Understanding Negative and Positive Shift Crystals

Most people do not understand why there is a need for positive and negative shift crystals.  I have had many people tell me that all crystals should be the same as they are exact on the frequency and crystals do not have a polarity.  Well, this is true, but as Paul Harvey would say," and now for the rest of the story." 

First, crystals do not affect the "Shift" of a transmitter or receiver.  The "Shift" of a transmitter is determined by the digital signal sent by the TRANSMITTER!  If you were to take the "digital signal" from a modern transmitter and place it on an oscilloscope, it would be represented by spikes or "pulses" that moved off of the centerline of the scope.  In a Futaba transmitter, for example, the "shift" is to the negative side of the centerline of the signal.  On a JR transmitter, the "shift" is to the positive side of the centerline of the signal.  So a receiver that is set to receive negative "pulses" from a Futaba transmitter just does not know what to do with a positive pulse from a JR transmitter, so it usually does nothing.  But when the receiver that is set to negative shift sees a negative pulse from a Futaba transmitter, it turns that pulse into a signal that is used by the servo to move the servo arm.  The converse of all this is true for a positive shift transmitter like the JR or Airtronics. 

Second, it is true that crystals are exactly on the frequency they are designed for and it is true that crystals have no polarity.  What is not known by most of the modeling public is that your receiver crystal is not the frequency that is marked on the can or tape.  This is because the receivers utilize a device called a crystal filter (or similar circuit) to help with the selectivity and stability of the receiver frequency.  For example, in most single conversion receivers the crystal filter is 455 Kilocycles.  The manufacturer knows that this device, while helping to block out unwanted radio frequencies, will either add to or subtract from the circuit frequency ( I know that you technical guys will have a lot to say about this, but I am trying to keep it simple.)  So if you have a negative shift crystal, say on channel 44 (which is 72.670 Mhz) the crystal is actually 72.215 Mhz.  It requires the addition of the 455 Kc from the crystal filter to bring it to the same frequency as the transmitter (72.215 + .455 = 72.670.)

Positive shift crystals go the other way.  Our same channel 44 would offset to the positive side so the 72.670 Mhz would actually be   73.125.  In this case the crystal filter is used to subtract 455 Kc from the receiver crystal to bring it inline with the transmitter frequency (73.125 - .455 = 72.670). 

From these two simplified examples you can see why a negative and positive shift crystals are not interchangeable.  Dual conversion (or double tuned) receivers can actually have crystals that are around half the value for a single conversion crystal, which is why a dual conversion receiver crystal on channel 44 is not interchangeable with a single conversion crystal on channel 44.

This also explains why channel 44 crystal from one brand may not work with a channel 44 receiver from another brand.  Often the manufacturers, for design and competitive purposes, will use offsets that are different than norm.  One manufacturer, who shall go unnamed here, even uses different offsets in the same line of receivers, depending apparently on the price of components at the time of manufacture.  A crystal from one receiver may not work on a like  receiver from this manufacturer for this reason. 

So there you have it.  As Paul Harvey would say, "now you know the rest of the story."

There is a similar affair going on in the transmitters, so two crystals that are marked for the same frequency and channel from two different manufacturers may be very different in their actual pre offset frequency.  This is why a transmitter crystal from one manufacturer may or may not work in another manufactures transmitter.  The only rational approach is to use crystals that are designated for use with equipment that you intend to use it with.

Feigao single conversion receivers use a 455 KHz crystal filter negatively so the crystal that is placed in the receiver must have a positive 455 KHz offset.  The receiver does not care if the transmitter is of positive pulse shift or negative pulse shift.  The shift select switch in the receiver allows for the proper selection of pulse shift.  THE RECEIVER CRYSTAL IS THE SAME REGARDLESS OF THE MAKE OF THE TRANSMITTER!  The transmitter crystal that is shipped with a Feigao receiver crystal is specific to either a Futaba Transmitter or A JR Transmitter, hence the Negative designation for the Feigao Futaba Compatible Crystal Set (negative transmitter digital pulse shift) and Positive designation for the Feigao JR Compatible Crystal Set (positive transmitter digital pulse shift.)  One last time, the receiver crystal is the same regardless of wheather or not you have a Futaba, JR, Hitec, GWS, Airtronics or some other make of transmitter.  The Feigao Futaba Compatible Transmitter Crystal is not compatible with the Hitec or GWS-F transmitter even though both the Hitec and GWS-F transmitters are a negative digital pulse Design. The Feigao Futaba Compatible Receiver Crystal works perfectly with both of those designs.   The Feigao JR Compatible Transmitter Crystal is not compatible with the Airtronics or the GWS-J transmitters even though both the Airtronics and the GWS-J transmitters are a positive digital pulse.  Again, the Feigao JR Compatible Receiver Crystal works perfectly with both of these designs.  Finally, for the Feigao receiver, the receiver crystal is exactly the same in both Futaba or JR compatible sets.

No PCM only type transmitter will work with the Feigao receiver, as it is only a PPM type (FM).  PCM receivers that can be set to PPM mode will work with the Feigao Receiver in the PPM mode.

Well, I hope that I have not muddied the waters to much.  Hopefully I have helped you understand a little bit about Transmitter Shift, and about crystal shift.  I also hope that I have cleared up what you can expect from one of our crystal sets.  If you still have questions about which crystal to select, send me an email with your phone number and when you will be available and we will happily give you a call to try to "clarify" our explanations.

Now for you technical guys who don't like my explanation, if you wish to send me a better and more understandable explanation, I would be happy to incorporate it or use it in its' entirety.  I respect our technical guys, but you would be surprised at just how much email an explanation like this can generate.  Just send it to our email site at Rinconflyer@RCS-Sports.com .

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