RC India

We all keep talking about  the transmission frequencies  of the radio  but I was recently wondering about 2-3 points with regards
 to the radio :

1)  How is channel separation achieved : eg  at take off throttle , rudder and elevator  channels are used  suppose I use 72.010 mhz radio then how are these   separated and conveyed to the receiver. I understand the pwm signals the receiver conveys the servos/esc etc but that is internal communication in the receiver .

2) What is  the difference between single and dual conversion

Any leads to reading material will be hepful

Sai

1) That is where PPM and PCM come in, in the case of FM.  The PWM signal is converted to one of the two (there are other variants also) for transmission.   If you read up the difference between PPM and PCM, you will get the answer you are looking for.  Basically there are "frames" (think of them as blocks) of data transmitted together from all the different channels.  The frames have separation markers (typically a long duration pulse).

2) http://www.rcindia.org/radios-and-receivers/fm-receiver-choices/msg11772/#msg11772 

Well this is very simple !
At the transmitter end a contant pulse is generated by oscillator circuit and these are given to the counter IC through potentiometer (which controls width)in counter every pulse is arranged in a such a way according to channel !example a you have 4 ch accordingly each pulses are arranged and converted into train of 4 pulses(encoding) and these digital pulses are given to transmitting circuit which transmitts the signal to rx end where another counter is used to decode the pulse and drive particular servo!
http://radio-control-schematics.blogspot.com/2007/07/basic-radio-control-encoder.html
well this may help you sir!

Regards
Praveen

The difference between PPM and PCM is this:

The signals from the control pots are analogue. In PPM mode this is converted into a signal whose pulse width varies proportionately (typically from 1 - 2 msecs.) with the stick movement . In PCM mode, the signal is "digitised" and converted into a 9, 10 or 11 bit (depending on 512, 1024 or 2048 resolution) code. This is considered more robust than PPM mode and also allows for features such as "fail safe".

The receiver decodes this to a PWM signal for the servos, in either mode.

Sai is looking specifically for how the different channel signals (like aileron, elevator etc) are laid out (or separated) in the transmission.

This picture of a PPM frame should help.

http://www.aerodesign.de/peter/2000/PCM/frame_ppm.gif

That is where PPM and PCM come in, in the case of FM.  The PWM signal is converted to one of the two (there are other variants also) for transmission.

My post was in reference to this.

Sure. I was hoping that he would find interesting links on "frames", "frame separation marker" etc while researching PPM and PCM.

In PPM/PCM radios (not sure about 2.4G), each channel data is transmitted one at a time. It is simple Time Domain Multiplexing (TDM). Since each channel has its own turn to transmit its data there no further requirement to have a channel separation. Note that there is a finite time delay between two consecutive data from the same channel. Between this time delay the rest of the TX channel data are inserted. How the data is represented depends on the modulation scheme (PCM or PPM) and the difference is well pointed out by Sushil.



This is more of a technical stuff. The most widely adopted receiver designs are based on super-heterodyne principle. It means translating the received frequency to an intermediate frequency so that it would be easier for the transmitted signal recovery.

In super-heterodyne principle the more the number of intermediate frequency conversion the better the receiver sensitivity and better the image signal rejection. (Image signal in simple terms is a bogus signal that the receiver sees. Although it is same as the transmitted signal it would be comparatively weaker and you don't want to lock on to that)

Having said that, the answer is pretty obvious - Dual conversion is better than single conversion. If you fly in a confined space like a park go either for dual or single conversion. If you fly large glow/gassers dual conversion is highly recommended.

One might wonder - why not use a triple or quad conversion.. Well, if I remember it right, the benefits that you get from the stages kind of saturates at some point. For most of the general use dual conversion is more than enough. For instance, the old pocket radio (fondly called transistor) most probably is a dual conversion super-het receiver.

-Ismail

Not true. They are single conversion types with an IF of 455kHz. Dual conversion is used in the type of sets used for communication, "Dx ing" and SW listening. Their first IF is 10.7 MHz. One of these is the SONY ICF 8600 which has been in production for more than 20 years with incremental changes.

You may be right. There are more single conversions pocket radios which makes sense as it reduces the BOM cost.

However, I have built pocket radios (from kits) in the past and used 3 stages of IFTs. Each stage tuned to different IF frequencies. The final one is obviously 455KHz the other ones are about 10 - 5ish MHz. Couldn't recollect the figures. The IFT color codes are Yellow, Grey and Green.. and there is another with Red which is the osc coil. So, I am pretty much sure that the ones that I built are dual conversion ones.

I also had a pocket radio from National which uses loads of IF stages.. May be a superior version. Or they have dedicated IF stages for MW, SW etc...

-Ismail

I have also built many radios, including valve sets. ALL were single conversion types. The different colour codes were for impedance matching (e.g. the final one had to feed the detector, and not another stage).

Any set that uses/d dual conversion would need 2 sets of oscillator coils, one for each band and one fixed for the second IF. I can categorically state that the kits were single conversion types.

Thats good. Do you know the frequency range of the IFT color codes? I was trying to get hold of them but could not find it in the net.

Izmile

They are all the same, i.e. 455kHz. It is only the primary/secondary impedance ratio that is different.

Hmm... I see. But for some reason I feel that there is a difference in frequency response. Anyway, sometime later I would check that with a VNA. Thanks for the pointer. Its been quite a long time.. 8-)

http://www.electronics-tutorials.com/filters/if-amplifier-transformers.htm

Maybe of some help.