
9-33
HFA3101
The use of the HFA3101 as modulators has several advan-
tages when compared to its counterpart, the diode double-
balanced mixer, in which it is required to receive enough
energy to drive the diodes into a switching mode and has
also some requirements depending on the frequency range
desired, of different transformers to suit specific frequency
responses. The HFA3101 requires very low driving capabili-
ties for its carrier input and its frequency response is limited
by the FT of the devices, the design and the layout tech-
niques being utilized.
Up conversion uses, for UHF transmitters for example, can be
performed by injecting a modulating input in the range of
45MHz to 130MHz that carries the information often called IF
(Intermediate frequency) for up conversion (The IF signal has
been previously modulated by some modulation scheme from
a baseband signal of audio or digital information) and by inject-
ing the signal of a local oscillator of a much higher frequency
range from 600MHz to 1.2GHz into the carrier input. Using the
example of a 850MHz carrier input and a 70MHz IF, the output
spectrum will contain a upper side band of 920MHz, a lower
side band of 780MHz and some of the carrier (850MHz) and IF
(70MHz) feedthrough. A Band pass filter at the output can
attenuate the undesirable signals and the 920MHz signal can
be routed to a transmitter RF power amplifier .
Down conversion, as the name implies, is the process used
to translate a higher frequency signal to a lower frequency
range conserving the modulation information contained in
the higher frequency signal. One very common typical down
conversion use for example, is for superheterodyne radio
receivers where a translated lower frequency often referred
as intermediate frequency (IF) is used for detection or
demodulation of the baseband signal. Other application uses
include down conversion for special filtering using frequency
translation methods.
An oscillator referred as the local oscillator (LO) drives the
upper quad transistors of the cell with a frequency called
ωC. The lower pair is driven by the RF signal of frequency
ωM to be translated to a lower frequency IF. The spectrum of
the IF output will contain the sum and difference of the fre-
quencies ωC and ωM. Notice that the difference can become
negative when the frequency of the local oscillator is lower
than the incoming frequency and the signal is folded back as
in Figure 7.
NOTE: The acronyms RF, IF and LO are often interchanged in the
industry depending on the application of the cell as mixers or
modulators. The output of the cell also contains multiples of the
frequency of the signal being fed to the upper quad pair of transistors
because of the switching action equivalent to a square wave
multiplication. In practice, however, not only the odd multiples in the
case of a symmetrical square wave but some of the even multiples
will also appear at the output spectrum due to the nature of the actual
switching waveform and high frequency performance. By-products of
the form M*ωC + N*ωM with M and N being positive or negative
integers are also expected to be present at the output and their levels
are carefully examined and minimized by the design. This distortion
is considered one of the figures of merit for a mixer application.
The process of frequency doubling is also understood by
having the same signal being fed to both modulating and
carrier ports. The output frequency will be the sum of ωC and
ωM which is equivalent to the product of the input frequency
by 2 and a zero Hz or DC frequency equivalent to the differ-
ence of ωC and ωM. Figure 7 also shows one technique in
use today where a process of down conversion named zero
IF is made by using a local oscillator with a very pure signal
frequency equal to the incoming RF frequency signal that
contains a baseband (audio or digital signal) modulation.
Although complex, the extraction or detection of the signal is
straightforward.
Another useful application of the HFA3101 is its use as a
high frequency phase detector where the two signals are fed
to the carrier and modulation ports and the DC information is
extracted from its output. In this case, both ports are utilized
in a switching mode or overdrive, such that the process of
multiplication takes place in a quasi digital form (2 square
waves). One application of a phase detector is frequency or
phase demodulation where the FM signal is split before the
modulating and carrier ports. The lower input port is always
90 degrees apart from the carrier input signal through a high
Q tuned phase shift network. The network, being tuned for a
precise 90 degrees shift at a nominal frequency, will set the
two signals 90 degrees apart and a quiescent output DC level
will be present at the output. When the input signal is fre-
quency modulated, the phase shift of the signal coming from
the network will deviate from 90 degrees proportional to the
frequency deviation of the FM signal and a DC variation at
the output will take place, resembling the demodulated FM
signal.
The HFA3101 could also be used for quadrature detection,
(I/Q demodulation), AGC control with limited range, low level
multiplication to name a few other applications.
Biasing
Various biasing schemes can be employed for use with the
HFA3101. Figure 8 shows the most common schemes. The
biasing method is a choice of the designer when cost, ther-
mal dependence, voltage overheads and DC balancing
properties are taken into consideration.
Figure 8A shows the simplest form of biasing the HFA3101.
The current source required for the lower pair is set by the
voltage across the resistor RBIAS less a VBE drop of the lower
transistor. To increase the overhead, collector resistors are
substituted by a RF choke as the upper pair functions as a
current source for AC signals. The bases of the upper and
lower transistors are biased by RB1 and RB2 respectively.
The voltage drop across the resistor R2 must be higher than
a VBE with an increase sufficient to assure that the collector to
base junctions of the lower pair are always reverse biased.
Notice that this same voltage also sets the VCE of operation of
the lower pair which is important for the optimization of gain.
Resistors REE are nominally zero for applications where the
input signals are well below 25mV peak. Resistors REE are
used to increase the linearity of the circuit upon higher level
signals. The drop across REE must be taken into consider-
ation when setting the current source value.
Figure 8B depicts the use of a common resistor sharing the
current through the cell which is used for temperature com-
pensation as the lower pair VBE drop at the rate of -2mV/oC.
Figure 8C uses a split supply.