This version (30 Aug 2022 19:43) was approved by Doug Mercer.The Previously approved version (07 Feb 2022 15:23) is available.

Activity: Poly Phase Filter Circuits - ADALM1000

Objective:

The objective of this lab activity is to examine poly phase filter circuits as a quadrature generation technique and to extend the differential tuned amplifier to create a poly phase amplifier or filter that can produce all four quadrature ( 90º increments ) phases of an input signal source.

Notes:

As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current –V is added as in CA-V or when configured to force current / measure voltage –I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage –H is added as CA-H.

Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.

Background:

The use of quadrature frequency conversion is common in modern wireless transceiver architectures, because both amplitude modulation and phase modulation, Quadrature Amplitude Modulation, (QAM) are deployed in today's digital communication systems.

Figure 1 shows a simplified first order poly phase circuit, as implemented in many quadrature demodulators such as the ADL5380. This simple poly phase circuit consists of complementary RC subcircuits. A low-pass transfer function from the input to one output shifts the phase by -45º at the corner frequency, and a high-pass transfer function to the other output shifts the phase by +45º at the corner frequency. The net phase difference between the two outputs will be 90º. If the R and C values of the two paths are matched, then both paths have the same corner frequency and, more importantly, the phase of one output tracks the other with a 90° phase shift for all frequencies. The relative amplitudes of the two output signals, ( LO I 0º and LO Q 90º ) will be only equal at the -3dB corner frequency of the two RC paths.

Figure 1. Simplified First Order Poly phase Filter

Generation of quadrature local oscillator (LO) signals is an important functional block in sideband rejection heterodyne receivers. Quadrature accuracy, that is the phase accuracy of the in phase and quadrature 90º phase-shifted signals, directly determines the image reject ratio (IRR), an important specification determining the sensitivity of a receiver.

Materials:

ADALM1000 module
Solder-less breadboard, and jumper wire kit
2 – 0.1 uF capacitors (marked 104)
2 – 1 KΩ resistors

Directions:

Build the poly phase filter circuit shown in figure 2 on your solder-less breadboard.

Figure 2, poly phase filter circuit

Hardware Setup:

The green squares indicate where to connect the ALM1000 module AWG, and scope channels. Open the Bode Plotting software tool in ALICE. Configure the frequency sweep to start at 100 Hz and stop at 20 KHz. Set CHA AWG Max value to 4.0 and the Min value to 1.0.

Check the box to sweep Channel A as in the Bode plotting window to measure the phase of one output path with respect to the other.

Procedure:

Calculate the expected RC corner frequency based on the R and C values you used. Run a single sweep of the frequency and be sure to save your data to a .csv file for later use in Excel or other data analysis software.

Questions:

Making all four quadrature phases:

One copy of the quadrature filter generates one pair of outputs 90 degrees apart, plus and minus 45 degrees with respect to AWG A. If we build a second quadrature phase filter and drive its input with the output of AWG B but set AWG B to be the complement, that is 180 degrees shifted with respect to AWG A, the quadrature filter generates a second pair of outputs plus and minus 45 degrees with respect to AWG B or plus and minus 135 degrees with respect to AWG A.

Figure 3, Two filters generate all four quadrature phases

Figure 4, AWG settings for generating a differential signal

The AIN and BIN scope input channels can be used to display two of the quadrature output at the same time but only 2. If we use an external 4 into 1 analog multiplexer to expand the BIN input we can display the AWG A input waveform and all four of the quadrature outputs.

Figure 5, Using analog Mux to display all four quadrature phases

Figure 6, Analog Mux controls

Figure 7, All four quadrature phases plotted using Phase Analyzer

Effect of Component tolerance:

Questions:

How do small differences in the component values change the relative phases?
How would you tune or trip the circuit to account for these variations?

Single-ended to Differential conversion:

Transformers are often used at high frequencies to convert a single ended, ground referenced, signal to a differential signal. The LTspice schematic shown in figure 8 shows a simulation using the HPH1-1400L 6 winding inductor as a 1:2 transformer with a center tapped secondary. The differential output of the transformer is connected to the same quadrature filter from figure 3.

Figure 8, LTspice simulation schematic

Additional Materials:

1 HPH1-1400L 6 winding transformer
1 47 Ω resistor
4 470 Ω resistors
4 47 nF capacitors

Using the HPH1-1400L six winding inductor we can connect it as a 1:2 center tapped transformer as shown in figure 9. The HPH1-1400L has a lower useable frequency limit and the values of the R and C in the quadrature filters needs to be scaled to a higher frequency.

Figure 9, Using BALUN transformer, single ended to differential conversion

Figure 10, Time display of quadrature outputs

Figure 11, Phase display of quadrature outputs

Differential Poly Phase Tuned Amplifier

By adding second order L-C and C-L low and high pass filter sections as differential output loads in a NPN differential amplifier we can generate all four 90º phases ( i.e. 0º, 90º, 180º and 270º ) of an input sine wave signal. Such a tuned amplifier is shown in figure 12.