R3131A Spectrum Analyzer 9KHz-3GHz
« Reply #199 on: April 20, 2016, 01:41:51 pm »
As another quick follow up...

When we source from an external unit a 30 Mhz output we need to up the amplitude to ~ -3 db to get the Spec A to register -20 (as it should be seeing from the Cal Out).

Now, I may be wrong but I assume that when the unit performs the "Self Cal" that it will make any adjustments/offsets to that 30 Mhz @-20db signal. So I am not that surprised that things are reading off. After all this unit was sitting for quite some time, and had a dead CPU battery so any correction that may have been programmed into it have  been lost.

My real question here is when you attempt the "Self Cal" there is a mechanical clicking noise that I assume preparing the unit receive the expected Cal Out signal.

Does anyone think there might be ab issues with some sort of relay that might not be functioning properly and causing that to not happen?
 


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