ELEC 477L
Topics in Wireless System Design
Spring 2007
Lab #9: Direct Conversion Receiver Front End
Introduction
One of the simplest receiver architectures is the homodyne, or direct conversion, design. A block
diagram of a basic direct conversion receiver is shown in Figure 1. Instead of downconverting
signals to one or more intermediate frequencies, the direct conversion (or “DC”) receiver
translates the signal frequency directly to baseband. For example, in a receiver designed to pick
up voice signals, an incoming SSB or DSB-LC (standard AM) signal at RF is converted directly
to audio. The first few stages right after the antenna are often collectively called the “front end,”
and they have the greatest impact on the overall noise and strong signal-handling capabilities of a
receiver.
Many mixers, especially diode rings mixers, work best when the IF port is terminated in a
specific impedance (usually 50 ) at all frequencies, not just the IF. 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 called a diplexer and then lay out a PC board for a
diode ring mixer and the diplexer circuit. The mixer/diplexer board will be combined with the
band-pass filter you constructed in Lab #7 to form the front end of a simple aircraft band
receiver.
(RF)
Mixer
BPF
(audio)
to
speaker
or headphones
Diplexer
Antenna
Local
Oscillator
Low-Noise
High-Gain
Audio Preamp Audio Amp
Chain
~
Figure 1. Block diagram of a basic direct conversion receiver designed for
reception of CW (Morse code), SSB, DSB-LC, and some types of FSK/PSK
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 while at the same time the spurious products must 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
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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, where the spurious signal
energy is dissipated.
In the case of a 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 2 below 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
products at frequencies well above the audio range. All of these signals have to be terminated in
an impedance of 50 . In principle, a buffer amplifier with an input impedance of 50  at all
frequencies could be used ahead of the audio stages instead of a diplexer, 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 audio subsystem. A high-pass filter with the same cut-off
frequency directs the spurious signals to a 50- resistor. Not all mixer designs require diplexers
or triplexers, and even those that perform better with them can be used without them if the
resulting degradation in performance is acceptable.
audio signal is passed to
to following stages (with
50- input impedance)
LPF
IF
RF
HPF
LO
50 
spurious products are
dissipated in matched
load
Figure 2. Diplexer block diagram.
The diplexer circuit does not usually have to be complicated. The simple circuit shown in
Figure 3 on the next page 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-
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off frequencies of the filters are the same (or close) 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
Rt
Ra = input
impedance
of audio
subsystem
Figure 3. 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.
Experimental Procedure

Design a suitable diplexer for your direct conversion receiver. (That is, find appropriate
values for L and C in Figure 3.) Assume that Ra = Rt = 50  and that the desired cut-off
frequency for both the low-pass response and the high-pass response is 1 MHz. Briefly but
concisely explain your design procedure.

Lay out a printed circuit board using the ExpressPCB software for a Mini-Circuits TUF-1
mixer, the diplexer circuit, and three SMA connectors, using one-third of a standard
“MiniBoard” (which is 2.5 x 3.8”). Don’t forget to include the termination resistor Rt. The
diplexer circuit should use SMT components for L, C, and Rt, since it has to be effective well
above the aircraft band. Also be sure to place 50- microstrip lines (0.1” wide traces) where
appropriate. PC board outlines for SMA connectors and the TUF-1 mixer will be provided.
Show your layout to the instructor before finalizing your design.

Each lab group should turn in:
o details of diplexer design (i.e., brief analysis and computed values for C and L)
o ExpressPCB file of PC board layout for mixer/diplexer system
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