ELEC 477/677
Topics in Wireless System Design
Spring 2003
Lab #11: 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. These include the image and intermodulation (IM) 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 that spurious signal energy
could reflect back into the mixer and create significant additional intermodulation products. The
problem then is to create a wideband impedance-matched termination for all of the significant
output products of the mixer. In this lab exercise you will design one type of wideband
termination. You will also complete the construction of your direct conversion receiver 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 we want
the IF signal to pass through the IF filter, but we also want to keep the spurious responses out of
the IF subsystem. That means that the IF signal and the spurious signals have to follow two
separate signal paths; otherwise, we could simply connect a 50- resistor to the output of the
mixer and be done with it! What we need is a circuit called a diplexer, which is designed to
direct signals in one frequency range along one signal path and direct signals at all other
frequencies along another path (which usually ends in a simple 50- resistor).
In the case of the direct conversion receiver, the design of the diplexer is simplified somewhat
because we want to pass audio signals only and terminate all signals above the audio range. In a
superheterodyne receiver, by contrast, it is usually necessary to pass signals that lie in a narrow
range of frequencies and terminate all signals above and below that range. In the latter case, a
band-pass/band-stop filter combination is required (band-pass for the IF; band-stop for
everything else). However, in the direct conversion case, a low-pass/high-pass combination is all
that is required.
Figure 1 may 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
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low-pass filter with a cut-off frequency above the audio range directs the audio signals to the
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 converted to heat.
audio signal passed
to following stages
(with 50- overall
input impedance)
LPF
IF
RF
HPF
LO
spurious products
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 should perform 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 cutoff frequencies of the filters are the same and appropriate for the design goals. Thus, the
unknowns in the diplexer circuit are the values of L and C. Their values are determined by the
two constraints of the 50- input impedance at all frequencies and the desired cut-off frequency.
L
C
Zin = 50 
(all freq’s)
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 on a separate sheet(s) of paper. Turn in one set of
notes for your group at the end of the lab session. All members of the group will receive the
same grade.

Design a suitable diplexer for your direct conversion receiver. (That is, find appropriate
values for L and C.) 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.

Construct the diplexer on the same circuit board that contains the mixer. Add a RG-58 test
cable to the output of the diplexer to carry the audio signals to the audio subsystem circuit
board (which will be supplied to you). The braid of the circuit board end of the cable should
be soldered directly to the ground plane (or to the common grounding point), as should the
grounded side of resistor Rt. The other end of the cable should have a BNC connector
attached to it.

Adjust one of the benchtop function generators to produce a 7 MHz sine wave (or some other
frequency inside your receiver’s passband) at a power level of +7 dBm. This generator will
serve as the local oscillator for the receiver. Adjust another benchtop function generator 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
of the attenuator is around –100 dBm. This will be the RF signal.
Connect the output of the receiver’s audio subsystem to an oscilloscope, and adjust the LO or
RF frequency until you observe an output frequency of around 1 kHz. Increase the level of
attenuation until the sine wave is lost in the noise as displayed on the oscilloscope. The input
signal level just above that at which the output disappears into the noise is one definition of
minimum detectable signal (MDS). Record the MDS specification for your receiver.

Now decrease the attenuation and observe the output signal waveform carefully. Keep
decreasing attenuation until the output signal level compresses by approximately 1 dB. This
occurs when a 1-dB increase in input power results in less than 1 dB increase in output
power. Estimate the input level at which 1-dB compression occurs as well as you can. From
this result and your MDS measurement, calculate the blocking dynamic range of the receiver.

Remove the function generator and attenuator that served as the RF signal, and replace them
with an antenna. Remove the output connection to the oscilloscope and connect an amplified
speaker in its place. Tune around and have fun!

Each lab group should turn in:
o
o
o
o
details of diplexer design (i.e., brief analysis and computed values for C and L)
measured MDS of the receiver
estimated blocking dynamic range of the receiver
a list of one or two shortwave stations that you monitored
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