Last month we offered you a video and audio-stereo scalable to 4 channels on 2.400, 427, 454 and 481 GHz. This article aims to let you know how to modify the transmitter and receiver to increase the number of available channels to 256 and to extend the limits of band 2 to 2.7 GHz.
The basic system (ELM 23 page 8 and following) consists of a transmitter and a receiver with 4 channels, whose frequency is controlled by a microcontroller, specifically, an integrated circuit microcontrollers.
In this version, an integrated circuit OTP (not reprogrammable) is used. This circuit allows our system to operate on all 4 frequencies classics: 2400, 2427, 2454, 2481 MHz.
In what follows, we explain how to act on either the TX or RX on, to change the working frequency, moving between the boundaries much further.
More channels, more frequencies
During testing, we had no difficulty in moving us between 2 and 2.7 GHz, without significant reduction in power from the TX, or reduced sensitivity on the part of the RX.
To change the frequency of work, we have eliminated, on both decks, the microcontroller original and in its place, with a slight modification of the program, we used the PIC16F84.
This is made possible because the manufacturers of RF modules provide clear information for use in particular as regards programming internal PLL (Figure 1).
Both RF modules are controlled by an I2C bus, then, using only two lines (SCL and SDA clock, data).
Using this protocol, it is possible to change almost all parameters of the divider, to obtain the frequency of interest.
We will see changes to the program but, above all, we will dwell on routines to allow everyone, to reprogram its system as it wishes.
The program allows implemented, using a dip-switch with 8 DIP switches to select 256 different frequencies, spaced 1 MHz entreelles and both the base frequency of 2400 MHz.
We know, of course, that to avoid interference between a television channel and another, it is necessary to have a spacing of at least 10 to 20 MHz between them.
The purpose of this section is not that of a solution well defined as to what to do to program the frequency (or frequencies) by simply changing some parameters of the program.
modules transmit and receive audio / video
Figure 1a 
Figure 1b Figure 1: The transmission and reception of audio / video.
Pin layout of the two modules to 2.4 GHz radio frequency, used in the proposed system audio / video transmission up to 256 channels (ref. FM2400TSIM for the transmitter and receiver for FM2400RTIM8). The nominal operating frequency (2 400 to 2 483 MHz) can be greatly exceeded, both down than up.
During testing, we were able to operate between 2.0 and 2.7 GHz without significant loss of power for the transmitter and without reduction of sensitivity for the receiver.
Change TX
Let's start with ourselves with the TX, which we reproduce the layout diagram of the components in Figure 2.
As we explained in the previous section, the TX has to control the frequency of a dipswitch to 8 micro-switches, which controls the original microcontroller.
In this dip-switch, only the first two channels are active, the other 6 must be positioned to ON.
In our case, we have eliminated the original microcontroller and we replaced it with a PIC16F84, programmed with the software reported in Figure 3.
To make this process easier, we used a small circuit board on which we mounted a long-legged support, type "wrapper" (see Figures 4, 5, 6 and 7).
In addition to the microcontroller on the printed circuit, we find a resistance of 10 kilohms, which maintains high, the SDA line of control that is on leg 1 of the microcontroller.
In this case, the inputs of the microcontroller is arranged to read the 8 DIP switches found on the deck.
The microcontroller reads, permanently, the status of micro-switches and generates a corresponding sequence of bytes that go through an I2C bus, program the PLL communication module.
In practice, the microcontroller must send a sequence of 4 bytes, the first and the last are always identical.
The first byte identifies the device to be controlled (RF module), the fourth byte enables new parameters in the second and third octet.
Have recent data, the most significant, which contain the values to assign to the programmable divider and allow the VCO to generate the carrier frequency.
The frequency of the TX, is positioned by adjusting the dip-switch with 8 channels present on the plate. Starting from the base frequency of 2.400 GHz, it is possible to increase it up to 2.655 GHz in steps of 1 MHz.
The value of adding to the base frequency depends on the micro-switches that are set to ON.
Each micro-switch has a "weight" which is increasing from left to right. For example, the first micro-switch (if turned ON) is 1, the third is 4, the sixth is 32, and so on.
By adding the "weight" of different micro-active switches, we get the number to be added to the base value of 2400 MHz for the frequency generated.
The examples in Figure 16 can remove the remaining doubts.
The same procedure is used for the receiver.
If we observed the program listing soaps implemented in the microcontroller of the transmitter (Figure 3) and more specifically the part HAND, we observe that the value of PORT B (in practice the dip-switches) is used to increment values PLL core.
The minimum programmable divider is 125 kHz, therefore, to obtain 1 MHz steps, we must multiply by 8 the data obtained (PLL = PLL base + TMP * 8).
If we had not sought to obtain 500 kHz, we should multiply by 4 the TMP variable.
Similarly, if we wanted to obtain frequency values less than 2 400 MHz or even higher, we should change the line of the program as follows:
PLL = PLL based - TMP * 8
Really simple!
The next line of the program performs the actual writing in the RF module, via the I2C bus, sending four bytes of command among those called HI and LO, which contain the value to assign to the PLL and, in practice, determine the frequency generated.
Figure 2: Implementation of the platinum issue before modification (see ELM 23 page 8 and following). program for the transmitter
The program implemented in the microcontroller which controls the RF module transmitter is very simple.
It is easily modified to suit your requirements. In our case, we chose to generate 256 different frequencies from the base frequency of 2400 MHz with 1 MHz steps exactly. The purpose of this program is to read the status of a dip-switch with 8 channels and send, using an I2C protocol, the commands necessary to PLL RF module.
In practice, each possible combination of dip-switch, must match a frequency dif ferent.
For all this, the microcontroller must send a sequence of 4 bytes, in which the first and the last are always identical. The first byte is the address of the device (ADDR1 = $ C2), fourth ($ 8E) activates the parameters sent with the second and third octet. These last two (LO and HI) thus contain the information for the PLL. In fact, how the PLL must move in frequency, relative to a reference value (PLLBASE = & 4B00) equivalent to the frequency of 2400 MHz exactly. Taking this into account and the fact that the PLL moves in steps of 125 kHz, interpret the program is very simple. In the body of the program, the value of micro-switches is assigned by the TMP variable. Subsequently, the value obtained is multiplied by 8 to obtain 1 MHz is then added to the value of PLLBASE (PLL PLLBASE + = TMP * 8). We obtain the values of the corresponding two bytes (LO and HI) and we can send the RF module, the frame of complete program: I2C write DT, CK, ASSR1 (HI, LO, $ 8E).
DEFINE OSC 4 DEFINE I2C_SCLOUT 1 @ DEVICE RC_OSC SYMBOL DT = PORTA.2 SYMBOL = CK PORTA.3 SYMBOL DIP1 PORTB.0 SYMBOL DIP2 PORTB.1 SYMBOL DIP3 PORTB.2 DIP4 SYMBOL = PORTB.3 SYMBOL = DIP5 PORTB.4 SYMBOL = DIP6 PORTB.5 SYMBOL = DIP7 PORTB.6 SYMBOL = DIP8 PORTB.7 ADDR1 VAR BYTE TMP VAR BYTE PLLBASE VAR WORD PLL VAR WORD LO VAR PLL.LOWBYTE HI VAR PLL.HIGHBYTE Input DIP1 Input DIP2 Input DIP3 Input DIP4 Input DIP5 Input DIP6 Input DIP7 Input DIP8 Output CK Output DT ADDR1 = $ C2 PLLBASE = $ 4B00 MAIN: 255-TMP = PORTB PLL = PLLBASE + TMP * 8 I2CWrite DT, CK, ADDR1, [HI, LO, $ 8E] Pause 500 GoTo MAIN
Figure 3: The program for the transmitter.
Figure 4: Diagram of the modification of the transmitter. To change the number of channels, just replace the microcontroller which controls the PLL, with a suitably programmed PIC16F84 (see listing in Figure 3).
Figure 5: Diagram of location of components of the replacement card microcontroller TX. 
Figure 6: Photograph of a prototype replacement card microcontroller TX. 
Figure 7: Drawing on the scale of a daughter card for the transmitter. Component List TX
R1 = 10 kW
U1 = PIC16F84-MF371T
Others:
1 Support 2 x 9 pin wrapper
An integrated circuit ref. S371T
Change TX
Now for the review of changes to the receiver, we also defer installation diagram of components in Figure 9.
As can be seen in Figures 11, 12, 13 and 14, in this case, it is not sufficient simply to substitute a new microcontroller integrated circuit as the home plate receiver is not equipped with a dip switch.
The deck with the new integrated circuit must, therefore, also provide a dip-switch with 8 tracks, plus the resistance of 10 kilohms on the SDA line, which in this case, coincides with the lug 18 of the microcontroller (see Figure 11).
On the old microcontroller, some legs are equipped with a pull-up resistor, some not, some are connected to four LEDs.
To avoid problems, we have assigned a high level, through the program, all interested legs of the microcontroller replacement.
The one leg that is not connected to the support of the motherboard is the pin 7 as the resistance that is on this map has a value too low, which, at the close of the micro-switch because the girl you correspondent, could cause a prohibitive power consumption.
This change is made (removal of the pin 7 of the support-wrap), and after removing the old and inserted the microcontroller circuit board replacement, control of the receiver is performed by the program implemented in the new PIC16F84.
In Figure 10, plus a few lines of explanation, we reproduce the listing to allow understanding of the functioning of our system and thus be able to make personal changes.
In this case also, the program will read the status of micro-switches, so as to obtain a variation of the PLL frequency step of 1 MHz. Here too, this frequency will be added to that of the PLL core, so as to obtain a value between 2400 and 2655 MHz.
If we observe the "MAIN" program, we note that here too, the TMP variable is multiplied by 8, the internal PLL of the receiver is still not 125 kHz.
As in the case of TX, if we create more by 4, we would get a step of 500 kHz and so on (Figure 16).
Note, too, that the values of PLL based TX and RX are different: in the first case, the value is $ 4B00, $ 3C00 in the second. Obviously, these values always determine a base frequency of 2400 MHz.
At this point, you'll certainly understand how to adapt the program to your own requirements, and even then, how to operate the RF module at the desired frequency simply by programming the microcontroller with suitable values. That said, it only remains to put into practice what we have learned, realizing the two circuit boards and programming both microcontrollers using the listings provided in Figure 3 for TX and 10 for RX.
Figure 8: Focus on the couple motherboard / daughter card transmitter. The modification of the TX is very simple, because we simply replace the original microcontroller with a small plate on which is mounted a PIC16F84 programmed according to the listing shown in Figure 3.
Dip-switch, through which it is possible to select the 256 new channels, is already installed on the deck of the transmitter.
The use of a device equipped with a FLASH memory allows you to quickly change the program.
Figure 9: Diagram of implantation of the plate receipt before modification (see ELM 23 page 8 and following). receiver program
The program implemented in the microcontroller which controls the receiver module is very similar to the transmitter. In this case also, the program's goal is to get 256 different frequencies from the base frequency of 2400 MHz with 1 MHz steps exactly.
In practice, each possible combination of dip-switches must match a frequency dif ferent.
As with the TX, the microcontroller must send a sequence of 4 bytes, in which the first (ADDR1 = $ C2) and last ($ 8E) are always identical.
The second and third bytes (LO and HI) thus contain the information for the PLL. In fact, how the PLL must move in frequency, relative to a reference value (PLLBASE = & 4B00) equivalent to the frequency of 2400 MHz exactly. Taking this into account and the fact that the PLL moves in steps of 125 kHz, interpret the program is very simple.
In the body of the program, the value of micro-switches is assigned by the TMP variable. Subsequently, the value obtained is multiplied by 8 (to obtain 1 MHz) and added to the value of PLLBASE (PLL = PLLBASE + TMP * 8). We obtain the values of the corresponding two bytes (LO and HI) and we can send to the module RF reception, the frame complete program: I2C write DT, CK, ASSR1 (HI, LO, $ 8E).
It is obvious that if we want to increase or decrease the distance between channels, we simply multiply by a value other than 8, the TMP variable or, if we want to lower frequencies, we must subtract the value of PLLBASE , that of TMP.
DEFINE OSC 4 DEFINE I2C_SCLOUT 1 @ DEVICE RC_OSC SYMBOL DT = PORTA.1 SYMBOL = CK PORTA.0 SYMBOL DIP1 PORTB.0 SYMBOL DIP2 PORTB.1 SYMBOL DIP3 PORTB.2 DIP4 SYMBOL = PORTB.3 SYMBOL = DIP5 PORTB.4 SYMBOL = DIP6 PORTB.5 SYMBOL = DIP7 PORTB.6 SYMBOL = DIP8 PORTB.7 ADDR1 VAR BYTE TMP VAR BYTE PLLBASE VAR WORD PLL VAR WORD LO VAR PLL.LOWBYTE HI VAR PLL.HIGHBYTE Input DIP1 Input DIP2 Input DIP3 Input DIP4 Input DIP5 Input DIP6 Input DIP7 Input DIP8 Output CK Output DT Poke $ 81, $ 7F 'ABILITO I PULL UP-PER PORTB IL (DIP SWITCH) Pause 500 ADDR1 = $ C2 PLLBASE = $ 3C00 MAIN: 255-TMP = PORTB PLL = PLLBASE + TMP * 8 I2CWrite DT, CK, ADDR1, [HI, LO, $ 8E] Pause 500
Figure 10: Listing of the receiver program.
Iist RX
R1 = 10 kW
U1 = PIC16F84-MF371R
Miscellaneous:
1 Support 2 x 9 pin wrapper
An integrated circuit ref. S371R
Figure 11: Diagram of the modification of the receiver. In the receiver, in addition to the new MCU, there should be a dip-switch with 8 channels, with which we can later, select the 256 channels.
Figure 12: Diagram of location of components of the replacement card microcontroller RX. 
Figure 13: Photograph of a prototype replacement card microcontroller's RX. 
Figure 14: Drawing on the scale of a daughter card for the receiver. 
Figure 15: Focus on the couple motherboard / daughter card receiver. The amendment to the RX is slightly more complicated than the TX.
Indeed, the receiver does not have dip switches to set frequency, it is necessary to incorporate a deck replacement on the microcontroller and voila.
To avoid problems, some tabs are brought to high level. However, the pin 7 should be excluded from this system. So be sure to cut the wrapper on the support after having soldered daughter, of course!
How to program the frequency
The frequency of the TX and RX are parameter acting on the dip-switch with 8 channels now directly on the circuit of the transmitter and the receiver daughter card.
Starting from the base frequency of 2.400 GHz, it is possible to increase the value up to 2.655 GHz in steps of 1 MHz. The value of adding to the base frequency, depends on the micro-switches that are in position (Figure 16a).
Each of them has a "weight" which is increasing from left to right.
For example, the first micro-switch (if it is placed in position) is 1, the fifth is 16 and so on. By adding the "weight" of dif ferent micro-active switches, we get the value (to be added to 2400 MHz) frequency generated or received. For example, if we set the dipswitch as shown in Figure 16b, we get a working frequency of 2541 MHz (2400 + 41) by cons, if we set the dipswitch as shown on the Figure 16c, we get 2427 MHz (2 400 + 27).
Figure 16a 
Figure 16b 
Figure 16c Figure 16: How to program the frequency
data to be sent to the TX and RX modules
We report in this table, the most significant part of the I2C protocol, required for programming the operating frequency of RF modules. In practice, the microcontroller must send a sequence of 4 bytes, the first and the last are always identical. The first byte identifies the device to be controlled (RF module), the fourth byte enables new parameters in the second and the third octet. Have recent data, which contain the most significant values to be assigned to programmable divider and enabling the PLL to generate the desired frequency (in the case of the transmitter) or tuned frequency (in the case of the receiver).
Figure 17a. 
Figure 17a. Figure 17: Data sent to the TX and RX modules.
Figure 18: With a frequency quencemètre" amount "to nearly 3 GHz, it will be easy measure the frequency of your sys-tem for video transmission. In practice
To make the modification proposed in this article, you must perform or obtain printed circuits of Figures 7 and 14 and go up the few components visible in Figures 5 and 12.
If you do not have a programmer and if you're not particularly savvy in this discipline, the microcontrollers are available pre-programmed course.
The substitution of old by new microcontrollers, should be done with the modules off, unplugged power (obviously!) And paying attention to the proper placement of racks wrapper in existing media.
Remember that pin 7 of circuit board replacement MCU is not connected to the support of the motherboard (cut it like that, you forget it).
Now, connect the video inputs and audio transmitter and receiver can monitor the power on.
For the TX, it is necessary to use a voltage between 13 and 15 volts for the RX, 12-volt agree, provided that they are fully stabilized (in this case, delete the 7812 on the deck).
Set all DIP switches OFF and verify that the signal is received by the receiver perfectly. In this case, both devices run on 2400 MHz exactly.
Now try several combinations, following the example of the table given in Figure 16.
You will find that our system works perfectly, whatever the chosen frequency. If you have a suitable frequency, you can also check the exact frequency of issue.
In an upcoming issue, we present more projects with these modules, in particular, a scanner audio / video operating between 2 and 2.7 GHz.
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