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Single/Dual RX Crystal

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Old 07-03-2007, 09:03 PM
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pupmeister
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Default Single/Dual RX Crystal

Hi, I have a Futaba Single Conversion RX.

Due to the fact that my LHS did not have any genuine Futaba crystals in stock for my receiver, I put them on order to have them delivered to me when they arrived. I have just received them and while I was inspecting them I noticed that they were dual conversion crystals. I just want to confirm that I can not use dual conversion crystals in the single crystal rx, before I go through the trouble of sending them back to have them replaced.

I understand the difference in technology for the actual receiver between single and dual conversion rx, but I am not to sure what the actual difference in the crystal is, since my (very) basic understanding of radio technology is that all the crystal provided was a reference frequency that the receiver used to decode the signal.

Thanks
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Old 07-03-2007, 09:52 PM
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Default RE: Single/Dual RX Crystal

What model RX do you have?
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Old 07-03-2007, 09:59 PM
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Default RE: Single/Dual RX Crystal

[link=http://www.futaba.com.au/article.asp?contentId=6698]AR136[/link]

I do not think they are available in the US

Cheers
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Old 07-03-2007, 10:27 PM
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Default RE: Single/Dual RX Crystal

the whole single / dual conversion receiver thing is how they go about getting them "narrow" enough to listen to only your frequency.

The oscillator in the receiver is "mixed" with the incoming RF signal. When they mix, the output of the mixer is both the sum of the two frequencies, and the difference between the two. Usually a receiver (not just R/C) will use the difference frequency, since it's the lower of the two.

The whole mixing thing is used to make it easier to build narrow filters. Trying to build a filter that is only 10,000 Hz wide (10kHz) at our typical 72 million hertz (MHz) would be next to impossible to do in a practical manner. So, the "local oscillator" is used to mix the signal down to a lower one.

In a dual conversion receiver, the first mixer usually brings the signal down to a 10.7 MHz IF. 10.7 is used because lots of commercial radios (AM, FM, stereo, etc) use 10.7, and parts are readily available. It's still a little tough to get all of the selectivity, or "narrow band" filtering out of a 10.7 MHz filter, so there's a second mixer that is used to bring the signal down to typically 455 kHz (455,000) - where it's pretty easy to get ceramic filters that are "narrow band".

The single conversion receiver normally skips the first step, and mixes straight down to 455kHz.

OK - how about a little math...

Channel 17 here in the US is 72.130 MHz. If we used a "local oscillator" that is 10.7 MHz away, the LO crystal (channel crystal) would have to be (for example) 72.130 - 10.7, or 61.43 MHz. When these two frequencies mix, the result is both 10.7 MHz, and 122.86 MHz. (difference and sum..) - the 122.86 MHz is pretty easy to filter out, since it's so much higher than what we're looking for.

THe 10.7 is then mixed with either 10.245 or 11.155 to get you the 0.455 MHz (455khz) difference.. hence the "dual conversion" ..

In a single conversion, the 72.130 is mixed with either 72.585 MHz, or 71.675 MHz to create the 0.455 MHz (455 khz) difference. You can see the "channel crystal" is a drastically different frequency than it is in the dual conversion receiver case, hence one won't work in the other, etc.

OK - more theory.. you've heard of "image" response..

Say we have a 72.130 MHz receiver, single conversion, that uses a 72.585 MHz "local oscillator". THere are two frequencies that will mix with the local oscillator to create 0.455 - our desired 72.130, and .455 ABOVE the LO signal, or 73.04 MHz. This other signal is the "image response" frequency of the receiver. The RF filters in the front of the receiver are designed to filter out these image responses. Without the front end filter, the receiver would respond equally well to either of the two frequencies.

In a dual conversion receiver, remember the LO is something like 61.43 MHz - and the image would be 10.7 MHz on the other side of that, or 50.73 MHz. It's easier to filter out a signal that far from our desired channel. That's why dual conversion receivers are used.

That's not to say that a single conversion receiver isn't good enough - there's an awful lot of them out there, that do work quite well. Less parts and less crystals, but it's a little more challenging to the designer to insure that the image response isn't a problem

Whew - probably more than anyone really wanted to know...

Anyone get any good out of this? Hope at least some folks were interested enough to read through to here...
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Old 07-03-2007, 10:40 PM
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Default RE: Single/Dual RX Crystal

Assuming you already have a crystal for the receiver, compare the model number on it to the one the hobby shop got for you. They should be the same.

Edit: the chart on this page: http://www.futaba.com.au/browse.asp?ContainerID=1838 appears to show single and dual conversion FM receiver crystals.
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Old 07-03-2007, 11:18 PM
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Default RE: Single/Dual RX Crystal


ORIGINAL: fizzwater2

the whole single / dual conversion receiver thing is how they go about getting them "narrow" enough to listen to only your frequency.

......

Anyone get any good out of this? Hope at least some folks were interested enough to read through to here...
I have read your response about 5 times now and I am finally starting to understand what you are saying. You are correct in saying that it was more than I originally wanted to know, but it has allowed me to understand the difference in Single and Dual Conversion receivers to a lot greater degree than normal statement that single conversion filters once and dual conversion filters twice; these statements always confused me and I guess I always equated them to the type of multi filtrations done on liquids and gases.

If I wanted to translate your description into something that I could explain to somebody that has not done any courses or studying of radio systems can I summarize it as the following

Dual Conversion - First conversion does a rough conversion out of the signal and then does the fine conversion
Single Conversion - Only does one conversion to get the final signal
And finally it is a lot easier to do the dual conversion system than a single conversion to get a accurate signal from the TX to the servos

ORIGINAL: piper_chuck

Assuming you already have a crystal for the receiver, compare the model number on it to the one the hobby shop got for you. They should be the same.

Edit: the chart on this page: http://www.futaba.com.au/browse.asp?ContainerID=1838 appears to show single and dual conversion FM receiver crystals.
Thanks for the help piper_chuck, I did see this page when I was checking out the crystal that I received, however since at that stage I believed that I understood the difference between single and dual conversion receivers I could not understand the need for different crystals and I thought that maybe the LHS had made the mistake on purpose. Since reading fizzwater2's post I think I can safely say that the LHS made a mistake and that I should send the crystals back and have them replace it with the correct crystals.

Thanks for all the input
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Old 07-04-2007, 01:39 AM
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Default RE: Single/Dual RX Crystal


ORIGINAL: pupmeister Hi, I have a Futaba Single Conversion RX. Due to the fact that my LHS did not have any genuine Futaba crystals in stock for my receiver, I put them on order to have them delivered to me when they arrived. I have just received them and while I was inspecting them I noticed that they were dual conversion crystals. I just want to confirm that I can not use dual conversion crystals in the single crystal rx, before I go through the trouble of sending them back to have them replaced. I understand the difference in technology for the actual receiver between single and dual conversion rx, but I am not to sure what the actual difference in the crystal is, since my (very) basic understanding of radio technology is that all the crystal provided was a reference frequency that the receiver used to decode the signal.
Thanks Pupmeister
Correct, AM/FM single conversion receivers crystals only work in AM & Single converson FM receivers. Dual conversion are for use only in Dual Conversion RX of same brand.
Best 101 explanation, refer to "Receiver - FAQ, guides and aids to best reception" on my web page, e.g. Single Conversion vs Dual Conversion - fact sheet. see also sub section "Crystals, Oscillators & Piezo".
Regards
Alan T.
Alan's Hobby, Model & RC FAQ Web Links
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