AN1776| Application Note
Maxim/Dallas > App Notes > WIRELESS, RF, AND CABLE Keywords: MAX1470, superheterodyne receiver, 315MHz, 433MHz, 315MHz-433MHz receiver
Oct 22, 2002
APPLICATION NOTE 1776
MAX1470 Superheterodyne Receiver FAQ
Frequently asked questions concerning the MAX1470 superheterodyne receiver.
Frequently Asked Questions for the MAX1470 315MHz Low-Power, +3V Superheterodyne Receiver How do I select the right crystal for my MAX1470 application?
The key is to find out how much capacitance exists between XTAL1 and XTAL2 on your board and to get a crystal which is designed to oscillate at that capacitance. For example, the MAX1470 evkit presents a capacitance of about 5pF between XTAL1 and XTAL2. Therefore if a crystal designed to oscillate with a 5pF load is used, no additional capacitance is needed and you can "short" the series capacitors. If a crystal designed to operate at a higher load capacitance is used, the crystal will oscillate at a higher frequency, and additional capacitance is needed to pull the crystal to the correct frequency. As a rule, parallel capacitance is needed to pull the crystal lower, while series is needed to pull it higher. Ultimately, the best test is to monitor the IF frequency (after the filter) on a spectrum analyzer. The deviation of the IF from 10.7MHz will determine how much capacitance needs to be added, keeping in mind that the added capacitance will affect "cold" start-up time and the shutdown current. Please refe
r to the application note: "How to Choose a Quartz Crystal Oscillator for the MAX1470 Superheterodyne Receiver."
How critical is the ESR requirement of 70 ohms max for the MAX1470 crystals? I am trying to get some samples quickly and one source I have found cannot provide an ESR of less than 150 ohms. Would this work?
The design of the control loop is such that it could accommodate such a high ESR. A high ESR will increase current consumption (slightly) and will increase "cold" start-up time (not an issue with the MAX1470 since the oscillator circuit is running continuously even in shutdown mode) Having said all that, in the long term a lower ESR value would be preferable.
I have a TDA based design that I would like to convert to the MAX1470. Can you help?
Please refer to the application note:"How to Modify an Existing Design from a TDA5200 or TDA5201 to a MAX1470 Superheterodyne Receiver."
The MAX1470 startup time is listed as 250usec. What does that include?
The 250usec start-up time is from the positive edge of the PWRDN signal to valid data, with approximately 100msec of power cycling. This time is unaffected by minimum sensitivity and includes the time it takes for the PLL to lock. The PLL lock time is actually more in the 100usec range. It should be noted that the start up time from "cold" could be up to 10ms. This is due to the crystal start up time and any pulling capacitors that it may
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