6
The use of the HFA310 1 as modulators has several
advantages when compared to its counterpart, the diode
doublebalanced 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 capa bilities for its carrier input and its frequency
response is limited by the fT of the devices, the design and
the layout techniques being utilized.
Up conversion uses, f or UHF transmitters f or e xample , can be
perf ormed by injecting a modulating input in the range of
45MHz to 130MHz that carries the inf ormation 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 injecting
the signal of a local oscillator of a much higher frequency range
from 600MHz to 1.2GHz into the carrier input. Using the
ex ample 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) f eedthrough. A Band pass filter at the output can
attenuate the undesirable signals and the 920MHz signal can
be routed to a transmitter RF powe r amplifier.
Down conversion, as the name implies, is the process used
to translate a higher frequency signal to a lower frequency
range con s erving the modul a ti on inf ormation cont ai n ed in
the higher frequency signal. One very common typical down
conversion use for example, is for superhetero dyne 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 con version 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
frequencies 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 2.
NOTE: The acronyms R F , 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
s witching wav ef orm and high frequency performance. By-products of
the form M*ωC + N*ωM with M and N being positive or negative
integers are also ex pected to be present at the output and their le vels
are carefully examined and minimized by the design. This distor tion
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 equiv alent to
the difference of ωC and ωM . Figure 2 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
s witching mode or ov erdrive , such that the process of
multip licatio n tak es place i n a quasi digi tal f orm (2 square
wa v es). 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 alwa ys 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 lev el will
be present at the output. When the input signal is frequency
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 v ariation at the
output will take place, resemb ling the demodulated FM signal.
The HFA3101 could also be used for quadrature detection,
(I/Q demodulation), AGC control with limited range , lo w le vel
multiplication to name a few other applications.
Biasing
Various biasing schemes can be employed for use with the
HFA3101. Figure 3 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 3A 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 tr ansistor . To increase the overhead, collector resistors
are substituted by an 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 re verse 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 sign als are well below
25mV peak. Resistors REE are used to increa se the linea rity
HFA3101