joelacrane

Awhile back i broke my daul conversion airtronics 6 channel reciver in a crash so just bought a 4 channel 72mhz micro receiver from airtronics and i want to use the crystal out of the broken receiver in the new one, but on the side of the new on it says "WARNING! Use only AIRTRONICS Single Conversion Crystals". Can i use the crystal out of my Daul conversion receiver in the single conversion rx?

hilleyja

My Feedback: (3)

NO!!!!

You have to follow the instructions.

Steve Lewin

It won't work. Single conversion and dual conversion crystals actually work internally at completely different frequencies.

BTW the crystal is usually the first thing to break in a crash anyway so that's yet another reason why it's not worth trying.

Steve

joelacrane

So do i just have to buy 1 dual conversion crystal for my tx, and one single conversion fo my rx?

hilleyja

My Feedback: (3)

No.

1) You don't need to and should not change your crystal in your TX.

I'm not an expert here but here is my understanding about the difference between single conversion and dual conversion:

Each component in the circuit (resister, inductor, capacitor, crystal, etc) plays a part in establishing the center frequency of the signal the RX is sensitive to.

In a single conversion RX, a single circuit combination with crystal will react to a certain frequency generated by a TX.

In a dual conversion RX there are basically 2 separate circuit combinations, the crystal reacts to 2 separate frequencies interpreted by the circuits. These two circuits are then compared with each other to arrive at a concensus of the center frequency the RX is sensitive to.

I'm sure my explanation above would not satisfy my electronics instructor but it has been 30 years since I took basic electronics.

In addition to a center frequency the RX must be sensitive to the same type of modulation. There are currently 3 different modulations being used in most RC radios: AM (amplitude modulation -- information is interpreted from the signal by measureing the change in the amplitude of the signal.) FM(frequency modulation -- information is interpreted by minute changes in the frequency of the signal around the center frequency) and PCM(pulse code modulation -- information is interpretted by measuring the width of signal pulses)

Hope this helps with your understanding -- maybe someone more current can provide a better layman's explanation.

In summary, your RX only needs to match you TX by frequency (channel) and modulation. Crystals for RX must be matched to whether they are for single conversion or dual conversion. You must also use a specific manufacturer's crystal. Only use Futaba crystals for Futaba RXs and only use Hitec crystals for Hitec RXs.

jgiles-RCU

My Feedback: (1)

I'm looking at a similar situation with dual conversion crystals. However, my question is: will any dual conversion crystal work? Specifically, can I put a HiTec crystal in a Futaba receiver? Why does Futaba list a separate dc crystal part # for every receiver type they sell?

joelacrane

Ok, thanks a lot! Now, does anyone have a single conversion Rx crystal (Channel 15)?

http://www.rcuniverse.com/showthread...09&forumid=250]

HarryC

My Feedback: (1)

In dual conversion they don't work like that, there is no concensus comparison. An Rx generates its own internal frequency, controlled by the crystal. It's a little bit different to the incoming frequency from the Tx. The two waves are mixed together, producing a third wave which is much lower frequency, being the difference between the Tx and Rx's own frequency. It's the same effect that makes the wah wah noise of two engines on a twin that are not quite at the same revs. That wah wah, called the Intermediate Frequency or IF is typically 455khz in radios, compared to the many Mhz of the transmitted wave, but it still contains all the data. That low frequency is much easier to amplify then filter off than the high Tx frequency. That mixing of the two frequencies, one from the Tx and one from the Rx, is the single conversion.

The snag is, an incoming frequency of 455kHz above, and 455kHz below the Rx's own internal frequency will both get through. One of them is your Tx, and one is an image frequency from some other transmitter that is 0.9Mhz away from your Tx. Also there can be images from half and whole multiples of your frequency. your Rx sees them as being perfectly valid IFs. Dual conversion stops this by doing the mixing process twice. The Rx generates two internal frequencies. The first is mixed with the incoming frequency to make an IF usually of 10.7Mhz, which is then mixed with the second internal frequency to make an IF of 455kHz. So it does nothing to block interference on your frequency, what it does is block images caused inside the Rx by frequencies quite well removed from your own.

Your radio has only 2 modulations. AM or FM. PCM is not a radio modulation, it is a method of data encoding and is nothing to do with radio per se. We use 2 methods of data encoding, PPM or PCM. We could send these down a wire, or switch a light beam on and off etc. We choose to send by radio, so the PPM or PCM data is impressed onto a radio signal by means of AM or FM. Your radio is one of: PPM AM; PPM FM; PCM FM. You could make a PCM AM if you really wanted to but no-one bothers. In American modelling it is very common to talk about FM when you really mean PPM which is why you often run into confusion. Your PCM radios are FM. You can not choose between PCM and FM because they are different things. You can choose between PPM or PCM, and between AM or FM.

PCM is not interpreted by measuring the width of the pulses. PPM uses the analogue time length, often referred to as the width, of the pulses. PCM is binary encoding and is interpreted simply as "is it on or off?" as the data is sent as a binary number in a series of on-offs.

Harry

KLRico

Well explained Harry!

Rodney

Typically dual conversion receivers use two internal frequencies. The first crystal (the one you instal) is 11.7 MHZ different from the transmitted frequency. There is a second crystal (or frequency generator) that then mixes it'sfrequency with the 11.7MHz to come up with 455 KHz. The 455 KC signal is then demodulated to get the servo information whether it PPM or PCM. The same transmitter can be used for either dual conversion or single conversion--the dual part is in the reciever only. The prime advantage of dual conversion is the it eliminates any third order harmonic distortion that can occur with single conversion.

MO_Radio_Tech

Good PPM/PCM/FM/AM discussion.

To answer another question, don't mix crystal brands. Sometimes you can, sometimes you can't. Different manufacturers have different ways of arriving at the frequency that we would recognize as the channel. As you can see from the discussion, the Heterodyne and Superheterodyne receiver designs are kinda complicated.

Just pay the $15 from the receiver manufacturer and get the right crystal for you receiver.