Tag Archives: harmonic mixer

Microwave

HP 11517A aka 08747-60022 Harmonic Mixer: a little study of a very intriguing device

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As part of a HP R8747A 26.5-40 GHz reflection/transmission test unit (for the 8411A network analyzer; 6300 USD in 1973 — about 40 kEUR today), I got two HP 08747-60022 harmonic mixers, one didn’t seem to work right, the diode has just 0.2 V voltage drop. These were fairly fragile devices, only designed for 1 mW of power, and very static sensitive, point contact devices.
In addition to the regular 11517A, the 08747-60022 has a bias connection (needs about 1.5 V DC bias, center positive).

The main unit can work from 12 to about 40 GHz, with a set of adaptor waveguides.

The unit can be taken apart, all precision machined.

The diode is pressed in, on some holder (haven’t tried to remove it from the case).

There are several other precision parts, a coaxial resistor, held on to the diode with a spring.

There is also a spacer, with a very flat capacitor, a DC block. The spacer is modified to connect the center conductor to a surface at the perimeter (used for DC bias).

Further up, there is a low-pass, machined from a single piece and gold plated.

The N-type connector, stainless, is screwed on.

The DC bias uses a small 1.5 kOhms resistor, and a custom connector, so that the resistor is pushed onto the spacers’s connection.

Here, a quick schematic. Seems a lot of engineering went into this device…

Finally, we need a microscope to study further here, the diode (the square), about 0.25 mm side length. It is isolated from the case by an air gap.

The diode is made by a contact junction, a small tungsten(?) whisker.

Probably, this whisker needed some adjustment during manufacturing. I tried to adjust it a bit, but this didn’t change the diode characteristics of my broken unit, unfortunately (anyway, these devices are more for study than for use; currently using only cartridge-based diode mixers).

Also, looking into the waveguide of the assembled mixer, with good long-range optics, I could get a shot of the actual point contact in action. Very interesting historic technology.

Various

HP Fundamental/Harmonic Mixer 5086-7285 (22 GHz): digital bias control

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In an effort to build a 2-18 GHz down converter, a HP mixer 5086-7285 needs to be controlled. This is one of a group of 22 GHz mixers, all used in earlier HP spectrum analyzers. These mixers are very linear, and useful both at fundamental and harmonic frequencies.

That’s the little magic thing, and the frequency list-harmonics:
5086-7285 mixer
5086-7285 mixer harmonics

All in all, at a first glance, pretty easy to use – it only needs +10 and -10 V power supply and bias for the diode.

Well, bias, after looking through the schematics, this is the assembly taking care of it: a board full of resistors and amplifiers, with no less than 22 (!) adjustment pots.
08565-60023 bias assembly

The interesting part are the bias drivers itself –
hp bias circuit for harmonic mixer
– the linearization, etc., this can all be done easily by using digital memory and a DAC nowadays, but the drivers, we still need them.

The bands B3 and B5, the even harmonics, the things are clear and as expected – a voltage source, and a resistor. Easy enough. But, what did HP do for the odd harmonics?? – the are a few extra resistors around the opamps, and these resistors make it a tricky thing. Too tricky to make it easy to understand. Some kind of negative resistance circuit/kind of a voltage to current converter, which depends a bit on the load resistance.

So, what do you do to understand such things better – build a little test circuit, here we go:
mixer bias test circuit
-it is essentially the same circuit, as for the B1/B4/B2 bands, U6B of the HP circuit- just left out the switching transistor.

It works pretty well, and as a U to I converter, see here:
bias driver test 200 mv-div ramp 1 mA-div current
– ramp voltage is the drive signal, 800 mV p-p, 200 mV per div (center line is zero). During the negative signal period, the output is active – current signal is 1 mA per div (center line is zero).

Having the basic functionality of the ciruit confirmed – some calculations with LTSpice, one of the best general purpose analog simulators around, Thank You, Linear Technology!

Here the files, in case you want to investigate it yourself:
hp mixer bias

This is a typical result, mixer bias current, vs. input voltage of the circuit, at resistance (of the mixer), of 950 (steepest)-1050-1150-1250 ohms.
r6-92 1-9 bias rscan vs Vi
So, this cirucit really is a U to I converter, with the slope depending on the load resistance.
Also note the model circuit of the mixer internal resistor and diodes. The two diodes and the 970 Ohm resistor are the result of bias current vs. bias voltage measurement. Bias voltage is in the range of -1 to -7 volts, about 0 to 8 mA.

With these findings, next step will be to build a driver circuit that can work fully digitally controlled, with no adjustment pot at all (series resistors will be manually selected).