RF video reciever - digital tuning

[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

[Ron Marcus]

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

[RYTECH]

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

[Jerson]

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

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

[sinoteq]

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.

[Jumper]

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!!!

[Ron Marcus]

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

[RYTECH]

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

[Jerson]

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