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Hi everyone,
the nanoVNA V2_2 (SAA-2N) has finally arrived. I wanted to use it to fine tune the four-cavity filter (BPF) at 2200 MHz, I was thinking of connecting CH0 to the 2nd LO input of the TR503 and the CH1 to the IF output of the oscillator at 2095 MHz, or it is better to connect the CH1 at the mixer output, by disconnecting the semi-rigid cable from the input of the oscillator at 2095 MHz ?
First of all I put the cables and adapters together and connect them between CH0 and CH1 and perform the calibration of the nanoVNA by setting the center frequency to 2200 MHz and the span to 20 or 30 MHz.
Honestly I am not very familiar with VNA, if you have any advice and suggestions I would be grateful.
Thank you.
-- Cheers
Attilio
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Thanks Ed,
I did not think about it. I'll read the instructions in the 7L13 manual.
--Cheers
Attilio
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Attilio,
Before you worry too much about the tracking frequency issues, check the 7L13 manual and its adjustments. There should likewise be a crystal oscillator in there that sets parts of the frequency plan, and it too may need adjustment. The two systems have to be reasonably close together and stable, to get proper tracking range.
Ed
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Thanks to you Ed and Miguel if I was successful.
There is still work to be done, but the most is done.
You are right, I have to reread all the procedures of the TR502, but I have the feeling that the IF bandwidth at 105 MHz is narrow compared to what is indicated in the manual, I have to understand why.
Yes Ed, I put the 18 Vdc zener between the control voltage and ground, as a protection for any excess voltage.
I checked the frequency range of the Tracking Adjust command and it is as per the manual, at center stroke the crystal oscillator is at 52.500 MHz.
I also thought that further reducing the Iosc current also decreases the power supplied by the oscillator.
-- Cheers
Attilio
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Hi!
At the 7L14 manual end is the 2.5ghz mod explained
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Congratulations Attilio! I knew you could do it, even with the equipment limitations. So, it looks like the collector tuning was totally limited out, and changing the emitter bias was key to success.
Regarding the 2LO BPF tuning, I think now that you have the LO working and locking OK, and you have a higher external supply available, that doing the BPF adjustment according to the manual (except now the external tune supply is different) should work. But, which tuning procedure? Since you've now effectively converted it from a TR503 to a TR502, then that's the one to go with. I don't know how much (if any) difference there is between them, but it would be good to compare to make sure you're not missing any details about how it should work. BTW, did you add a protection Zener as recommended? It's good to have some protection, especially for manual tuning with an external supply.
The tracking should be easy to tweak up. You just need to (after the 2LO BPF is properly adjusted) center up the tracking range, stability, and coverage. That's all in the crystal oscillator. Again, you should be using the TR502 manual for reference, and also look for differences versus the TR503. I remember seeing something about a variable trimmer cap in the oscillator, called "mode adjust" or something like that, that seemed to be pretty important. You can trim the crystal frequency a little either way, once you know which direction it needs to go. Look back through the whole tuneup procedure for both models.
Regarding further reduction in emitter bias, it looks like that would drop the required supply/tune voltage some more, making it easier for the PLL. But, it also decreases the LO output power available, so you don't want to go too far down. I'd say leave it right where you have it for now, and see how everything works through all the calibration/adjustment steps.
Ed
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Hi everyone,
I reassembled the TR503, connected it to 7L13 (SA) and I have the signal on the screen of the SA.
With the MAX span / div the trace drops a bit from 1 GHz to 1.8 GHz but it is in the 2 dB, so I think acceptable, with 50 MHz of span / div the trace on the screen is a straight line. I can get a decent adjustment of the TRACKING ADJUST on the TR503 with the resolution at 300 Hz on the 7L13, but at 30 Hz the adjustment is not enough, maybe some calibration needs to be revised.
I also have two doubts: when I redid the calibration procedure of the BPF four cavity filter, I got the curve centered, but very narrow, from the manual it seemed about 20 MHz wide while to me it is around 2 MHz and I was unable to widen it acting on the BPF filter settings. The other question is about the 7L13: is the PHASE LOCK active with the selector positioned up or down?
When I place it high below a white dot appears.
Thanks for your help
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Hi everybody,
I have some positive news: I added a 180 ohm resistor in series to R414 and thus 2095 MHz is reached with 15 Vdc of control voltage (21 mA of absorption, against 28 mA of before). The voltage on the Iosc terminal is now 1.21 Vdc.
I reconnected P446 and the IF output of the oscillator to the input of the amplifier at 100 MHz and connected the 2nd LO output of the 7L13 to the 2nd LO input of the TR503: the control voltage from the PLL is stable at 15 Vdc, the IF frequency is 105.009 MHz and the IF signal level is around 77 mVpp, this after adjusting the four-cavity BPF filter to get the maximum IF signal.
How can I continue now?
Should I try further increase the resistance in series to R414?
Thanks for your help.
--Cheers
Attilio
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Here I am,
I restarted with another external power supply and the frequency meter connected to the IF output of the oscillator and I have these results:
with control voltage at 17.5 V dc the frequency is 2095.050 MHz, with 20 Vdc it is 2097.600 MHz.
I made four more measurements: with a control voltage of 8.3 Vdc the frequency is 2072.019 MHz, with 12 Vdc it is 2085.740 MHz, with 15 Vdc it is 2091.650 MHz, with 16.5 Vdc it is 2093.122 MHz.
The absorbed current is constant at 28 mA.
I don't know, but I think the control voltage output from the PLL is a bit at the limit.
I suppose 20 Vdc for the oscillator is a limit not to be exceeded (with 20 V it reaches 2097.6 MHz), but the PLL stops at 17.6 V at the maximum.
--Cheers
Attilio
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Hi everybody,
sorry for the delay, I made some tests following Ed's instructions. By connecting the oscilloscope to the output of the oscillator control voltage coming from the PLL with the P446 connector disconnected, there is a signal that starts at + 1.6 Vdc rises up to 17.6 Vdc it remains there for 8 ms then it gradually drops to + 1.6 Vdc in 8ms then after about 200ms it repeats and so on. By connecting the P446 connector and the IF output to the input of the amplifier at 100 MHz, the signal on the P446 connector remains the one described above, then connecting the 1st LO output of the 7L13 to the 2nd LO input of the TR503 and tuning the center frequency of the 7L13 to have on the 1st LO output about 2200 MHz (FC at 109 MHz) the signal on P446 changes into an almost square wave that varies from + 14.6 Vdc to + 17 Vdc giving the idea that the PLL is working.
By connecting the 2nd LO output of 7L13 to the 2nd LO input of the TR503, the signal on P446 rises to + 17 Vdc and remains a straight line as if the output were saturated at the maximum level.
I think the problem is the signal level at the input of the 2nd LO of the TR503 (the 1st LO of the 7L13 comes out with + 10 dBm, while the 2nd LO comes out with - 5 dBm).
As soon as I can I do other tests, the problem is that I don't have an adapter to connect to the IF output of the oscillator to measure the frequency.
--Cheers
Attilio
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Thanks a lot Ed.
I try in the day.
--Cheers
Attilio
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Hi Attilio,
Yes, keep your new crystal (52.5 MHz) in place from now on. For the first test, you want the setup you already were using at some point before, with the counter in high band, looking at the IF output, where it previously showed the LO frequency, and with the LO running from +15 V, which was the limit of your external supply, as I understand. This should show something around 2095 MHz on the counter, as before. The PLL should be running open loop without any IF feedback, which is what you want. Then you want to confirm that the PLL tune output voltage can rise and stay at somewhere around +17 V, which is about where I think it should go when open-loop. But, if the search oscillator kicks in, then it will need to be disabled. If all goes well, the tune voltage will go up and stay near 17 V.
Presuming the first test works as planned, the next step is to remove the external supply, and reconnect the internal tune voltage via P446, so now the LO is running near 17 V, and should be at a higher frequency, which should show on the counter. That tells you how high it can go without additional modifications, and it's hopefully well over the 2095 MHz result at 15 V. If it is, then there's a very good chance that when you close the PLL loop, it will run the LO up to the desired 2095 MHz and lock, and the tune voltage should end up somewhere near 15 V (where you got about 2095 MHz before), and be stable.
The last test finally closes the loop by restoring everything to normal - putting the 2LO in from the 7L13, connecting the IF out to the PLL, and enabling the search oscillator. Now you're looking for a stable tune voltage around that 15 V, which would show that the PLL is in control, and the LO should be locked at 2095 MHz. The counter isn't necessary, but you can measure the IF if you want, with the scope probe and counter setup as you did before, tapping into the IF amplifier signal, but be sure to not interrupt the IF to the PLL.The IF should be at the new 105 MHz, or at 52.5 MHz after the divide by two circuit. Now everything should be running about right, if the 2LO BPF isn't so far out of adjustment that there's not enough signal to get a good IF level.
In reality, you could skip the first test, but it's good to start from a stable point you know, where you had it set up and measuring the LO frequency. Also, if the tune voltage driver does not operate as I expect, then that issue would need to be addressed before the next steps can work..Good luck.
Ed
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Many thanks Ed,
I have read everything you wrote to me, I only have a couple of questions: when I reconnect P446 and the PLL commands the oscillator at 2072 MHz, the frequency meter where I connect it, considering that if I have to make the PLL work the IF output connector will be connected to the 110 MHz amplifier input ?
I could connect the frequency meter to the 110 MHz amplifier input.
When I connect the 2nd LO of the 7L13 (SA) to the 2nd LO of the TR503, on the oscilloscope should I see a line positioned at about + 15 Vdc?
Now I have the 52.5 MHz crystal on the PLL reference oscillator, should I leave it or put the 55 MHz one back on? I think the 52.5 MHz one should be left.
Tomorrow I'll try to carry out all the tests, then I'll tell you.
Thanks again.
-- Cheers
Attilio
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OK, Attilio, here you go:
I looked at the LO drive circuits and circuit descriptions and calibration info in the TR503 and TR502 manuals. I think you've been hung up on this 2LO BPF adjustment issue, where you're manually driving the LO with an external supply. Forget about that for now. You don't need to redesign the circuits to get higher tune voltage - it's already capable of reaching around 17 V, maybe a little more, which hopefully is plenty enough. At the plus extreme, U365A's output should rail at somewhere around +18 V or so (you should look up the NE5558 specs to see just how high it can get), since it's powered from +20 V. Q420 and the current limiter should drop about a volt, so the output will be able to get to around +17 V, and intrinsically limited there. Q445 and Q430 form a search oscillator to make sure the tune voltage sweeps around until phase lock is achieved, but apparently only kicks in if the tune voltage drops below +3 V. In the BPF tuning procedure with external supply drive, it says something like "do not exceed 20 V," so the capability of the circuit is within the limit.
In your test setup, you already have the PLL open-loop since you've run the IF into the counter, and the PLL tune voltage is disconnected. So, using your current setup, with the LO powered from the +15 V external supply, and measuring about 2095 MHz on the counter, check the PLL output tune signal with a scope - it should be either stuck high near 17 V, or sweeping up and down between there and some low value around a couple volts.
If it's stuck near 17 V, then that's perfect, just what you want for the next step. Now shut everything down and disconnect the external supply, and reconnect P446 so the PLL will run the LO. Fire it up and observe the frequency on the counter - it should be stable and well above 2095 MHz. Continue to monitor the tune voltage on the scope, and make sure it's still up around 17 V, not sweeping. If it is sweeping, then disable the search oscillator by grounding Q445's collector. You should now see the maximum tuning frequency available, on the counter. Hopefully, it will have a comfortable margin, maybe 2100 MHz or higher. If so, then shut down again, and hook everything back up to normal, and connect the 2LO from the 7L13. Continue to monitor the tune voltage. Fire it all up, and see if the PLL locks, indicated by the tune signal being constant, somewhere near +15 V. If it is, then you've lucked out, and the 2LO BPF is close enough to get a good signal through, the IF and PLL are working, and the LO is running around 2095 MHz. The rest is then a matter of perfecting the changes and tweaking it up. At this point it would be good to shut down, and solder an 18 V, 1 W, 5% Zener diode onto the LO's power supply/tune connection, from there to ground, to serve as an absolute clamp around +18 V. It will also provide reverse voltage protection. The physical location can be at the module (best) or wherever convenient, as long as it's on that line, and can't be disconnected accidentally. This will give some protection against accidents during any situations where external driving is used - later during tweaking - or if a failure in the tune driver (like if Q440 were to short) lets it go over-voltage.
Whether this all works, or doesn't, please report the results, and we'll see what to do next. Good luck.
Ed
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Hi Miguel,
OK, tomorrow I'll try, I act on R412.
Thank you.
--Cheers
Attilio
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Hi Attilio As Ed said tray to play with R412 R414, increase value, R412 at 470 for example, to see if the LO varies -----Mensaje original----- De: [email protected] [mailto: [email protected]] En nombre de Attilio Enviado el: domingo, 7 de febrero de 2021 0:19 Para: [email protected]Asunto: Re: [TekScopes] Spectrum analyzer Tektronix 7L13 on mainframe Tektronix 7603 Hi Ed, thanks for the support you are giving me. Honestly I'm afraid to tweak the inside of the 2072MHz oscillator it's all so microscopic and it feels delicate. The line partially covered by the Iosc input pin is straight and unbroken. For the idea of ??increasing the control voltage between 15V and 20V I am concerned about approaching the maximum limit of the transistor, I would not want to break it. Also I don't know how to change the control voltage which now goes from 5V to 12V, which has to go from 15 to 20V, with less voltage variation (5V vs 7V). Iosc is connected to the 2072MHz oscillator transistor emitter, I could try to modify the Iosc circuit to see if the frequency varies. Thanks again Attilio Scanned by McAfee and confirmed virus-free. Find out more here:
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Hi Ed,
thanks for the support you are giving me.
Honestly I'm afraid to tweak the inside of the 2072MHz oscillator it's all so microscopic and it feels delicate.
The line partially covered by the Iosc input pin is straight and unbroken. For the idea of ??increasing the control voltage between 15V and 20V I am concerned about approaching the maximum limit of the transistor, I would not want to break it.
Also I don't know how to change the control voltage which now goes from 5V to 12V, which has to go from 15 to 20V, with less voltage variation (5V vs 7V).
Iosc is connected to the 2072MHz oscillator transistor emitter, I could try to modify the Iosc circuit to see if the frequency varies.
Thanks again
Attilio
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Hi Attilio,
I wouldn't worry about trying to tweak the filter yet, until the oscillator is right. Even if you manage to adjust the filter with a separate equipment setup, you'll have to redo it anyway in the final steps. The manual mentions that the RF sections interact quite a bit during adjustments, when everything is hooked up for normal operation. There is very little isolation between them, so you can't treat them as independent functions that can be perfected separately - in other words, it doesn't matter yet.
The oscillator will also act differently when the 2LO filter and mixer are hooked up (mostly due to the mixer load), for better or worse, frequency-wise. The main thing now is to get that tuning range to where you're confident it can get well past 2095 MHz, with the mixer load included, which I think has been the situation all along. The 2LO signal is small compared to the LO mixer drive, so its effect will be fairly small. Also, unless it's possible to have the BPF grossly misadjusted, most of any LO power at the mixer's 2LO BPF port will be reflected off the filter and return to the mixer, adding or subtracting to the drive, depending on phase. These effects are attenuated by the isolation of the mixer and the coupler, which may be 20-30 dB in total.
Did you get a chance to inspect the end of the resonator line? If you can somehow get a decent view (it's blocked from top view by a feed-through post), it would be good to see what it looks like, and see if there are any trim features, as I mentioned in my previous post.. Also, can you estimate the clearance from the post to the substrate? Don't do a mechanical measurement that risks damage from poking around in there - just a visual assessment, looking from as far to the side as possible.
The reason for this info is consideration of more drastic options if necessary. Besides blocking the view, this feed-through post is important. First, if it turns out that trimming the resonator end becomes necessary, the post may be a good reference surface for tool control and leverage. Second, deforming the post may provide a good - but risky - mechanical tuning option. I can't quite figure out if the resonator is supposed to be approximately a quarter-lambda or half-lambda, with all the stuff going on in there, but either way, that open end should be hot, RF voltage-wise. This means that the post, being close to the end, may have a fairly large effect on the frequency, due to its capacitance to the line. If there's very little clearance as-is, then bending it upward away from the substrate may have a very large effect, letting the frequency rise. This can be very risky of course, depending on its diameter and what it's made of. I think they are (unfortunately) usually fairly hard steel wire, which you can confirm by checking with a small magnet, being very careful to not let anything slip and damage the guts - you don't want a magnet getting loose and jumping in there trying to stick to something else. Bending the post also risks damage to the attached bond wire, and the feed-through insulator at its base, besides the risks associated with grabbing and prying on it. But, it is an option for later on, if necessary. Don't do anything in there yet unless the other options are exhausted.
The easier options are external, like your plan for upping the tune/supply voltage, as discussed earlier. I think that if the original design allows for it to go toward 20 V before the protection clamping kicks in, then you have some room for more, but study the circuit descriptions and schematics thoroughly to be sure. As I mentioned earlier, the tuning effect (df/dV) diminishes, the higher you go, so you may have to get the voltage up fairly far beyond 15 V (where it reaches 2095 MHz now, right?). The transistor power dissipation will go up in proportion, and get a little warmer, which tends to oppose rising frequency. It shouldn't be too much of an effect, since the alumina substrate provides excellent thermal conductivity. During your experiments with tuning, allow some time for things to thermally stabilize at each data point.
If you can get the voltage tuning to work with comfortable margin, then I'd say that's it, and go for it. It's fairly easy to modify the circuits to get whatever you need, once you know what that is. Once you get everything working, including the PLL, don't be surprised if you may need to make slight changes in the PLL for stability. It probably will be just fine, but since the VCO (LO) tuning sensitivity may be much different (less) than designed for, it could change things a bit.
Another possibility is to change the transistor current a little bit, and see if that can move the frequency enough that the original tuning range can work. In all the models, the manuals say the transistor runs at about 25 mA, and they all have the same bias arrangement. You could try changing the current by a reasonable amount, say up to 10-20 percent, so maybe run somewhere between 20 and 30 mA, being especially careful to not slip up and allow it to get beyond the upper limit. I think in this situation, increasing the emitter bias current should raise the frequency, but it could go the other way around. First try upping the current a little (maybe 10%) and see what happens.
So, between modifying the collector voltage, and the emitter current, I think you should be able to arrive at a workable external solution. If it turns out that you have to increase both the collector and emitter by quite a lot, then you may have to worry about the transistor's SOA and power dissipation, but unfortunately there's no way to know for sure how far it can be pushed, without the part's identification and info. If all of this is still not enough, then the more drastic options should be considered, but I doubt it will come to that. The saving grace in all this sort of stuff is that your instruments likely won't be required to perform over the original specified commercial temperature range - they should be nice and comfortable with you in your lab.
One last thing - on the cover(s) of the LO/RF module, are there any microwave absorbers attached? These would look like thick (maybe up to 1 mm) pieces of rubbery stuff glued to the inside of the cover, in certain places, to suppress cavity resonances inside. If there are any in the LO section, that's yet another place that tuning can be affected, but can be very complicated.
In all the experiments, you should just look at the LO frequency on the counter, directly from the IF port, as before, and leave the 2LO mixer/BPF connected as normal, and do not connect the 2LO from the 7A13. As you get the tuning perfected, you can experiment with the various other pieces to get an idea of how much interaction to expect.
After all this is done and working, then you can finally tweak that 2LO BPF that's been bugging you all this time, with the procedure in the manual, as intended.
Ed
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Hi everybody,
I don't know what to do with the 2072 MHz oscillator, to bring it to 2095 MHz, having no adjustments. I wait for the nanoVNA to arrive to recalibrate the four cavity filter at 2200 MHz.
--Cheers
Attilio
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Thanks Ed and Miguel for the help you are giving me.
Miguel, I'll send you more photos under the microscope today.
At the left end of the meandering resonator is a double comb which I think is a capacitor which is not on the schematic, the other end of the resonator is open.
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Attilio