JR receiver R790

IIRC, All JR receivers except one are single conversion receivers. This means they have as their only intermediate frequency 455 KHz. A single conversion receiver will receive any transmitter that is transmitting on a frequency that is 2 X the IF frequency from their actual frequency. This means that channels 57 through 60 are the image frequencies of channels 11 through 14. If they don't use 11 through 14 then the image frequency is eliminated for that receiver. Basically if you were using 14 and another flyer turned on on channel 60, you may have a problem.

Any single conversion receiver has this possibility. For example, the small park flyer receivers will also have this problem.

Yup, you did recall correctly, and thanks! The Berg Rx's are a favorite for me, and while I don't use the channels where image freq is a potential problem, I don't know what channels other guys in my club that I have recommended this Rx to (the 4L, don't have the 7 Ch yet) are using. I'll pass the caution I read into your post along.

Abel

That is correct fellow , but why that the others single conversion JR receivers, like the R600 that are not synthesized works fine in the

Maybe because JR just recently got smart about the issue and are looking out for their customers?

Abel

Hummm.....seems like you already knew the answer. I answered the question you asked and now there is a follow up question. I didn't say the other receivers wouldn't work on the low channels. It is just they will have a possible image problem if some one turns on your image channel. Are you sure JR supplied the 11 to 14 crystals?

Seems to me you need to address your questions to JR.

That is correct fellow, Horizon Hobby ansewered my question:" JR does not use channels 11-14. These channels are close to mirror image frequency's of some of the higher channels. This is the reason JR has decided not to use these channels. " Thank's a lot. Best regards

Mano

You are sort of right - but the selectivity of the RF stage in the receiver is a major factor in how well the 'mirror' frequencies are rejected

David

IFLYJ3 wrote:

Frankly though, its bugger all over the band with a SC.

However the SC does not pick up +- 2xIF Its either plus, OR minus. depending on whether the LO is above or below the incoming.

That means that in practice only SOME channels will be interfered with by SOME others, and many manufacturers simply don't sell sets or crystals in those channels that are liable to interference. As the original guy said.

In fact the largest amounts of shoot downs are on neighboring channels due to poor FINAL IF selectivity. You really need a couple of 6 pole filters to be safe. Many cheap sets only have one 4 pole..

I'm wondering what selectivity of the front end has to do with anything when the image frequency is right on the frequency the Rx is tuned to. If the RF stage could practically be made selective enough, there would be no need for down conversion to IF, whether once in the case of SC, or twice for DC. Just go directly from RF to baseband - and good luck on selling your receivers.

Abel

This is true and it allows a simple audio low pass filter to shape the band.

The biggest problem is that the LO itself tends to swamp the receiver, you have very little chance of any pre mixer amplification.

So you have to start with a mixer, and do all the amplification at baseband.

And doing all that amplification at one frequency on a small board is a recipe for instability.

Its possible, but its tricky.

How things are done is a reflection of the need to produce fairly simple designs of adequate performance cheaply. Its always possible to do better..but not easily or cheaply.

It's rather simpler than that from a practical standpoint. In the US, the Rx front end has to pass everything from 72.01 to 72.99 Mhz plus or minus IF offset(s) to accomodate the range of xtals that might be plugged in to clock the LO(s). A MHz+ bw ain't all that selective.

Abel

Geez I must be getting old - my comment does apply to radio sets that have a tunable front end, but of course for RC receivers that aint the case as the front end is designed to accept all the frequencies in the band it is designed for.

Oh, well, back to the nursing home, if I can find my way :-)

David

Abel Pranger wrote:

I hear you, David. My bread-and-butter thing has taken me from ELF to EHF comms (and well above that to the optical part of the spectrum). Sometimes you just have to ask yourself "where am I at now."

Abel