Download AN-102A APPLICATION NOTE Interface Circuits for the QD Series

AN-102A
APPLICATION NOTE
Interface Circuits for the QD Series Quadrature Demodulators
by Lowell Hoover
INTRODUCTION
The two differential I/Q output ports, I / I
The QD series quadrature demodulators provide
Q / Q , each have 100 Ω differential source
exceptional linearity and demodulation accuracy for
impedance. Internally matched lowpass filters on the
direct conversion and low-IF receiver applications.
I/Q ports limit the demodulator’s noise bandwidth.
This application note presents several circuit ideas
for interfacing the differential I/Q outputs of the QD
series demodulators to single-ended loads and
modern differential A/D converters.
and
The I/Q outputs can be modeled as voltage sources
with 100 Ω of differential source resistance. When a
sinewave RF input signal at +3 dBm is demodulated,
the resulting I and Q differential output signals are
DEMODULATOR PORTS
approximately 400 mV p-p centered at 0 V (ground).
Figure 1 shows the block diagram of a QD series
This is equivalent to four single-ended baseband
demodulator. For proper operation, the demodulator
outputs supplying –10 dBm each into a 50 Ω single-
should be supplied with an LO drive signal having a
ended load.
power level within the demodulator’s specified LO
To minimize amplitude imbalance and quadrature
power range. The LO signal should have low 2nd
harmonic content to achieve the rated LO-RF
isolation. The LO and RF ports are single-ended and
matched to 50 Ω.
phase error, the baseband outputs should be
terminated into a matched load impedance over the
processed frequency bandwidth.
The DC voltage
offset present at each baseband output is typically
within a few mV of ground and may be blocked with
LO
DC-blocking
capacitors
without
degrading
demodulator performance.
0°
90°
INTERFACING TO SINGLE-ENDED LOADS
RF
A simple differential-to-single-ended amplifier circuit
1
based on the Analog Devices AD8130 is shown in
0°
Figure 2. This circuit converts differential I and Q
0°
outputs from a QD series demodulator into singleended I and Q signals capable of driving lowimpedance loads. The Vo input provides a means of
level-shifting the I and Q signals to any convenient
I
I
Q
DC voltage level. This circuit has excellent amplitude
Q
and phase tracking from DC to 100 MHz.
Figure 1. Quadrature Demodulator Block Diagram
Tel:
P o l yp h a s e M i c r o w a ve I n c .
1983 S Liberty Drive
Bloomington, IN 47403
USA
+1 (812) 323-8708
Fax: +1 (812) 323-8709
© 2008 Polyphase Microwave Inc.
Web: www.polyphasemicrowave.com
AN-102A
The transformer interface of Figure 3 has excellent
QDxxxxB
Q
amplitude and phase tracking from 100 kHz to over
49.9 Ω
300 MHz. It can drive 50 Ω single-ended loads with
AD8130
Q CHANNEL
OUTPUT
less than 1 dB of insertion loss.
49.9 Ω
Q
DUAL A/D CONVERTER INTERFACE
Vo
In many direct conversion and low-IF receivers, the
baseband I and Q outputs from the QD series
I
demodulators must be amplified and lowpass filtered
49.9 Ω
before digitization. The QD series demodulator’s I
AD8130
I CHANNEL
OUTPUT
49.9 Ω
I
and Q differential outputs often must also be levelshifted to the A/D converter’s positive reference
voltage. The differential amplifier circuit shown in
Vo
Figure 4 provides +14 dB gain and has a lowpass
Figure 2. Active Interface to Single-Ended Loads
The transformer circuit shown in Figure 3 provides
an all-passive method of converting the differential I
and Q outputs from a QD series demodulator into
single-ended outputs. Since this method is ACcoupled, it cannot be used in applications that
require I and Q DC response. The DC-blocking
capacitors prevent self-modulation and degraded
cutoff of 30 MHz. It also moves the common-mode
voltage to the A/D converter’s reference voltage
(Vref). The gain of the Analog Devices2 AD8132
differential amplifier is set by the ratio of the
feedback resistor to the series input resistor. A 30
MHz differential lowpass filter is formed by the output
resistors and capacitors.
51 Ω
Q
IIP2 performance.
499 Ω
100 pF
51 Ω
499 Ω
1 μF
MINI-CIRCUITS
Q CHANNEL
OUTPUT
Q CHANNEL
A/D CONVERTER
AD8132
51 Ω
QDxxxxB
Q
Vref
2.49 kΩ
QDxxxxB
Q
Vref
51 Ω
100 pF
2.49 kΩ
Vref
2.49 kΩ
51 Ω
I
499 Ω
100 pF
51 Ω
Q
1 μF
499 Ω
ADT2-1T+
I CHANNEL
A/D CONVERTER
AD8132
51 Ω
I
Vref
51 Ω
100 pF
2.49 kΩ
I
1 μF
MINI-CIRCUITS
I CHANNEL
OUTPUT
Figure 4. A/D Converter Interface
REFERENCES
[1] Analog Devices, Inc., “AD8129/AD8130
I
1 μF
Product Datasheet, Rev. 0”, 2001.
ADT2-1T+
[2] Analog Devices, Inc., “AD8132 Product
Figure 3. Transformer Circuit to Single-Ended Loads
Datasheet, Rev. C”, 2003.
Tel:
P o l yp h a s e M i c r o w a ve I n c .
1983 S Liberty Drive
Bloomington, IN 47403
USA
+1 (812) 323-8708
Fax: +1 (812) 323-8709
© 2008 Polyphase Microwave Inc.
Web: www.polyphasemicrowave.com