Maxim > Design Support > Technical Documents > Application Notes > Automotive > APP 1017
Maxim > Design Support > Technical Documents > Application Notes > Wireless and RF > APP 1017
Keywords: MAX1470, superheterodyne receiver, 315MHz-433MHz receiver, crystal oscillator, quartz
crystal oscillators
APPLICATION NOTE 1017
How to Choose a Quartz Crystal Oscillator for the
MAX1470 Superheterodyne Receiver
Aug 21, 2002
Abstract: Quartz crystals are available from various vendors in a variety of shapes and sizes, and can
range widely in performance specifications. Some of these specifications include resonance frequency,
resonance mode, load capacitance, series resistance, holder capacitance, and drive level. This
application note helps in the understanding of these parameters, allowing the user to specify the crystal
that is right for their application and one that will give the best results with the MAX1470
superheterodyne receiver circuit.
Quartz crystals are available from various vendors in a variety of
shapes and sizes, and can range widely in performance
specifications. Some of these specifications include resonance
frequency, resonance mode, load capacitance, series resistance,
holder capacitance, and drive level. Understanding these
parameters will allow you to specify the crystal that is right for your
application and one that will give you the best results with your
MAX1470 circuit.
Click here for an overview of the wireless
components used in a typical radio
transceiver.
The equivalent circuit of a crystal is shown in Figure 1. It consists
of the motional elements: resistor, Rs, inductor, Lm, and capacitor,
Cm, and a shunt capacitance, Co. The motional elements determine the series resonant frequency and
the Q of the resonator. The shunt capacitance, Co, is a function of the crystal electrodes, holder, and
leads.
Figure 1. Crystal models.
The following details the key performance specifications.
Resonance Frequency
The crystal frequency to be specified is dependent on the frequency you're interested in receiving. Since
Page 1 of 5
the MAX1470 uses a 10.7MHz IF with low-side injection, the crystal frequency is given by (all units in
MHz):
For 315MHz applications, the crystal frequency is therefore, 4.7547MHz, while for 433.92MHz a
6.6128MHz crystal is needed. Only fundamental mode crystals should be specified (no overtones).
Resonance Mode
Crystals have two modes of resonance: series (the lower frequency of the two) and parallel (or antiresonant, the higher of the two). All crystals exhibits both resonance modes at which they appear
resistive in an oscillator circuit. At series resonance, the reactances of the motional capacitance, Cm,
and inductance, Lm, are equal and opposite and the resistance is minimal. At the anti-resonance point,
though, the resistance is maximum and the flow of current is minimal. The anti-resonance point is not
used in oscillator designs.
A quartz crystal can be used to oscillate at any frequency between the series and anti-resonance
frequencies by adding external components (usually capacitors). In the crystal industry this is referred to
as the parallel frequency or mode. This frequency is above the series frequency but is below the true
parallel resonance of the crystal (the anti-resonance point). Figure 2 shows a typical crystal impedance
versus frequency plot.
Figure 2. Crystal impedance versus frequency.
Load Capacitance and Pullability
Load capacitance is an important specification when using the parallel resonant oscillation mode. In this
mode, the crystal's total reactance is slightly inductive and is in parallel with the oscillator's load
capacitance, forming an LC tank circuit which determines the oscillator frequency. As the value of the
load capacitance is changed, so does the output frequency. Therefore, the crystal vendor must know the
load capacitance employed by the oscillator circuit so that it can be calibrated at the factory using the
same load capacitance.
Page 2 of 5
If a crystal designed to oscillate with a different load capacitance is used, the crystal is pulled away from
its stated operating frequency, introducing an error in the reference frequency. Therefore, to pull the
crystal back to its desired operating frequency, external capacitors are added to modify the load
capacitance
Figure 3 shows the crystal within the MAX1470EVKit circuit. In this circuit, C14 and C15 are series
pulling capacitors, while C16 is a parallel pulling capacitor. Cevkit is the equivalent MAX1470, plus the
evkit PCB stray capacitance. Cevkit is approximately 5pF.
Figure 3. EVKit equivalent circuit.
Series pulling capacitors, will "speed up" the crystal, while a parallel capacitor will "slow" it down. Since
Cevkit is equal to 5pF, if a crystal with a load capacitance of 5pF is used, it will oscillate at it's intended
frequency, and no additional capacitance is needed (C16 is left open, while C14 and C15 are shorted on
the board). The evkit itself uses a 3pF load capacitance crystal, necessitating 2 x 15pF capacitors in
series to speed it up. To calculate the capacitance values needed use:
In the case of the evkit, if the 2 series capacitors are not used, the 4.7547MHz crystal will actually
oscillate at 4.7544MHz, causing the receiver to be tuned to 314.98MHz rather than 315.0MHz, an error of
about 20kHz, or 60ppm.
Therefore, the key is to match the crystal's load capacitance required by using either series or parallel
capacitors or even a combination of both (depending on the value of capacitors available). For example,
a 1pF parallel capacitor is all that's needed for a 6pF load capacitance crystal (or the following
combination: C14 = C15 = 27pF, C16 = 5pF).
Care must be exercised not to use too large values for C16, as it increases the current through the
oscillator circuit, causing it to fail. Figure 4 shows the relationship between parallel capacitance and
oscillator current.
Page 3 of 5
Figure 4. Crystal oscillator current vs. added parallel load capacitance.
On a custom PCB, if Cevkit is not known, it is possible to monitor the IF on a spectrum analyzer (make
sure to use a DC blocking capacitor before inserting the signal into the spectrum analyzer), and then use
the series and parallel capacitors to "tune" the IF back to 10.7MHz.
Series Resistance
A typical range of series resistance is 25Ω to 100Ω for most crystals. The crystal vendor usually
characterizes this resistance and specifies maximum values for series resistance. Do not exceed 100Ω
for the MAX1470 oscillator circuit.
Holder or Shunt Capacitance
This is the capacitance of the crystal electrodes, holder, and leads. Typical values range from 2pF to
7pF.
Drive Level
The power dissipated in the crystal must be limited or the quartz crystal can actually fail due to
excessive mechanical vibration. Crystal characteristics also change with drive level due to non-linear
behavior. The crystal vendor will specify the maximum drive level for a particular product line. Use
crystals with drive levels in the 1µW range.
These specifications will allow you to specify a crystal that best fits the requirements of the MAX1470
oscillator circuit, which in turn will improve the overall performance.
Related Parts
MAX1470
315MHz Low-Power, +3V Superheterodyne Receiver
Free Samples
MAX1472
300MHz-to-450MHz Low-Power, Crystal-Based ASK
Transmitter
Free Samples
MAX1473
315MHz/433MHz ASK Superheterodyne Receiver with
Extended Dynamic Range
Free Samples
Page 4 of 5
More Information
For Technical Support: http://www.maximintegrated.com/support
For Samples: http://www.maximintegrated.com/samples
Other Questions and Comments: http://www.maximintegrated.com/contact
Application Note 1017: http://www.maximintegrated.com/an1017
APPLICATION NOTE 1017, AN1017, AN 1017, APP1017, Appnote1017, Appnote 1017
Copyright © by Maxim Integrated Products
Additional Legal Notices: http://www.maximintegrated.com/legal
Page 5 of 5