Phasing Network Notes

by John Seboldt, K0JD, 
Milwaukee, WI

Last update: 1/10/05

RF phase shift network options for the R2/T2 are many and varied. Here is my favorite, and an elegant setup to bandswitch a group of them..

The Twisted-Wire Quadrature Hybrid

For more information and applications for this slick little circuit: see Reed Fisher (W2CQH), "Twisted-Wire Quadrature Hybrid Directional Couplers", QST, January 1978 (thanks to Eric, KC6SPN, for digging this up for me!)

It has a wide bandwidth, small number of components, takes care of splitting AND quadrature in one fell swoop. And, you can bandswitch by just opening and closing the A, B, and C ports onto common busses, making bandswitching relatively easy.

(In-phase is the +45 port, quadrature or -90 degrees is the -45 port.)

Assuming all ports are at 50 ohms,

VARIATION 1, as detailed in the schematic:
L1: bifilar twisted pair on toroid, reactance 50 ohms at center frequency.
C1, C2: 100 ohms reactance each at center frequency
Port D: terminate in a 50 ohm resistor.

You obviously have to calculate your values from the reactance/frequency formulas found in your ARRL Handbook or other references.

VARIATION 2 - above, but eliminate C2, and make your C1 twice the value (i.e., 50 ohms reactance at the chosen frequency). This is what I'm using lately to save variable capacitors!

This gives you 90 degree phase shift over a broad range. However, the amplitude ratio between the two outputs will vary as you depart from the design frequency. No big deal - just tweak your amplitude balance pot on the R2.

In addition, for a perfect null, I make the capacitance adjustable with tiny 1/4 inch trimmer caps I found at a hamfest.

Now, in the real world, not everything is 50 ohms; and for various reasons, the audio phase shift networks in the R2 and T2 are not exactly 90 degrees, either. So things may depart from the theoretical. Some reasons and solutions after the next section...

The "Quadrature Expressway" to switch several networksQuadrature Expressway pic

I decided to start upgrading my old "plug-in module" arrangement. For the phasing network, I came up with a "three-lane microstrip" board, one microstrip each for LO in, in-phase out, and quadrature out. The microstrips are about 1/16 inch wide, in the top foil of the double sided board, forming an approximately 50 ohm transmission line with the bottom ground plane foil. Then pads are provided for five of the above phasing networks, plus PIN diode switches. The extra pads near the microstrips probably disturb the perfect transmission-line characteristics, but it still works fine at HF, and is cleaner than my old haywire setup for sure.

Components are a mix of surface-mount and leaded components. Don't shy away from surface mount - with a little care, a good magnifier, and some silver-bearing solder (even Radio Shack has the silver solder now), it's a compact and flexible way to do homebrewing. Paralleling components to improvise the right value is a matter of just stacking your chips! (And I don't mean at the casino...) A hamfest purchase of a whole reel of 3.9k resistors, plus little packs of 500 chip capacitors (180 pF and .1 uF), a kit of assorted NP0 chip caps from DigiKey, and miscellaneous other hamfest purchases make it possible to come up with the necessary values.

The LO input goes through an attenuating pot, then a 2N5109 amplifier stage (not shown on schematic), to provide a uniform source impedance and adequate drive power. This feeds the center microstrip.

The networks do their thing, then send their signals to the outer microstrips. A single PIN diode in each of the A, B, and C legs of each network makes or breaks the connections to the microstrips. I use the MPN3700, but the lower-voltage MPN3404 would do fine. (These are former Motorola parts, now made by ON Semiconductor, and available from Allied Electronics). Forward bias on the diodes (about 6 mA each) switches them on, and isolation is good enough for this application (about 40 dB at HF) by simply removing the bias. (Reverse bias would increase the isolation, but is probably overkill except for more critical applications like front-end filter switching.)

I did, just for fun, try garden-variety 1N4148 diodes in one network. With the values above, there was definitely an audible loss, so I didn't research it further. My guess is they'd need a lot more forward current to turn them on better. Even if that helped when "on," I don't know if the isolation would be adequate when "off." It's probably worth shelling out your shekels for the real thing.

Why did I have to mess around so much with the theoretical values for a perfect null?

I've had Glen Leinweber's R2a writings for some time, waiting for the right time to implement the ideas or pieces of them. He makes his audio phase shift network tweakable, and provides a simple schematic of an accurate audio quadrature generator to adjust it right on the nose.

In addition, I started playing with PSpice (as provided in the free OrCad Lite v. 9.2 from Cadence Design Systems - see my simulation primer), modelling everything in sight to find out the whys and wherefores of how things work, what happens if I tweak "widget X", etc. In modelling, I found (with perfect parts) that:

  • the network behaved very well, with a precise 90-degree phase difference between the output ports over a wide frequency range
  • changing the source or load impedances changed only the amplitude, not the relative phases, of the outputs of the above network.
  • changing only one or the other of inductance or capacitance, however, led to changes in the phases from the theoretical 90 degrees, that varied with frequency.
This led me to the conclusion that there must be some phase errors in my audio phase shift network - precisely what Glen was addressing with his adjustable (and lower-noise) network.

I built up his audio quadrature generator, in preparation for building the R2a phasing network... and thought maybe I should check my current implementation of the original R2 network. Sure enough - over the communications audio band, there was quite a bit of audible difference in rejection, sometimes quite good, sometimes not so good. And, of course, the typical CW listening tone of 750 Hz was one of those not-so-perfect points!

So, clearly, I had been working hard to get a perfect null at that frequency, tweaking networks, adding capacitors, switching coil windings, and on and on... little knowing that my pursuit of the perfect null led to compromises in the total picture - like good null over the audio bandpass, and closer-to-perfect nulling over a wider RF tuning range.

Now I've finally built part of Glen's R2a: the tweakable audio phase shift network. After adjusting it precisely with a quadrature audio generator made from CMOS chips, any tweaking of the RF phase shift network is fairly minimal. Nice!

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John Seboldt, K0JD, Milwaukee, WI
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