The Stepped Phase Shifter
The Stepped Phase Shifter is a device which upon command delays the phase of a signal
by a fixed increment. The target signal is generated by dividing down a Clock input,
which is a digital signal in the 10-100MHz range. There is a divide by 2 prescaler on the
input, and a subsequent divide ratio jumper selected from 2 to 256. The output signal is
thus a divided-down version of the Clock. A positive level asserted on the Skip input
causes the output signal to be delayed by an increment equal to two Clock periods. No
matter how long the Skip input is held high, only a single phase shift is made. Multiple
Skip pulses can be used to accomplish multiple phase shifts.
The Skip command is implemented by causing the output counter to skip a single count
pulse. Because of the prescaler on the input, the skipped count causes a delay in the
counter output equal to two Clock periods.
Using an 80MHz Clock and setting the output jumpers for divide-by-16, the output will
be 2.5MHz and each Skip command will cause a phase delay of 1/16 of a cycle, or 22.5
degrees. Higher divide ratios will give lower frequency output which can be shifted in
smaller increments.
The Shifter can be used for calibration of a Vector Network Analyzer, if the master clock
of the VNA is used to clock the Shifter, as described in more detail below.
The schematic and PCB layout for the Shifter are shown at the end of this document in
Figures 4-6.
Figure 1 shows a photo of the assembled Shifter on a 1.5” x 2” PCB. The shifting
mechanism described so far is actually located in the upper left quadrant of the board.
The remaining components implement additional features useful for testing, and are
described below.
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Figure 1—Assembled Stepped Phase Shifter
Actual Shifter is in upper left. Optional phase detector is on right. Optional
Reference Counter is in bottom center. Optional Skip pulse generation
mechanism is in lower left. SMA connectors are located on back of board.
The board in Figure 1 also contains a pushbutton and signal input which can be used to
generate one-time or periodic skip pulses. With a 2kHz signal attached to the SQR input,
the output will shift 22.5 degrees each 500us. The board also contains a Reference
Counter, which simply divides down the prescaled clock input in the same manner as the
Skip Counter, but does not react to the Skip command. Thus, it provides a constant phase
reference.
Figure 2 shows a dual trace oscilloscope display of the shifted output (top) and the
Reference Counter (bottom trace). The top trace is effectively an overlay of the shifted
output in all of the 16 different phase positions resulting from a divide-by-16 output.
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Figure 2—Reference Counter (bottom) and Shifted Output (top)
The output is being rapidly shifted through 16 positions, so it appears as an overlay of all
16 positions. Each low/high transition is visible as a faint vertical line—the transition
locations are more clearly marked by the overshoot glitches on the top trace. Those
transitions divide the top trace into 16 sections for each full period of the bottom
Reference Clock.
The 16 phase positions shown in Figure 2 are very equally spaced, although that cannot
be determined with precision on a scope display. Therefore, an XOR detector was added
to the Shifter for testing purposes, to measure the phase between the Shifted Output and
the Reference Counter. The average output of the XOR will be maximum (3.3V) when
the signals are 180 degrees out of phase, decline in equal steps as the phase is shifted
until it reaches 0V when the signals are at 0 degrees, and then increase back to maximum
when the signals are -180 degrees out of phase (which of course is the same as 180
degrees phase).
Figure 3 shows a dual trace display of the XOR output (top) and the Skip commands,
with overlapping traces. It can be seen that it takes 8 Skip commands to move from the
peak at the left (180 degrees) to the low in the center (0 degrees), and 8 more to move
back up to the peak at the right. Each phase shift occurs immediately as the Skip pulse
goes high.
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Figure 3—XOR phase detector output (top) and Skip commands (bottom)
The two parallel dashed lines are the Skip square wave; their transitions cannot be seen.
The traces are overlapped to show the alignment of phase change with the Skip
commands.
To precisely determine that each phase increment is identical, the XOR output was
attached to a DVM, the jumpers were set for divide-by-64 and the push button was used
to shift the phase. The voltage was measured at each step. Each of the 64 steps measured
either 0.103 or 0.104 volts, which corresponds well to the theoretical 3.3V/32=0.103125
volt step size. The minimum level was exactly 0V and the maximum was exactly 3.3V,
matching the supply voltage.
The original design actually included a multiplexer after the XOR, intended to be sure the
digital signal level switched precisely between 0V and a precise supply level. It turned
out, however, that the XOR switches precisely between the supply rails, and the
multiplexer, due to DC leakage, did not. So the multiplexer was cut out of the circuit.
The reason a precise and consistent phase shift can be achieved is that the signal path is
identical for all phases, so the signal encounters identical trace lengths and parasitics no
matter what the phase. There are no components switched into or out of the circuit to
achieve the phase shift.
Use of the Shifter
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The Shifter can be used for VNA calibration. For this purpose, the Reference Counter,
XOR phase detector and pushbutton are not needed, so about 2/3 of the PCB can be cut
off.
Assume that the Shifter is set for divide-by-64 on its output, to give 64 phase increments.
The master clock is used as the Clock input of the Shifter, and the Shifted Output is
connected to the VNA input, perhaps through a lowpass filter (space for which is
provided on the board). The VNA test signal output is not used.
With a 64MHz clock, this gives an output of 500kHz (remember the input prescaler).
The VNA is tuned to 500kHz. The VNA will now measure it’s input as being at some
phase relative to it’s own 500kHz output, which is not being used. We don’t care what
phase is displayed; the important point is that it will be some stable value.
The VNA can now issue a series of Skip commands to cycle through the 64 phase
positions, recording the phase measured after each increment. Each increment should be
measured as 5.625 degrees. If not, the VNA can make appropriate adjustments to its
calibration table. If each increment comes out correct, the VNA can dispense with a
calibration table. If the VNA uses internal phase shifting to avoid inaccurate zones of its
own phase detector, these measurements can be used to determine where those inaccurate
zones are.
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FIGURE 4—SCHEMATIC OF STEPPED PHASE SHIFTER
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FIGURE 5—SCHEMATIC OF STEPPED PHASE SHIFTER ADD-ONS
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FIGURE 6—PCB LAYOUT
Actual Board is approx. 1.5” H x 2” W
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