Batboard

When viewing the Per Channel options, there is an option named "CHANGE RX INJ" with options of HI and LOW. What does this do? Should I leave it alone?

The version I am using is R03.01.00

Paul

The RSS will default that level as it see fit for the frequency in each mode.

DO NOT touch that setting unless (1) you have a compelling reason and (2) you know the effects of changing it. Bad things happen when the RX injection is wrong.

The normal user will not need to change that level against what the RSS sets it at.

Rx injection refers to the frequency that the 1st local oscillator makes so that, when mixed with the desired receive frequency, the beat produc will be exactly the input frequency of the second mixer.

For instance, say you want to listen to 460.0000 and your 1st IF is 10.7 mHz. The 1st LO will generate 470.7 mHz. Mix 460.0000 and 470.7 mHz and the beat product is 10.7 mHz.

The equation for the beat frequency is |A-B|, where the notation "||" means absolute value. (This is the same thing as saying the difference between the two, regardless of which is larger.)

So, in our example above, instead of using a 1st LO frequency of 470.7 (which is 460.0000 plus 10.7), we could also use 449.3 (which is 460.0000 minus 10.7, if my math is right).

If the LO freq is higher than the desired beat, this is called "high side injection;" if lower, "low side injection."

Now, here's the problem. Let's assume we want to listen to 460.0000 and our receiver uses high side injection. Let's also assume our front end is a bit loose, and also that there is a readable station in the area that is transmitting on 481.4. When some of that 481.4 gets into my 1st mixer and is mixed with 470.7, what is the resulting beat? 10.7. Some some of the modulation on that unwanted channel will get through my receiver. If I can change my LO to low side injection, the LO frequency will not produce a 10.7 beat with the offending interference, and I'm home free.

Some receivers enable injection switching, then, in order to be able to avoid the effect of nearly unwanted signals that just happen to be the right distance away from what I want to listen to (which is 2 times my first IF) to cause interference. Frankly, I've only seen this on service monitors and sophisticated wide band communications receivers, but if you tell me it is an option on the MT1000, so be it.

All of the foregoing is a longwinded way of seconding 007's advice: don't mess with the injection side unless (a) you really have a problem that needs to be solved and (b) you can handle the unintended consequences.

Good info...made me think about LO freq mixing again

The RSS does indeed have that option, but in all the MT1000's I've eitehr owned or programmed, I've only seen the default value changed once for a weird low-power itinerent channel that had 4 places to the right of the decimal point (xxx.4675 or something...)

Thanks for all of the help guys! I recall changing the injection on one portable and after observing no discernable difference, changing it back. I think I will leave it alone now, the radio works just fine.

Paul

This Is a Good Question And We Always Go Back
To The Fact That If You Play Put It On A Service Monitor.