RF video reciever - digital tuning

Anybody know what i can do to be able to "tune" the frequency of my RF video receiver using my pic.

Basically i want to replace the knob so that i can control the receiver using a pic... what do i need to do this... it's for one of these wireless video recievers...

I have multiple cameras which are on slightly different frequencies.

I'd like to be able to "select" a camera from my software program, but need to know how to replace the knob so that i can tune using a pic.

Please let me know if you know :)

Is the knob a potentiometer or a switch? Do you have any info on the receiver?

I believe it's a potentiometer, its a turning knob with 3 wires leading out of it...

Quote:

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Originally Posted by RYTECH

Anybody know what i can do to be able to "tune" the frequency of my RF video receiver using my pic.

Basically i want to replace the knob so that i can control the receiver using a pic... what do i need to do this... it's for one of these wireless video recievers...

I have multiple cameras which are on slightly different frequencies.

I'd like to be able to "select" a camera from my software program, but need to know how to replace the knob so that i can tune using a pic.

Please let me know if you know :)

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I think you need to use a fractional N PLL chip to slect the frequency of your choice. Off the top of my head, I cannot recollect the chip I used. This was in 1995. If you need more, just let me know, I can dig out the part number for the chip.
EDIT : I do think you are lucky -- the tuner I used was ALPS TEKE9X094A pll tuner with 50Khz resolution to build a set top channel converter. If you want a PLL chip, try using the MC 145191


Jerson

One way is to replace the potentiometer with a Digital potentiometer. There are many different kinds so start Google and you will find..... If you want a simple to make ( but requires some calibration after it is made) take something from here http://www.maxim-ic.com/DigitalPotentiometers.cfm and replace the old one. The tricky part will be the resolution. If you get 2 or 3 with different values you can connect them in parallell and the combined setting will be used. Bit more trouble but bettet resolution.

If the potentiometer is changing the reference voltage and not the resonance frequency of the reciever you might be able to replace the current potentiometer with an OP amp. Use the pic pwm function to create a DC level, boost it with the OP and off you go. This requires some filters on the PWM output.

There are some potentiometers used in mixers and automation that are a stepengine combined with a pot, these are a bit expencive but a simple solution. Step the engine with the pic and change freq.

Just attach any off the shelf step engine to the potentiometer you have now. Turn it all the way to zero and count how many steps you have to the maximux value. Then it is just to make sure you never step outside these min and max values.

To be safe, don't screw the potentiometer straight on the engine axis, instead align the axis parallell to the Knob and put som rubberbands as a driving belt. That should give you room for minor misstakes in the software development :-)

Not the neatest solution but definatly the fastest one.

And you can always just remove the stuff and you are back to where you are now.... Hey hey hey - a true Krusty the clown design!!!

My suggestion is to measure the DC voltage going to the pot.It is usually found on the two outer pins. The center is almost always the wiper assembly. The way the pot works, is as a variable voltage divider feeding a diode that changes reverse capacitance as the voltage fed to it is changed. You need to replace the pot with a D to A chip. They are cheap and pretty easy to use. An 8 bit chip will give you 255 steps between 0 and 5 volts or so. A higher res chip will give you smaller increments and finer tuning but may have some noise issues and is more difficult to program. With the cheaper receivers, the unit is "tuned" with a pot because there is little frequency stability, and it is cheaper and lots easier than a fractional N synthesizer. The down side to replacing the pot is that step "55" which originally corresponded to channel 1, may not take you back to the exact frequency after a few hours of use. It should be a fun project, and will reward you with alot of knowledge if not a satisfactory product.

Ron

a digital to analog solution sounds appropriate. i'll investigate this option more...

Rytech

I am not sure if you will be able to acheive the precision required for tuning using digital pots or D/A converters since there may be an issue of fine tuning the frequency. I seriously recommend the PLL fractional-N synth approach so that you can numerically get the frequency you desire. Moreover, it is not difficult to program the device to a particular frequency. You could build the entire CCIR frequency table as divisors and set the frequency by channel.

Of course, this may be overkill for a one-off solution where you just need to get something done; fast.

Jerson

Hi,

A budget solution:

Use the Hardware PWM in the PIC together with a filter (2 caps and 2 resistors) to get an adjustable DC level by changing the duty cycle of teh HPWM. Feed this in to an OP if you need more current. This is excactly the same as having a 255 step DA converting chip by only adding a few cents of components. If you dont have a HPWM, it is possible to make a SW one but pick a PIC that has.

Buy a LM1881 (sync separator chip) for a dollar or two, connect the video source from the receiver to it. Then connect the V-sync signal from this chip to your PIC.

Let the software check the V-sync signal period to make sure it is correct, if it is not correct, adjust the duty cycle and voila ,hopefully :-)


Then you have a kind of PLL, since you are locking the HPWM dutycycle to the v-sync signal from this chip. The LM1881 has been around for a long time so it is nothing special.


Total cost less then 10 USD with PIC and all.

/me

Quote:

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Originally Posted by Jumper

Hi,

A budget solution:

Use the Hardware PWM in the PIC together with a filter (2 caps and 2 resistors) to get an adjustable DC level by changing the duty cycle of teh HPWM. Feed this in to an OP if you need more current. This is excactly the same as having a 255 step DA converting chip by only adding a few cents of components. If you dont have a HPWM, it is possible to make a SW one but pick a PIC that has.

Buy a LM1881 (sync separator chip) for a dollar or two, connect the video source from the receiver to it. Then connect the V-sync signal from this chip to your PIC.

Let the software check the V-sync signal period to make sure it is correct, if it is not correct, adjust the duty cycle and voila ,hopefully :-)


Then you have a kind of PLL, since you are locking the HPWM dutycycle to the v-sync signal from this chip. The LM1881 has been around for a long time so it is nothing special.


Total cost less then 10 USD with PIC and all.

/me

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I like the idea of this, but how does the PIC know which way to tune, if it loses the sync? I bet that there is an AGC point on the circuit board, which will give a measurable voltage change with signal level. This might give a better tune indication than the sync pulse. If you get a D to A chip which is a resistive network, rather than a charge pump, your noise level will be much easier to minimize. You could also use the eight lines of an I/O register to create an R2R network to accomplish the same thing with lower noise.

Ron

When the PIC senses that the signal has been lost (or getting bad) it could try to tune up a certain number of steps, if that didn't work it tunes down instead. The signal must be somewhere, or it just sweps the entire band for a signal.

Probably will the FQ drift mostly in one direction and then we start tuning that way.

Feeding the AGC point into an AD converter in the PIC would be neat but it requires som reverse engineering to find it.

A proper filtered HPWM should have a nice DC level out but as you say, a resistor network from the I/O will do just as fine.

I've got my D/A chips from maxim.. i'll report back if the digital tuning solution using D/A works out...

tuner works using max518's.. :)

the 8 bit DAC is sufficient for tuning in the various frequencies.