Master Ganzenko V.A, Dr.Eng., prof. Kotserzhynskyy B.O
National Technical University of Ukraine
"Kyiv Polytechnic Institute", Ukraine
Quadrature
modulation as a way to transfer the signal spectrum of integrated synthesizers of
direct digital synthesis to higher frequencies
Among
the integrated synthesizers of direct digital synthesis (ISDDS) that are
produced by the leading company Analog Devices, there are so-called quadrature complete
synthesizers of direct digital synthesis, such as ISDDS AD9854 (CMOS 300 MSPS
Quadrature Complete DDS AD9854). They have the analog outputs of the two DAC
(Fig.1). The AD9854 outputs have the following signals: OUT1-unipolar current
cosine output of the I component, IOUT1-complementary unipolar current cosine output
of the I component, IOUT2 - unipolar current sine output of the Q
component, IOUT2 - complementary unipolar current sine output of
the Q component.
Fig.3. Single-sideband upward transformation
Switching
of the output ISDDS quadrature signals enables to select the required sideband,
and suppression of the unnecessary sideband is corrected with amplitudes of ISDDS
quadrature signals (Fig.3). While using quadrature modulator AD8346, the signals
must be passed through the filters of low frequency.
The
proposed transformation was implemented by the company laboratory using adjusted
boards of ISDDS and the modulator with some modifications [1].
Testing
confirmed the expected results when the signals from ISDDS were adjusted to
compensate the phases and amplitudes unbalance (Fig. 4). Phase unbalance occurs
in devices between ISDDS and the modulator. It can be adjusted by changing the cable
length. Signal amplitudes are regulated by DAC amplifiers. Fig.4 shows the
central frequency of 1.05 GHz and the sideband at the distance of 25 MHz. The
upper band is suppressed for 40 dB compared to the lower one that corresponds
to the phase unbalance of 1 degree. The amplitude of the component of central
frequency exceeds the upper side one (suppression of 36 dB). Its level can be
reduced by increasing the amplitudes of the input AD8346 signals.
Fig.4. Signal spectrum at the AD8346 output
The paper
determines that the quadrature modulation is a well-made and economical method
for transferring ISDDS spectrum into the VHF and UHF wavebands without loss of
the advantages of direct digital synthesis (DDS) and signal quality. Integrated
quadrature modulator AD8346 simplifies the process and conforms AD9854. Yet the
main fact is that the quadrature modulation doesn’t prevent the usage of various
modulation techniques of ISDDS signals.
Quadrature
modulation can be achieved not only by specific ISDDS, such as AD9854 and
AD9857, but also using a pair of mutually synchronized ISDDS (AD9954 (clock
frequency
AD9910 (
Analysis of different alternatives in realization of
quadrature modulation
Let
the two
then
Let
the two
then
If
the signals are put together in quadrature modulator
then
the component with the frequency sum
(upper sideband), slightly weakened component with the
If
the
If
the
Now
let the input modulator mixer receive such pairs of signals:
For
the first pair
For the second pair
Only
the second harmonic of
Now
let the input modulator mixer receive such pairs of signals:
For
the second pair
The
sidebands and the second harmonic of the ω1
frequency disappear at the quadrature modulator output
(signals ³6 and ³8)
Summary
The
required output signal of the modulator can be achieved by choosing the right
combinations of phases of input harmonic signals. Modulation of complex frequency-modulated
oscillations requires further analysis.
Bibliography
1.Cushing R. Single-Sideband Upconversion of
Quadrature DDS to the 800-to-2500-MHz Band/ Analog Dialogue 34-3(2000).
2. Brandon D. Synchronizing
Multiple AD9850/AD9851 DDS-Based Synthesizers/AN-587, Analog Devices,
www.analog.com.