Tag Archives: Avantek

HP 8753C Network Analyzer: an Avantek fix

Still continuing on the repairs of the HP 8753C. No luck with the Watkins-Johnson YTO, maybe one could get it to work after some severe modification of the phase lock assembly, but this is not my intention. So I rather try another YTO, a Avantek part, purchased for 25 USD.

Some changes are necessary – at least the dampening of the YIG needs some work. Set the 8753 to the source tune mode, and determined the capacitor value to get stable current regulation (in the source tune service mode, the PLL is open).
From about 100-120 nF onwards, in parallel to a 100 nF-2.2 kOhm network, perfectly stable. Added 200 nF, ceramic capacitors, directly at the YTO coil, and modified the R-C network on the A11 assembly a bit (replaced the 1 k resistor with a 2.2 k).

Here a quick schematic, the voltage regulators, and a few Zener diodes to protect the YTO driver. The main modifications are:

*1 cut a trace on the A11 assemly, re-route to the current sense resistor, to allow for some adjustment (sure you can use fixed resistor, once the necessary current has been found.

*2 replace the current sense resistor – most YTOs have 50 mA/GHz current requirement, about double of the original part.

*replace the YTO driver transistor, with a part that can be more suitably mounted on a heatsink, or provide some heatsink to the existing TO-3, if you can manage.

*adjust the R-C/C network across the YTO main coil, good values are 100-200 nF in series with 1-5 kOhm, in parallel with 10-250 nF. Check for clean and fast current regulation, with the PLL open (source tune mode in the service menu).

After the preliminary checks (adjust the current of the YTO for about 4 MHz output, with the source tune mode set to 300 kHz), immediate success – there are absolutely no PLL issues, phase lock is established smoothly.

There is a 3 dB pad, the YTO has plenty of power, and it will help to reduce any frequency pull.

Below, to adjust the pretune DAC, you need to remove the EEPROM write protection, this picture from the web is quite useful.

At first try – the pre-tune test passed!!

Even better – all internal test passed!

Some check at 1 GHz, the spectrum looks clean, there are no parasitic oscillations or sidebands.

The analog bus of the 8753 series, input 16, is quite useful to check the tuning voltage and YTO status. It needs to be a line (the voltage is directly proportional to the YTO coil current).

To test the unit, measured a 1.9 GHz bandpass. All good.

Also the GPIB interface and plot function, working well.

After all the electronics, a little bit of work on the lathe – the cable that connects the test set to the VNA had broken screws, two new long screws fabricated – I used a bit thinner round stock, to make the task easier. Note that the cable is still available from Keysight, EUR 238 a piece.

Remaining task – to add the options 010 and 002, time domain analysis, and harmonic analysis, both are software options and can be added with a secret key code….

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.

Avantek AFT-4231-10F 2-4 GHz Amplifier: some characterization and modeling

The task for today – characterization of a bunch of microwave amplifiers, Avantek/HP AFT-4231-10F. These are quite rugged and affordable components, widely available surplus, and hermetically sealed – will last forever, if things are not messed up completely.

aft-4231-10f under test

The specification however, it’s not quite clear, and no detailled information could be found on the web. That’s why I have been asked to come up with measurements and a calculation model that allows to estimate the gain (and the actual maximum output power, and the necessary input power, to reach close to maximum output), at any given frequency and input power. Also, it needs to be checked how far above 4 GHz this device still works.
Last item is to measure the supply voltage sensitivity of the gain, to get a feeling on the required stabilization, to avoid incidental AM on the signal.

The datasheet –
aft series amplifier

The only equipment at hand at my temporary workshop here, a microwave source, EIP 928, and an HP 8565A spectrum analyzer was used to measure the gain at various input levels. Accuracy of this setup is about 1 dB.

Some of the results (0 dBm input: blue diamonds; 10 dBm input: green triangles):
aft-4231-10f pout at 0dbm and 10 dbm pin vs frq

To get a proper continuous description, these data were fit to a non-linear function, fractional polynomial term (fits are done using Tablecurve 2D, an excellent program, highly recommended, but doesn’t come cheap):
gain fit
The gain fit (0 dB input) can also be used to describe the maximum power, with some scaling factors – this considerably reduces the number of parameters needed, and the calculation effort later, when implemented in a microcontroller. Black lines in above diagram show the fit results.

For the gain compression, a 2nd order polynomial is used, and scaled for the 10 dBm input gain.
aft-4231-10f gain compression vs pin at 3 GHz

Once this is all established, no big deal to see the full picture.

Gain, at various input power levels, Pin:
aft-4231-10f gain vs frq at various pin

Output power, Pout, at various input power levels, Pin:
aft-4231-10f pout vs frq at various pin

Accordingly, no problem to get 18 dBm+ in the 1.8 to 4.5 GHz range, perfect for the application requirement.

The final item – supply voltage impact on gain: tested at 3 GHz, 0 dBm input power.
Using a Micro-Tel 1295 test receiver, the reference level was set to 0 dB at 15 V supply voltage, which is the nominal voltage.
Down to 9.0 V, the AFT stays within an excellent 0.01 dB variation. Output power slightly increases (0.15-0.25 dB) down to 6 V. At about 5 V, amplification cuts out. So the AFT can work with any voltage from 10 to 15 V, at about 80 mA, and seems to have pretty good internal regulation.

amp avantek aft-4231-10f

Figuring out the details of the Avantek S082-0959 YIG filter

For a small job, I need to design a digitally-controlled YIG preselector (a high-performance bandpass filter), for the 12.4 to 18 GHz range. The application is related to a test rig, and only 4 units are needed – at low cost, and controllable by USB. The control will be easy enough, just a programmable current source and some parameters, but first, finding a suitable YIG is quite a challenge – either only single pieces are available surplus, or they are new, and prohibitively expensive.

Remembering some earlier work, I had a look at the S082-0959 – these were made by Avantek, and are available, scavenged from old spectrum analyzers, for about 200-300 USD each, and still have one spare around here. The S082-0959 is also known as YF85-0107, or HP 0960-0473 (pinout may vary).

To get started, first the basics need to be figured out. Tuning sensitivity, bandwidth roll-off (need at least 12 dB/octave; and >50 dB spurious).
The thing has two pairs of connections: heater (2 wires) and coil (2 wires, this sets the magentic field – the tuning, via current – not voltage – control).

Looking at some spectrum analyzer schematics – the heater needs about 28 V. And, in fact, it works well and heats up quickly, drawing about 80 mA at 28 V, less with strong coil current applied (more during heating-up).

YIG filter Avantek S082-0959

The test setup – two power supplies, a counter EIP 545A, a microwave source EIP 928, and a microwave receiver Micro-Tel 1295. Signal level was 0 dBm.
The coil supply has a 4.7 Ohm current sense resistor, I’m measuring the voltage drop to calculate the current.

For 10 GHz, the tuning current was found to be about 132 mA, about 75.8 MHz/mA sensitivity.

Measurement result of insertion loss vs. frequency –
s082-0959 yig insertion loss vs frequency at 132 mA
– note that the passband is not well captured, but 3 dB bandwidth has been measured, by manual tuning, about 25-30 MHz. Recordering accurate values is a bit troublesome, would need to phase-lock the microwave source and receiver.
There is a spurious signal, about 350 MHz above the center frequency. This I will need to investigagte further. Note that the measuement points are not arbitrarily selected, but the YIG was actually tuned for the minimum loss, and the maximum response of the spurious.

Calculating the roll-off (25 MHz assumed 3 dB bandwidth):
s082-0959 yig roll-off at 10 ghz

As you can see, when doubling the bandwidth (e.g., from 2x to 4x – don’t look to close to the center frequency), the signal is about 20 dB down. That’s close to 18 dB per octave.
Without going into theory, which can be found elsewhere, a one-stage YIG filter will give (ideally) about 6 dB per octave. So the S082-0595 is most likely a 3 stage (3 sphere) filter. Well, limited accuaracy – the YIG will be fully characterized, once things are more advanced.