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.

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.