LINEAR TECHNOLOGY LTC5585 Technical data

Optimizing the Performance of Very Wideband Direct
Conversion Receivers

Design Note 1027

John Myers, Michiel Kouwenhoven, James Wong, Vladimir Dvorkin

Introduction

Zero-IF receivers are not new; they have been around
for some time and are prominently used in cell phone
handsets. However their use in high performance wire­less base stations has had limited success. This is due
primarily to their limited dynamic range and that they
are less well understood. A new wide bandwidth zero-IF
IQ demodulator helps relieve the dynamic range and
bandwidth shor tcomings for main as well as DPD (digital
predistor tion) receiver s, and enables 4G base stat ions to
cost effectively address the ever-increasing bandwidth
needs of mobile access. This article discusses how to
optimize performance by minimizing IM2 nonlinearity
and DC offset that reduce the dynamic range of zero­IF receivers, thus offering a viable alternative to an
otherwise challenging design.

Pushing Ever Wider Bandwidth

Until recently, most base stations needed to only deal
with a 20MHz wide channel bandwidth, typically allo­cated to various wireless carriers. Associated with this
20MHz channel is a companion 100MHz bandw idth DPD
receiver to measure intermodulation distortion spurs
up to 5th order for effective distortion cancellation.
These requirements can generally be met effectively
w i t h h i g h - I F ( h e t e r o d y n e ) r e c e i v e r s . N o w a d a y s t h o u g h ,
such designs are more challenging, with industry
trends pushing for base stations to support operation
over the entire 60MHz bands. Accomplishing this feat
has signifi cant cost savings implications for the entire
wireless manufacturing, installation and deployment
business model.

To accommo date the three times increase in ba ndwidth,
the DPD receiver b andwidth must increase from 100MHz
to 300MHz. In 75MHz ba nds, the DPD bandwidth grows
to a staggering 375MHz. The design of receivers that
can suppor t this bandwidth is not trivia l. Noise increases
due to the wider bandwi dth, gain fl atness becomes more

diffi cult to achieve, and the required sampling rate of
A/D converters increases dramatically. Furthermore,
the cost of such higher bandwidth components is ap­preciably higher.

The modest bandwidth of a traditional high-IF receiver
is no longer suffi cient to support the 300MHz or higher
DPD signals with typically ±0.5dB gain fl atness. The
300MHz baseband bandwidth would require choos­ing an IF frequency of 150MHz at a minimum. It is not
trivial to fi nd an A/D converter capable of a sampling
r a t e u pw a r d of 6 0 0 M s ps t h a t is r e a s on a b l y pr i c e d , e v e n
at 12-bit resolution. One may have to compromise and
resort to a 10-bit converter.

New IQ Demodulator Eases Bandwidth Constraints

The LTC5585 IQ demodulator is designed to support
d i r e c t c o n v e r s i o n , t h u s a l l o w i n g a r e c e i v e r t o d e m o d u l a t e
the aforementioned 300MHz wide RF signal directly to
baseband (see Sidebar: Theory of Operation of a Zero-IF
Receiver). The I and Q outputs are demodulated to a
150MHz wide signal, only half the bandwidth of a high­IF receiver. In order to attain a passband gain fl atness
of ±0.5dB, the device’s –3dB corner must extend well
above 500MHz.

The LTC5585 supports this wide bandwidth with a
tunable baseband output stage. The differential I and
Q output ports have a 100Ω pull-up to V

in parallel

CC

with a fi lter capacitance of about 6pF (see Figure 1).
This simple R-C network allows for the formation of
o f f - c hi p lo w p a s s o r b a n d pa s s fi l t e r n et w o r k s t o r em o ve
high level out-of-band blockers and equalization of gain
roll-off the baseband amplifi er chain that follows the
demodulator. With a 100Ω differential output loading
resistance in addition to the external 100Ω pull-up
resistors, the –3dB bandwidth reaches 850MHz.

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.

03/12/1027

Baseband Bandwidth Extension

A single L-C fi lter section can be used to further extend
the bandwidth of the baseband output. Figure 1 shows
the chip’s baseband equivalent circuit with baseband
bandwidth extension. With 200Ω loading, the –0.5dB
bandwidth can be extended from 250MHz to 630MHz
using a series inductance of 18nH and a shunt capaci­tance of 4.7pF. Figure 2 shows the variety of output
responses that are possible with different loading. One
r e s p o n s e i s w i t h d i f f e r e n t i a l l o a d i n g r e s i s t a n c e s o f 2 0 0 Ω
and 10kΩ. For 10kΩ loading, the – 0.5dB bandwidth can
be extended from 150MHz to 360MHz using a series
inductance of 47nH and a shunt capacitance of 4.7pF.

Second-Order Intermodulation Distortion Spurs
Matter

In a direct conversion receiver, the second order in­termodulation distortion products (IM2) fall directly
in-band at the baseband frequencies. Take, for example,
two equal power RF signals, f1 and f2, spaced 1MHz
apart at 2140MHz and 2141MHz, respectively, while the
LO is spaced 10MHz apart at 2130MHz. The resultant
IM2 spur would fall at f2 – f1, or 1MHz. The LTC5585
has the unique ability to adjust for minimum IM2 spurs
independently on the I and Q channels by using ex ternal
control voltages. Figure 3 shows a t ypical setup for IIP2
measurement and calibr ation. The differen tial baseband

Figure 1. Baseband Output Equivalent Circuit for
Bandwidth Extension with L = 18nH and C = 4.7pF

Figure 2. Conversion Gain vs Baseband
Frequency with Differential Loading
Resistance and L-C Bandwidth Extension

2