Tag Archives: YIG

HP 8753C Network Analyzer: spare YTOs

Not giving up on the HP 8753C Network Analyzer repairs. Especially, because these units are really top class in terms of specifications and serviceability, easy to use, and well handled by GPIB bus software. Not sure why you would by and later model, if you have a working 8753C or similar model.

Some study of the YIG oscillators used in the 8753 series analyzers, while these are nothing special neither in output or tuning range (3.8-6.8 GHz, and a quick test on the source assembly shows that everything above 8 dBm seems to drive the source mixer to saturation), they have a ~23 mA/GHz tuning current, much less than the 50 mA/GHz (20 MHz/mA) of many industry standard and common YTOs. No idea why that is, a bit less power consumption, a bit less copper?

Looking around, found this marvelous Watkins-Johnson YTO, for just about 15 EURs. It was a bit dirty with wires badly soldered to it, but easily cleaned up.

Power is good, measured through a 6 dB attenuator, and pretty stable all over the range (won’t even need it up to 8 GHz).

a href=”http://www.simonsdialogs.com/wp-content/uploads/2020/02/8753-yig-replacement-power.jpg”>

There were no data on the power supply, so I needed to test it out. Heater voltage, seems to work well with 15 Volts, supply voltages -5 and +12 V give stable operation with sufficient margin (there seem to be internal voltage regulators).

The tuning current 50 mA/GHz, about double of the 23 mAh/Ghz of the HP YTO. So, there will be more heat dissipated, let’s do some calculations around the YTO driver. Located on the A11 board.

Clearly, the transistor will have to dissipate more heat. Do avoid any big changes, let’s adjust the current sense resistor to about half the value – resulting in about the same voltage drop per GHz.

The driver transistor, nothing special, a NPN TO-3 part, used without a heatsink.

There is no good space to fit a heatsink for TO-3 transistor, so I replaced the transistor. Used a BD245C, with amplification hfe of 40, well good enough. The SOT-93 case, it can easily be mounted on a piece of sheet metal (aluminum) to provide enough cooling.

Everything well insulated and mounted. Using the TO-3 screws to hold the transistor/heatsink assembly.

The current sense resistor, the key part for any YTO driver, needs to be low drift, low thermal coefficient. Otherwise, there will be all kinds of drift. HP use in some of their equipment sense resistors specified to 2 ppm/K, the best I could get is about 15 ppm/K with Dale RH-10. One day I will check their actual thermal coefficient. Now I just burned it in a bit, and selected one that looked perfectly stable with any load changes up to 1 Ampere.

The A11 board can be modified fairly easily, even reversibly – replace the sense transistor, from 40 Ohms to 20 Ohms, cut a trace (the current sense voltage to the opamps, close to the board connector – the via is handy to attach a wire). And a 18 Ohm/390 Ohm/200 Ohm -10 turn pot arrangement to set the proper currents.

Another small modification, the 1 k/100 network across the main coil – replace the 1 k resistor by 2.2 k.

The 8753C provides +15 V and -15 V at the YTO connector. Assembled a small power regulator board, to provide the necessary +12 V and -5 V.

Testing…

Several adjustments of the 10T trimmer, but there are issues – no stable lock below 3.3 MHz, and the Service Function 58 won’t do the pre-tune corrections. Note that you can use the source tune mode to monitor the YTO with the PLL disengaged. Ideal for adjustments of the YTO slope. Still no stable lock. Working reasonably stable at times, but all a bit temperamental.

Various lock issues, and the self tests won’t work well.

Some study of the manual, and quite some time spent to add more components, changing PLL filters, and so on. But no luck. Below 3.3 MHz, there is a also a gain change of the PLL, by Q10 FET, also modified a bit there, some improvement, but not as stable as it needs to be.

Maybe, some specific issue of this YTO, at least after detail study of the tuning currents, some magnetic hysteresis, or similar. Not an uncommon problem. Not all YTOs are suitable for fast, precise sweeping and phase-locked operation.

After all, let’s give it another try, with another YTO. Found this beauty for USD 25, a great AVANTEK YTO, ASF-8347M, with solid output power. Hermetically sealed, in a magnetically shielded casing.

Avantek S081-0321 YIG oscillator: not oscillating at all

One of the best sources of microwave signals still are YIG oscillators/YTO. These do require a good amount of power, magnetic coils, etc, but provide stable and rather low noise output, and good modulation capability. Core element is a small YIG sphere, placed in a magnetic field.

However, for the current unit under investigation (from a 18-26 GHz frontend), type S081-0321, 8.0-13.4 GHz, all the magnetic field and effort is wasted – no output detectable at all, not even a faint signal (checked with various equipment). Knocking it with a (small!) hammer, no effect. Varying the coil current – no effect.
Current consumption on the 15 V rail is normal.

yig-test-no-signal

yig-s081-0321-defective

Well, with all the basics checked, what to do with such hermetically sealed unit, other than using it to satisfy my curiosity about its internals. Hope to trace the defect to some specific part.

But before we consider more destructive measures, let’s try to re-tune the YIG by slightly adjusting the YIG sphere. This is possibly throught the side opening, which is usually welded shut, but can be drilled up rather easily.

yig-adjustment-open

Still no luck, no signal, even after turning the YIG quite a bit.

To look inside, carefully removed the top weld seam on a lathe, and the you can pry the case open.

yig-osc1

What you can see is pretty straightforward, despite all the gold wires. There is an input voltage regulator, from +15 V rail, down to 8 volts (measured about 8.15 V), this is then distributed to the 4 active parts via resistors (the bluish elements). Voltage at the resistors is about 4.3 V, so all stages seem to be adequately powered and current flowing as usual. Still no signal. Also probed other parts of the circuit, with a thin wire, under the microscope. No obvious defect. The gold wires and contact point reveal a good amount of adjustment done by placing/removing bond wires as need to adjust bias currents, probably also frequency response, etc.

yig-osc-closeup

The coil – rather, the coils. The thick wire is the main tuning coil, which accepts 0.4~0.6 Amps, the small coil around the magnetic center pole is the FM modulation coil. This is for much lower currently but high bandwidth modulation. All is sealed and soaked with epoxy resin. Note the hand made labels which may explain the cost of these units if purchased new… looks like US style handwriting to me.

yig-internals-mag-coil

Well, seems that fixing this is beyond what I can do here with the tools at hand. So will need to look for a spare/used 8-13.4 GHz YIG/YTO somewhere.

HP 8566B Spectrum Analyzer: 2 partial units and some spare parts

This story starts with a set of rather valuable 8566B parts that I received for free a litte while ago:
8566b spare parts

A partial unit, stripped of of most of its RF parts, and missing some boards, and missing the OCXO.
8566b partial unit1

For a long time, I have been looking for another partial unit that can provide the missing boards, the OCXO, and some bits of hardware to complete the instrument. Not worried about the 85662A display units, because I have a perfectly working spare unit around, or could use the unit of the 8568B.
Finally, a unit showed up, also missing some boards and parts, but luckily, not the boards that I needed -except, also no attenuator, and no OCXO here.
8566b partial unit 2
8566b partial unit2

That’s the start, the empty space that is going to hold the RF treasures:
8566b empty space

The YTO assy, missing the YTO, and other bits.
8566b yto loop incomplete

This part, the 5086-7226, to do it fully justice, one would have to talk about it for a few hours. It is a not only gold plated inside and out, but HP used two kinds of solds, of different melting point, to assemble the inner workings in subsequent steps, without melting the already assembled parts….
8566b 5086-7226 YTF

Some more pictures – the YTF driver.
8566b ytf driver front
8566b yig driver back

The 1st and 2nd converter assy, ready for the semi rigid lines to be attached.
8566b 1st and 2nd converter

A high quality input relais and a band pass filter.
8566b input relais and bp filter

Well, unfortunatly, don’t have a spare 8566B/8568B OCXO around, and they go on xbay for no less then USD 50, often, no less then USD 100, that’s ridiculous.
May this unit, which is very low phase noise, very stable, from a HP 3585A analyzer, can be made fit? Ovenaire OSC 73-52.
8566b spare ocxo 0960-0465 ovenaire osc 73-52 10 mhz

After a LOT of fiddly work:
8568b assembly progress

8568b testing

A first signal!! Amazing! Frequency is off by 80/300 MHz – the unit will need a proper alignment, but the PLLs are all locked, which is a great start.
8566b a signal

And, the noise – the effect of harmonic mixing can be clearly seen, so the input stage and mixers seem to be all working!
8566b noise

More to come!

Wiltron 6659A Programmable Sweep Generator: 10 MHz to 26.5 GHz, in a single sweep…

This instrument just needed a few adjustments, still, very interesting to look at.
It is of a very classical design – 4 YIG oscillators (2-8, 8-12, 12-18, 18-26.5 GHz), a coupler, a detector, and all the driver and ALC circuitry to make this work.
There is also a Wiltron-brand downconverter that provides the 10 MHz to 2 GHz output, by conversion of a 4610-6600 MHz input, with a 4600 MHz LO.

The YIGs are all of best quality, Avantek parts.

The coupler, for the ALC loop, a Krytar ultra-broadband part, with a biased detector.

wiltron 6659a

6659a alc detector

wiltron 6659a yig assy

wiltron 6659a front

The generator provides ample power, 10 dBm or more, over most of the bands; about +6 dBm, at above 18 GHz – and, around 23 GHz, there is a dip in the power curve (see scope screen, showing the ALC/power signal vs. horizontal sweep). Checked the bias (was set at a constant +11 V) – changing it, and going up to +15 V, no change. Also checked the power directly at the YIG output – still, the dip. So it seems, nothing we can do about it, but for most practical purposes, about 0 dBm will be plenty, at any frequency.

wiltron 6659a power measurement

After some more alingments, the frequencies and bands are spot-on (no need to re-programm the linearizer EPROMs – all YIGs are still tracking perfectly fine), some some cleaning, using 50% isopropyl alcohol – done.

The only thing left to be done – a back-up of the 2716 EPROMs that still hold the firmware, after about 30 years.