Radio microphone on a TXC101 chip with VOX. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Microphones, radio microphones

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This design is assembled to operate in the 4XX MHz band. However, thanks to the use of the RFM TXC 101 microcircuit, the frequency range of the device can be 310 - 319 MHz, 430 - 439 MHz, 860 - 879 MHz, 900 - 929 MHz. And this is only in the case of using a 10 MHz quartz resonator provided by the manufacturer!

Practical tests have shown that using quartz above or below the frequency of 10 MHz, it is possible to “take away” the operating range of the TXC 101 within more than 100 MHz, from that specified in the documentation.

With 13 MHz quartz, the transmitter worked absolutely without problems at a frequency of 565.5 MHz, while the programmatically set frequency for it was 435 MHz.

The ability to programmatically set the operating frequency with a small step (2,5 kHz on the 4XX MHz subband and 7,5 kHz on the 8XX and 9XX MHz subbands) makes this chip quite attractive for various applications.

The device is controlled by the PIC12F675 microcontroller. It downloads data to the transmitter, controls the power amplifier and "monitors" the level of the low-frequency signal from the microphone amplifier.

The VOX function is implemented on the built-in comparator of the microcontroller. If the level of the low-frequency signal from the microphone amplifier exceeds the threshold (set by software in EEPROM), the controller "wakes up" TXC 101 and turns on the final stage for a specified time (set by software in EEPROM).

(click to enlarge)

Technical characteristics of the device:

• Current consumption in standby mode - 1 mA
• Current consumption in operating mode - 20 mA
• Transmitter power - 20 mW
• Modulation - WFM
• Supply voltage range - 2,2 ... 3,6 volts

Controller Programming

In the zero cell of the controller's memory, at the programming stage, a subband is written in which the transmitter will be used.

Number 88 ( zero cell EEPROM ) - selection of the sub-range of application of THS 101. 

The number 00 (the first cell of the EEPROM) is optional settings (they do not need to be touched).

There are only 4 subranges.

The number 80 - means that the crystal transmitter will operate in the sub-band 310 - 319 MHz 

The number 88 - means that the crystal transmitter will operate in the sub-band 430 - 439 MHz

The number 90 - means that the crystal transmitter will operate in the sub-band 860 - 879 MHz

The number 98 - means that the crystal transmitter will operate in the sub-band 900 - 929 MHz

From the zero and first memory cells of the controller, the Configuration register ТХС 101 is filled.

For a more detailed acquaintance with the transmitter registers, I recommend the RFICDA program.

The next two memory cells are occupied by the number A7D0

This is the exact value of the transmitter frequency within the selected subband.

If you select a frequency in the RFICDA Center frequency program window, for example 435.015, the number A7D0 will change to A7D6.

If you select a frequency in the Center frequency window, for example, 438.4 MHz, then the number in the Frequency Setting Register window will be AD20, and so on.

Thus, at the "prompt" of the RFICDA program you can know what to enter into the memory of the controller for the transmitter to operate at the desired frequency.

It must be remembered that this is true for the 10 MHz reference crystal used.

If the reference crystal frequency is not 10 MHz, the entire frequency grid will shift proportionally.

For example, the specified frequency at 10 MHz quartz is 435 MHz.

How to calculate the frequency with a reference quartz of 12 MHz?

Very simple …

Need 435: 10 = 43,5 (division ratio)

Then you need 43,5 x 12 = 522 MHz (desired frequency)

In any case, it should also be remembered that a varicap connected in series with the reference quartz “takes” it up a few kHz, which will eventually cause an error in the final calculations by a hundred or even more kHz.

The next address of the non-volatile memory of the controller is occupied by the number 0A

This is the amount of VOX delay. That is, the time for which the transmitter will turn on after activation by voice.

The maximum time is an FF number, which in 0,5 second increments will be 128 seconds.

0A corresponds to a 5 second delay.

During the period of transmitter activity, each sound exceeding the VOX threshold will extend the time specified in the device's memory.

The next value is AA.

This is the threshold level for triggering voice activation.

There are only 16 threshold levels. Required from A0 to AF.

A0 - maximum sensitivity VOX.

AF - minimum VOX sensitivity.

Precise level calibration can be done by selecting the R VOX resistor.

If the resistor R VOX is replaced with a capacitance of 1 μF, then the binding of activation by voice will be precisely to the variable component of the audio signal.

To operate the radio microphone in the UFM mode, it is necessary to install a capacitor in parallel with the varicap, which reduces the modulation index.

List of radio elements

audio79.rar

Author: Sergey (blaze), Kremenchuk, blaze2006@ukr.net

See other articles Section Microphones, radio microphones.

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