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Old 5th Dec 2018, 10:15 pm   #1
orbanp1
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Default 2GHz oscillator not working - faulty step recovery diode?

Hello Everyone,

I am in the process of repairing a pair of Polarad-632B spectrum analyzers and in one of them the second LO, a 2GHz oscillator is not working.

I am writing about the repair briefly here:
https://www.eevblog.com/forum/repair...25/#msg1917425

The second LO oscillator is based on a 100MHz X-tal oscillator which is doubled, amplified, and is driving the step recovery diode (SRD) based harmonic (comb) generator. The diode is followed by a three-chamber helical filter tuned to 2GHz.
Ultimately the second mixer would need 2dBm power at the LO port.
I am enclosing the schematic of the oscillator.

The base of the oscillator seems to be working, when loading the output with the input of an SA I am getting about -10dBm power at 200Mhz.
Attaching the SDR assembly, which is the input coupling to the following filter, does not produce any additional harmonics.
So I am wondering if the SRD is working or not.

The SRD is an HP-5082-0112 diode.
The SRD checks out OK at DC, the forward bias voltage is about 740mV.
The mounting is also fine, when running through about 10mA current in the forward direction the voltage drop is about 830mV.
I am enclosing a picture about the diode assembly.
The anode of the diode is soldered to the center-pin of a SMC bulk-head connector, the cathode is grounded, it is "glued into" a small hole in the frame of the bulk-head connector by silver paint.
I am also enclosing a couple of pictures of the oscillator module.

I am wondering what would be the best way to proceed?
I would like to try another SRD, but have not really found that HP diode available.

Any ideas or advise?

Thanks, Peter
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Old 16th Dec 2018, 10:51 pm   #2
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Peter:


I've only limited experience of using step-recovery diodes, and it was a long time ago!

Hoowever, as you can infer from the circuit, you have to pump quite a lot of RF power into them to get the snap to work to high frequencies. There are two calss C amps in series driving your diode, and given that R14 is rated at 2W, you should expect at least a few hundred mW from the 2N3866. That would be my first port of call.



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Old 16th Dec 2018, 11:45 pm   #3
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

The helical filter will have loss, the SRD multiplier will have more loss, so maybe your 200MHz power is too low to get things going.

Be careful about DC testing exotic diodes. Some can be ruined by just a tough with an ohmmeter, but SRDs are a bit tougher than tunnel diodes.

The man who improved the SRDs at HP was Zvonko Fazarinc.

HP's diode and MMIC business went with Agilent when the firm split. Agilent, strapped for cash at the time sold it off to a finance consortium. It ran well as Avago for several years, but recently they bought Broadcom (no not the other way round) then they changed their name to Broadcom and obsoleted all the diodes and MMICs. I'm one of the people having to redesign lots of products to get rid of the things before life buys run out. Grrrrr!

Next Avago/Broadcom rep to stick a head round our door gets boiled in oil.

Anyway that's why those things are in very short supple. I think theres a surplus dealer, Sphere or something like that handles HP surplus parts in N america.

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Old 17th Dec 2018, 8:45 am   #4
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

RFM sell them. https://www.rf-microwave.com/en/hewl...ine/5082-0112/

I buy quite a few things from there. Very good company.

Another option is replacing the entire signal source. There are lots of little ICs these days which can quite happily knock out a 2GHz signal with low phase noise and harmonic content.
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Old 17th Dec 2018, 3:57 pm   #5
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Thank you very much for all the comments, I really appreciate it!

I did check the datasheet of the HP-0112 SRD before subjecting it to the DC tests!
It is max 35V reverse voltage and 50mA forward current.

I have also been wondering about enough power to drive the SRD.
I did check all the transistors in the module and they are all fine.
R14, that 51 Ohm 2W resistor at the output has increased in value to about 60 Ohm! I would be surprised if that made that much difference, though it would give a mis-match to that short piece of 50 Ohm rigid co-ax cable on the PCB!
The other components that I measured, the resistors, those were within spec.

In that LO module I could "tune up" the first stages as per the manual, using a "sniffer probe", and adjusting the trimmer caps at 200MHz.
Adjusting the final three trimmer caps at the output filter did not make any difference.
Simulation shows, if it is to be believed, that network is a low-pass filter for 2GHz.

I am thinking about taking apart the working SA and measuring and comparing the drive power to the SRD of that module.
On the other hand, I am sort of leery about it, usually Murphy is with me all the time ;-)
I also would not want to open up that working sealed box either, to measure inside, at the edges of the box they used silver paint as RF-seal.
I am wondering if that repair paint that is for fixing rear-window defrosters in cars is suitable as replacement for this, if such repair paint is still available?

I also thought about replacing the whole LO-module with something newer as MrBungle suggests, though as I have no microwave experience, I would not feel confident about selecting a suitable replacement...

The December '64 issue of the HP-Journal and the HP App Notes 918 and 920 are a good source of info on those SRD diodes and their applications.

Thanks again for all the comments and help!

Regards, Peter
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Old 24th Dec 2018, 5:07 pm   #6
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Happy Holidays to Everyone!

Well, as suggested by you, the PA stage is not really working in that circuit!

Did some measurements with an RF-mV-meter, there is about 3V indicated signal at the collector of Q3, the doubler-stage, and at the collector of Q4, the PA stage, there is only about 230mV.
No matter how I look at it, that is not enough!
I took out Q4 transistor, and that is fine, it has a beta about 40 to 60 (can't recall accurately), as measured at DC.
There is not really that much to go wrong in that circuit.
Resistor values are in the ballpark. Did not check the caps and chokes.

The interesting thing, when modeling that section of the circuit, and if the modeling results can be believed, the simulation roughly mirrors the measurements.
It looks like the signal coupling to Q4 does not really work! When there is about 14Vpp at the collector of the doubler stage, there is only about 440mVpp at the base of Q4, and that is clearly not enough to drive a transistor in a C-class power amp.

I am just curious what gives?
Any obvious errors or omissions in the simulation?
(cc1 and cc2 outputs eliminate the DC components of the signals, so the simulator would calculate the average and RMS values correctly)

I am enclosing the simulation schematics and the last section of the (paper) schematics.
I updated the schematics with the inductance values that I calculated from the geometric values of the coils.

I am just wondering where do I go from here, what would be the best way to proceed?

Thanks, Peter
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Old 24th Dec 2018, 7:36 pm   #7
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Those tapped inductors aren't just two or three inductors wired in series, they're auto transformers, so I'd expect some K statements on the spice diagram to represent the coupling.

If that's LT Spice, have a look at the help file section on 'Why there isn't a transformer in LT spice'

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Old 24th Dec 2018, 7:55 pm   #8
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Hi David,

Thanks for the comment, keep them coming!

Yes, it is LTspice.

There are coupled inductors in the simulation.
The L4 tapped inductor in the paper schematics is represented by L16 and L17 in the spice simulation with its "k2 l16 l17 1" coupling statement.
The L9 tapped inductor in the paper schematics is represented by L9, L10, L11 coupled inductors and their coupling statements in the spice simulation respectively.
The only comment on that is that a coupling factor of 0.8 is used in the simulation (just a guestimate) and the partial inductances of the tapped inductor were calculated with a coupling factor of 1. On the other hand the complete coil is tuned by C10 (in LTspice) to the right frequency.

Regards, Peter
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Old 24th Dec 2018, 9:07 pm   #9
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Ah, just spotted them. I usually spread my schematics over somewhat more space.

With class C chains like that you need to start with enough signal to get a stage turned on and going and then get the same at each subsequent stage. Once you get one section low on gain, it just gets worse.

Be careful with measuring voltages, the impedance levels get changed so the power represented by a certain voltage changes.

Look at voltage swings on the bases and be sure that there is enough to get the transistor turned well on. Ideally this means a wideband DC coupled scope but in this case you'll be OK because there is a DC path to ground on each base through an inductor.

The day job has me designing class C transmitter strips to 300W at a GHz, and one of the early designs was bipolar.

David
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Old 26th Dec 2018, 1:02 am   #10
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Hi David,

Thank you very much for your comments, I find them extremely helpful, I really appreciate them!

I started measuring components of the final stage, so far nothing sticks out as the sole source of the problem.

I mentioned that the resistor values were in the ballpark, well, they are within 10% of the nominal value. Technically that means most of them are out of tolerance, the resistors used are 5% carbon-composite components.

There was also a surprise for me, a component that I believed to be a molded inductance, it turned out to be a capacitor!
It did not make any difference in the simulation, it was a "choke" in the base circuit of the final transistor, and it did not make any difference in the simulation results. It was the inductance parallel with C16.
I updated the schematics and enclosing it.

Some more findings from the simulations.
In the "A" version of this module the value of R15 was 1 kOhm originally, in this "B" version it is 330 Ohm. Increasing that value to 1K in the simulation also increases significantly the drive signal to the final PA stage! (The other component values are the same!)
The tank circuit in the collector of the final PA stage, or better, its tuning, is not that important, changing the values of the LC components there barely changes the output signal!
The tuning of the input matching circuit to the final PA stage is important, the correct value of C16 has quite an impact on the output signal. That was the cap with the piggybacked component on it. Most probably it was installed during manufacture when they brought up the module, and that was the component I thought was an inductance!
They could have used a trimmer cap there!

Interestingly, there is a trimmer cap at the output coupling, C20, the value of which does not make that much difference.
May be I should just swap C16 and C20...

In any event I will continue checking the components of the final stage, possibly will change R15 to 1K as in the previous version, and try to tune up the input matching circuit.
Hopefully that would make the module work, as so far I could not find any single reason, or failed component, that would render this module nonoperational.

Again, thank you very much for the help!

Regards, Peter
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Old 26th Dec 2018, 1:35 am   #11
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

L11 provides the DC path for the base.

I've got receivers, power meters and spectrum analysers covering these frequencies and I'm familiar with the technologies (ex design engineer in the spectrum analyser division of HP) The difficulty is describing how to do things the old indirect way with a multimeter!

Investigating the DC component in the base feed chokes might be one path.

David
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Old 26th Dec 2018, 2:10 am   #12
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

With the 2N3866 driven correctly, the 200MHz power at the o/p socket should be +20 to 25dBm so be careful with your SpecAn settings when it jumps into life! For checking DC levels on the devices I'd use a 10k-ish resistor as a probe, it will give only very slight detuning and loading
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Old 26th Dec 2018, 11:14 am   #13
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

I do have a few new 100% certain-not-fake 2N3866 if needed. Tens of thousands of them were distributed throughout the QRP gang, they're marked 4-247 with a Motorola logo.

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Old 26th Dec 2018, 4:07 pm   #14
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

I think it might be better to analyse and test each stage in turn to prove it meets its basic requirements. Otherwise, you could end up wasting loads of time trying to fix a (later) stage that isn't faulty.

It's fairly easy to determine the basic requirements of the oscillator Q1 because it has to produce enough power to drive the (non linear) buffer amplifier into a large signal condition.

If you look at the biasing resistors for the buffer amplifier Q2 it uses 3k9 and 120R. Assuming that these values are correct on the circuit, this will only produce about 0.35V DC at the base of the buffer amp Q2 so this isn't a linear buffer amplifier as it needs a large drive signal level to get it to start conducting. So realistically, the oscillator Q1 has to be able to produce at least 0.5Vpk into Q2 in order to get Q2 into its large signal state. 0.5Vpk into 120R is about 1mW so I think the oscillator probably has to produce a few mW to ensure Q2 goes well into its large signal state. By large signal state I mean when Q2 has something like >20Vpkpk RF at its collector. The difference between this and the small signal state where it only produces a few Vpkpk at the Q2 collector will be quite a narrow window in terms of drive level. This is because of the way Q2 appears to be biased. Note that if the 3k9 resistor R5 has aged maybe 10% high then the Q2 stage will require more drive from the oscillator to get into the large signal state. So it might be worth checking that the 120R and 3k9 resistors are OK in terms of tolerance because this part of the circuit looks a bit fussy to me from a design point of view.

The oscillator Q1 will have been designed to produce a narrow spike of negative resistance at its base at about 100MHz. T1 and C3 and maybe L1 will need to be adjusted to ensure this spike of negative resistance is centred on 100MHz so that the circuit can oscillate strongly at the 99.75MHz crystal overtone. I'm guessing that the crystal is a 5th overtone so Q1 should not produce any significant negative resistance at the other multiples of ~20MHz. So T1 and C3 need to be adjusted to achieve strong oscillation at 99.75MHz and there probably needs to be several mW drive into Q2 for the reasons given above.

You could measure the raw oscillator drive level by connecting C5 to a 50R spectrum analyser but with a 56R resistor in series with C5. I'd expect to see something like -3dBm on the analyser at 99.75MHz with the series 56R inline. Alternatively, just measure the Vpkpk at Q2 collector with a hi Z voltmeter (or simple RF probe) that has low self capacitance. There should be something like 20Vpkpk here. The other option is to measure the power level at the output of the T attenuator at R10 with a spectrum analyser. There should be several mW here at 99.75MHz. But the best test would be to make sure Q2 collector is up at about 20Vpkpk or so.

The remaining stages look quite simple so should be easy to faultfind. However, I wouldn't recommend trying to model L9 as an auto transformer. You would be making a lot of work for yourself because it will be tricky to get the L and K values correct and I think it would be easier to simply model it as a tapped resonator. This would be more than adequate for a simple circuit like this. But for now can you test/prove that Q1 and Q2 are behaving as described above?
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Old 27th Dec 2018, 4:39 pm   #15
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Is it possible to post up an image of the PCB underside? The layout looks a bit odd in places so it's not obvious how it all connects together.

For example, the PCB layout suggests that the resonator taps might be reversed compared to the schematic and your simulation?

At first glance at the real PCB it looks like the doubler connects to the coldest tap on the resonator and the 510R resistor connects to the next tap up the resonator. But the PCB tracking might be different on the underside because this doesn't agree with the schematic.

Also, is it possible to see an original image of the schematic that hasn't been edited?

I tried simulating the oscillator and managed to get over +10dBm at 100MHz (with ideal output coupling at T1) so hopefully the real circuit can drive the buffer Q2 quite strongly. I think the Q2 buffer can deliver quite a bit of power if driven hard and so it would definitely be worth measuring how much power is at the output of the T attenuator before the doubler.
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Old 27th Dec 2018, 6:50 pm   #16
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Hi Jeremy and Everyone,

Many thanks for the help!

I did some measurements as you suggested and planning to do some more!

I am enclosing the original and the updated schematics (I did find some more changes), as well as a pictures of the board.
The other changes are R3 110 Ohm from 220 Ohm in the orignal, the emitter resistor of the oscillator stage, and an extra 15pF capacitor from the base Q2 (buffer stage) to the ground.

Did measure the signals at the buffer stage.
The RF mV-meter is a Boonton 92B. Reading the manual it suggests to use the 50 Ohm adapter at frequencies above 100MHz.
The mV-meter measures true-RMS voltage up to 30mV, above that it goes over to peak-to peak detection with RMS indication for sinusoidal signals.

This time I used an active probe, the home built version (from a kit from the UK!) of the "Poor man's 1GHz active probe". It has about 0.1pF -0.2pF input capacitance, 50 Ohm output, and an attenuation of 17dB.

I measured the 100MHz drive signal at the base of Q2, the buffer amplifier. It was 2.5Vpp with the active probe, and 2.3Vpp with a Tek-454 scope.
The signal at the collector of Q2 was 8.9Vpp with the probe and 9Vpp with the scope.
These readings confirmed that the active probe roughly corresponds with the readings from the scope!
As that scope only works to 150MHz I did not use that for measuring the 200MHz signals at the following stages.

Simulating the buffer stage also gave me 26Vpp at the collector.
Removing the DC-bias totally in the simulation the signal decreased to 16Vpp.
This is clearly a discrepancy to what I measured!

Also continued chasing the signal from the collector of the doubler stage (Q3) to the base of the PA stage.
The signal at Q3 collector was 10Vpp with the active probe, and by the time it got to the base of Q4 it was only about 0.6Vpp.
Nothing like the output springing back to life!

As I followed the signal I also tuned up again C24 and C25.
Taking out Q4 again did not make any difference!
I did measure the beta of Q4 again, it is about 95!

I will do some more measurements, and test more components.
Previously I did test the Q2 buffer transistor, it was also OK at DC.

Thank for Jeremy and for everyone else for the help and the advise!

Regards, Peter
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Old 27th Dec 2018, 7:44 pm   #17
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

Thanks for posting up the PCB plots and the pdf.

It does look like the resonator taps are swapped. Therefore, the doubler feeds to the lowest/coldest tap. So the simulation would need to be updated to be the same. Also, I would suggest going for a basic tapped resonator model here to begin with. Your model values look a bit odd to me but then I don't use LTSPice.

I'd be tempted to use a classic 1N5711 schottky diode detector based RF probe (feeding to a DC DVM) to measure at Q2 collector because a x10 scope probe will have quite a low Rp (as in equivalent parallel resistance) up at 100MHz. It will also have about 12pF input capacitance. I'm not sure how low the Rp will be for the active probe (with a suitable attenuator at the front end for 20V pkpk?) but there is a risk that both probes will load Q2 a lot and give a false (low) reading.
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Old 27th Dec 2018, 8:10 pm   #18
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

A good place to probe the RF level from Q2 would be across the 82R resistor in the T attenuator because the probe loading will be negligible here. The active probe will still be prone to overload here but a decent compensated x10 scope probe with a short ground connection at the tip ought to give a reasonable indication of the RF level at this low impedance point in the signal path. But you would ideally need a scope with a flat response to 100MHz and the accuracy with this setup won't be that good.

Note that the diode detector based probe will only give a rough idea of the RF voltage especially if the waveform is non sinusoidal but it ought to give some indication of the peak RF voltage. Here, I would probably use my old Marconi TK2374 active RF probe with the x100 attenuator head on it for probing this circuit.

The other option might be to use a passive x31 probe consisting of a series 1500R resistor feeding into 50R coax feeding to your 50R Boonton meter. This would be reasonably flat to 200MHz with about a -30dB response (for 50R based circuits) but you would need to ensure there was a DC blocking cap somewhere inline. The input impedance would be about 1500R across LF to 200MHz as long as you used a decent/small metal film 1500R resistor with low self capacitance. You would also need to use a very short/direct ground connection at the probe tip end of the coax and the Boonton meter would need to be arranged to look like a 50R load at the far end of the coax.
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Old 27th Dec 2018, 9:03 pm   #19
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

To give some idea of what a classic x10 scope probe will look like at 100MHz you can do a few calculations based on the info below.
Because a x10 probe uses deliberately lossy coax the loading effect of the probe can be crudely estimated by a series equivalent model of the scope probe's tip capacitance in series with the coax Zo. So for coax that might be around 50R the model would be 11pF in series with 50R. It's then possible to do a series to parallel conversion on this to work out the equivalent Rp and Cp at 100MHz.

https://www.daycounter.com/Calculato...lculator.phtml

A typical scope probe Rp and Cp works out to be about 470R in parallel with about 10pF at 100MHz. So this would load the Q2 collector a fair bit and this assumes a direct grounding of the probe tip. It would be worse with a croc lead used for the probe ground.

Some scope probes use lossy coax up around 90R and this makes things even worse...
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Old 27th Dec 2018, 10:02 pm   #20
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Default Re: 2GHz oscillator not working - faulty step recovery diode?

I had a go at simulating the load impedance seen at Q2 collector at 100MHz and if the total series inductance of L3 and C9 and the PCB traces and Q2 connection is just over 300nH then the load presented to Q2 collector is about 400R at 100MHz. So if we go with your 9Vpkpk then the power delivered to that load will be (4.5*4.5)/(400*2) = 25mW.

This probably isn't enough to fully drive the doubler at L4 if we take into account the lossy T pad that is in the signal path? But I now doubt Q2 collector will get to 20Vpkpk as that would mean something like 100mW from Q2? I'm not familiar with these transistors so I don't know if this is possible with Q2...
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