Download ELEC 477/677L Topics in Wireless System Design Spring 2006

ELEC 477/677L
Topics in Wireless System Design
Spring 2006
Lab #9: Diplexers for Direct Conversion Receivers
Introduction
Many kinds of spurious signals accompany the desired IF (intermediate frequency) signal at the
output of a mixer circuit, including image and intermodulation distortion (IMD) products.
Proper design and operation of a receiver can limit the spurious products to very low power
levels, but even highly attenuated signals can still cause problems with mixer performance in
some cases. Diode rings mixers, for example, work best when the IF port of the mixer is
terminated in the proper impedance (usually 50 ) at all frequencies, not just the IF frequency.
If the termination impedance at the frequency of a spurious product is not 50 , then the balance
of the mixer could be disturbed, which might lead to additional IMD products. In this lab
exercise you will design one type of wideband termination matched to the system impedance for
all of the significant output products of the mixer. You will also complete the construction of
your direct conversion receiver front end and use it to pick up a variety of shortwave signals.
Theoretical Background
The design of a wideband termination for a mixer circuit is complicated by the fact that the IF
signal needs to pass through the IF filter, but the spurious responses must also be kept out of the
IF subsystem. This means that the IF signal and the spurious signals have to follow two separate
signal paths. What is needed is a diplexer, a circuit designed to direct signals in one frequency
range along one signal path and signals at all other frequencies along another path. The latter
path usually ends in a simple 50- resistor.
In the case of the direct conversion receiver, the design of the diplexer is simplified somewhat
because it has to pass only audio signals and terminate all signals above the audio range. In a
superheterodyne receiver, by contrast, it is usually necessary to pass signals that lie within a
narrow range of frequencies (the IF) and terminate all signals above and below that range. This
requirement calls for a triplexer, a circuit that provides three possible paths, two of which end in
50- resistors. Hence, a band-pass/low-pass/high-pass filter combination is required (band-pass
for the IF; low-pass or high-pass for everything else). In a direct conversion receiver a lowpass/high-pass combination is all that is required.
Figure 1 might help to clarify this. The output of a mixer in a direct conversion receiver contains
not only the desired audio signal (and its undesired audio image) but also spurious mixer
products at frequencies well above the audio range. All of these signals have to be terminated in
an impedance of 50 . A buffer amplifier with an input impedance of 50  at all frequencies
could be used ahead of the audio stages, but amplifiers that exhibit constant input impedance
over a wide range of frequencies can be difficult and/or expensive to design and construct. The
amplified RF energy could also cause intermodulation problems in the audio circuits. Instead, a
low-pass filter with a cut-off frequency above the audio range directs the audio signals to the
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audio subsystem. A high-pass filter with the same cut-off frequency directs the spurious signals
to a 50- resistor, where the spurious signal energy is dissipated.
audio signal is passed to
to following stages (with
50- input impedance)
LPF
IF
RF
HPF
LO
spurious products are
dissipated in matched
load
50 
Figure 1. Block diagram of a diplexer circuit suitable for use in a direct
conversion receiver.
The diplexer circuit does not usually have to be complicated. The simple circuit shown in
Figure 2 performs adequately in most cases. The input impedance Ra of the audio subsystem
combines with the inductor L to form a low-pass response, and the termination resistor Rt and
capacitor C form a high-pass response. The design goal is to make sure that the input impedance
Zin of the combined LPF/HPF circuit is 50  at all frequencies and that the cut-off frequencies of
the filters are the same and appropriate for the design goals. These two constraints determine the
two unknowns in the diplexer circuit, the values of L and C.
L
C
Zin = 50 
at all
freqencies
Ra
Rt
Figure 2. Simple diplexer circuit suitable for use in a direct conversion receiver.
Resistor Rt is a physical resistor that serves as a termination for spurious signals.
Resistor Ra represents the input impedance of the audio subsystem.
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Experimental Procedure
Record the results of the following procedures, and turn in one set of notes for your group at the
end of the lab session.

Design a suitable diplexer for your direct conversion receiver. (That is, find appropriate
values for L and C in Figure 2.) Assume that Ra = Rt = 50  and that the desired cut-off
frequency for both the low-pass response and the high-pass response is 75 kHz. Briefly but
concisely explain your design procedure.

Construct the diplexer on the same circuit board that contains the mixer. Don’t forget to add
the termination resistor Rt. The standard value of 51  should be close enough to the design
value to work properly.

Adjust the Agilent E4438C signal generator to produce a 14 MHz signal at a power level of
+7 dBm. This generator will serve as the local oscillator for the receiver. Be sure to
connect the LO to the receiver before applying an RF input. Adjust one of the bench-top
function generators to produce a sine wave near the LO frequency at any convenient low
power level. Connect the output of the generator to a step attenuator, and set the level of
attenuation so that the output power is around –50 dBm. This will serve as the RF signal.
Once the RF signal level has been adjusted, apply it to the input of the receiver.

Connect the output of the front end (the IF port) to the spectrum analyzer, and determine
whether the diplexer seems to be performing as designed. Explain how you made this
determination.

Now connect the IF port to the audio subsystem provided to you. Remove the function
generator and attenuator that served as the RF signal, and replace them with an antenna.
Tune the LO and have fun listening to the various shortwave signals you can pick up! The
“20-meter” amateur radio band occupies 14.000-14.350 MHz, with Morse code dominating
the lower 100 kHz or so and single sideband (SSB) voice above that range.

Each lab group should turn in:
o details of diplexer design (i.e., brief analysis and computed values for C and L)
o evidence that diplexer operates correctly
o a list of one or two shortwave stations that the group monitored
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