AN1993
High sensitivity applications of low-power RF/IF integrated
circuits
Rev. 3 — 13 August 2014
Application note
Document information
Info
Keywords
Abstract
Content
12 dB SINAD, 20 dB SINAD, crystal filter, ceramic filter, VHF, UHF,
image frequency, FM broadcast receiver, ASK, FSK
This application note discusses four high sensitivity receivers and
Intermediate Frequency (IF) strips that utilize intermediate frequencies of
10.7 MHz or greater. Each circuit utilizes a low-power VHF mixer and
high-performance low-power IF strip. The circuit configurations are:
45 MHz or 49 MHz to 10.7 MHz narrowband; 90 MHz to 21.4 MHz
narrowband; 100 MHz to 10.7 MHz wideband; and 152.2 MHz to
10.7 MHz narrowband.
Each circuit is presented with an explanation of component selection
criteria (to permit adaptation to other frequencies and bandwidths).
Optional configurations for local oscillators and data demodulators are
summarized.
NXP Semiconductors
AN1993
High sensitivity applications of low-power RF/IF integrated circuits
Revision history
Rev
v.3
Date
20140813
Description
Application note; third release
Modifications:
•
v.2
20140804
Updated
Figure 9, Figure 12, Figure 14, Figure 16,
and
Figure 20.
Application note; second release
Modifications:
•
•
•
•
•
•
•
•
•
v.1
19970820
The format of this application note has been redesigned to comply with the new identity
guidelines of NXP Semiconductors.
Legal texts have been adapted to the new company name where appropriate.
“SA602” changed globally to “SA602A”
Section 3 “The problem”: deleted (old) fourth paragraph
Section 4 “The solution”, first paragraph: fifth and sixth sentences rewritten
Section 4.1 “The mixer”, second paragraph, first sentence changed from “interference must
be correct” to “impedance/noise match must be optimized”
Section 4.3 “Basic considerations”, second paragraph, fourth sentence:
changed from “(1.5
nominal)” to “(1.5 k nominal)”
Figure 18 “Oscillator configurations”: pin numbers are corrected
Section 10 “Data demodulation”, second paragraph: (old) third sentence replaced with (new)
third and fourth sentences.
Application note; first release
Contact information
For more information, please visit:
http://www.nxp.com
For sales office addresses, please send an email to:
salesaddresses@nxp.com
AN1993
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2014. All rights reserved.
Application note
Rev. 3 — 13 August 2014
2 of 23
NXP Semiconductors
AN1993
High sensitivity applications of low-power RF/IF integrated circuits
1. Introduction
Traditionally, the use of 10.7 MHz as an intermediate frequency has been an attractive
means to accomplish reasonable image rejection in VHF/UHF receivers. However,
applying significant gain at a high IF has required extensive gain stage isolation to avoid
instability and very high current consumption to get adequate amplifier gain bandwidth. By
enlightened application of two low-power ICs, NXP Semiconductors SA602A and
SA604A, it is possible to build highly producible IF strips and receivers with input
frequencies to several hundred MHz, IF frequencies of 10.7 MHz or 21.4 MHz, and
sensitivity less than 2
V
(in many cases less than 1
V).
The SA605 combines the
function of the SA602A and the SA604A. All of the circuits described in this application
note can also be implemented with the SA605. The SA602A and SA604A were utilized for
this application note to permit optimum gain stage isolation and filter location.
2. The basics
First, let us look at why it is relevant to use a 10.7 MHz or 21.4 MHz intermediate
frequency. 455 kHz ceramic filters offer good selectivity and small size at a low price. Why
use a higher IF? The fundamental premise for the answer to this question is that the
receiver architecture is a heterodyne type as shown in
Figure 1.
mixer
pre-select
filter
IF filter
LO
IF amp
demod
audio
and/or
data
002aah473
Fig 1.
Basic heterodyne receiver
A pre-selector (band-pass, in this case) precedes a mixer and local oscillator. An IF filter
follows the mixer. The IF filter is only supposed to pass the difference (or sum) of the
Local Oscillator (LO) frequency and the preselector frequency.
The reality is that there are always two frequencies which can combine with the LO: the
pre-selector frequency and the ‘image’ frequency.
Figure 2
shows two hypothetical
pre-selection curves. Both have 3 dB bandwidths of 2 MHz. This type of pre-selection is
typical of consumer products such as cordless telephones and FM radio.
Figure 2a
shows
the attenuation of a low-side image with 10.7 MHz.
Figure 2b
shows the very limited
attenuation of the low-side 455 kHz image.
AN1993
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2014. All rights reserved.
Application note
Rev. 3 — 13 August 2014
3 of 23
NXP Semiconductors
AN1993
High sensitivity applications of low-power RF/IF integrated circuits
10
pre-selector attenuation (dB)
0
−10
−20
−30
LO
−40
−60
−40
image
−20
−10
0
pre-selector
filter
002aah474
10
pre-selector attenuation (dB)
0
−10
−20
−30
−40
−60
−40
LO
desired
frequency
−20
−10
0
image
002aah475
10.7 MHz IF
455 kHz IF
pre-selector
filter
desired
frequency
20
10
40
∆
pre-selector freq. (MHz)
20
10
40
∆ pre-selector freq. (MHz)
a. Attenuation of low-side image with 10.7 MHz
Fig 2.
Effects of pre-selection on images
b. Attenuation of low-side 455 kHz image
If the single conversion architecture of
Figure 1
were implemented with a 455 kHz IF, any
interfering image would be received almost as well as the desired frequency. For this
reason, dual conversion, as shown in
Figure 3,
has been popular.
In the application of
Figure 3,
the first IF must be high enough to permit the pre-selector to
reject the images of the first mixer and must have a narrow enough bandwidth that the
second mixer images and the intermod products due to the first mixer can be attenuated.
There is more to it than that, but those are the basics. The multiple conversion heterodyne
works well, but — as
Figure 3
suggests — compared to
Figure 2
it is more complicated.
Why then, don’t we use the approach of
Figure 2?
first
mixer
first
IF filter
second
mixer
second
IF filter
pre-select
filter
IF amp
first
LO
second
LO
demod
audio
and/or
data
002aah476
Fig 3.
Dual conversion
AN1993
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2014. All rights reserved.
Application note
Rev. 3 — 13 August 2014
4 of 23
NXP Semiconductors
AN1993
High sensitivity applications of low-power RF/IF integrated circuits
3. The problem
Historically there has been a problem: stability! Commercially available integrated IF
amplifiers have been limited to about 60 dB of gain. Higher discrete gain was possible if
each stage was carefully shielded and bypassed, but this can become a nightmare on a
production line. With so little IF gain available, in order to receive signals of less than
10
V
it was necessary to add RF gain and this, in turn, meant that the mixer must have
good large signal handling capability. The RF gain added expense, the high-level mixer
added expense, both added to the potential for instabilities, so the multiple conversion
started looking good again.
But why is instability such a problem in a high gain, high IF strip? There are three basic
mechanisms. First, ground and the supply line are potentially feedback mechanisms from
stage-to-stage in any amplifier. Second, output pins and external components create
fields which radiate back to inputs. Third, layout capacitances become feedback
mechanisms.
Figure 4
shows the fields and capacitances symbolically.
Z
F
Z
F
Z
F
mixer
band-pass
filter
Z
F
Z
F
band-pass
filter
Z
i
Z
i
Z
i
002aah477
Fig 4.
Feedback paths
If Z
F
represents the impedance associated with the circuit feedback mechanisms (stray
capacitances, inductances and radiated fields), and Z
i
is the equivalent input impedance,
a divider is created. This divider must have an attenuation factor greater than the gain of
the amplifier if the amplifier is to remain stable.
•
If gain is increased, the input-to-output isolation factor must be increased.
•
As the frequency of the signal or amplifier bandwidth increases, the impedance of the
layout capacitance decreases, reducing the attenuation factor.
AN1993
All information provided in this document is subject to legal disclaimers.
© NXP Semiconductors N.V. 2014. All rights reserved.
Application note
Rev. 3 — 13 August 2014
5 of 23
京公网安备 11010802033920号
EEWORLD
