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singal or dual conversion

Old 04-03-2003, 04:43 PM
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it is because the image (of a possibly interfering rc channel which is about 2x455 KHz away) falls in between two channels and is therefore, 10 KHz removed from the desired rc channel and is therefore, filtered out --- note i am talking about rc channel interference only, it being the most likely source...
peter berg
Old 04-03-2003, 04:55 PM
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Default singal or dual conversion

Phil, I'm splitting hairs here, but fwiw it's Futaba that's on the "wrong" shift. Everywhere else in the world (sans 72/75MHz), +ve shift is the "standard", Futaba included.

DavidO, thank you for the shift converter link!!
Old 04-03-2003, 08:48 PM
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Default singal or dual conversion

Peter Berg:

I suspected the narrow IF was how you avoided the image problem. I didn't think suitable filters would be available (adequate passband with steep skirts, and small and inexpensive). Berg 5s are pretty well priced, so you don't have much room for extravagent components. However, it seems the filters are available.

Peter Kohr:

Isn't that what I said?: you set the JR to the "wrong" shift so it will work with a Futaba Tx. I did mean "wrong" from the JR point of view rather from the "RC Standards" point of view.

It would be nice if the industry go together and set some standards. In my working world, most of the standards are set by bodies composed of manufacturers and customers, e.g. ATM Forum, DSL Forum, IETF. I'll admit the IETF is not a particularly good example, since a lot of their output coincides with what's convenient for one particular network equipment manufacturer.
Old 04-03-2003, 09:16 PM
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Default singal or dual conversion

Potifar quote:
"it is because the image (of a possibly interfering rc channel which is about 2x455 KHz away) falls in between two channels and is therefore, 10 KHz removed from the desired rc channel and is therefore, filtered out --- note i am talking about rc channel interference only, it being the most likely source... "


In our flying area a very likely source is an industrial user on one of the frequencies 10 kHZ offset from our own.
We have a lot of trouble with the ABC&W SC Jrs .

Is the Berg RX available on 50 mHZ ? If not ,is it being considered ? Are schematics available to Berg customers ?
I am interested .
Old 04-03-2003, 09:44 PM
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Default singal or dual conversion

LF, the Berg 5 is available on 50 MHz, despite what it says on the RC-Direct website. I have a couple.

I can't say how they work yet, since the models they're going into aren't complete.

I also have a Berg 6 (no longer available) and that seems to work pretty well. It's in a micro helicopter, so extreme range is not needed, but it puts up with a gyro and ESC mounted right next to it, and flying in a gym around the mercury vapour lamps.
Old 04-04-2003, 12:15 AM
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Default IF filter

Originally posted by Phil Cole
Peter Berg:

I suspected the narrow IF was how you avoided the image problem. I didn't think suitable filters would be available (adequate passband with steep skirts, and small and inexpensive). Berg 5s are pretty well priced, so you don't have much room for extravagent components. However, it seems the filters are available.

Peter Kohr:

Isn't that what I said?: you set the JR to the "wrong" shift so it will work with a Futaba Tx. I did mean "wrong" from the JR point of view rather from the "RC Standards" point of view.

It would be nice if the industry go together and set some standards. In my working world, most of the standards are set by bodies composed of manufacturers and customers, e.g. ATM Forum, DSL Forum, IETF. I'll admit the IETF is not a particularly good example, since a lot of their output coincides with what's convenient for one particular network equipment manufacturer.
Please explain how an IF filter can remove the IF produced by the image frequency when it is exactly the same as the IF produced by the transmitted frequency
Old 04-04-2003, 01:41 AM
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Default Re: IF filter

Originally posted by dirtybird
Please explain how an IF filter can remove the IF produced by the image frequency when it is exactly the same as the IF produced by the transmitted frequency
This has already been explained, but I'll try again:

If the image is due to another RC transmitter, the IF of that image is 10 kHz away from the IF produced by the wanted signal.

E.g. Your single conversion receiver is on Ch 11. The guy next to you on the flight line is on Ch 56 or Ch 57

Ch11 = 72.01 MHz.
LO = 72.01 + .455 = 72.465 MHz
Image Freq = 7.2.01 + 0.91 = 72.92 MHz

Ch 56 = 72.91 MHz
IF = 72.91 - 72.465 = 0.445 MHz

Ch 57 = 72.93 MHz
IF = 72.93 - 72.465 = 0.465 MHz


LF already pointed out that this is no good if the image is from one of the services interleaved between the RC channels, or something else in the 72 to 73 MHz range. Once you're well into the 72 to 73 MHz range, the front end can give you some image attenuation.

It's still a reasonable bet since most of the time, the image source will be another RC transmitter.
Old 04-04-2003, 02:18 AM
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Default image frequency

You are talking about frequencies close to the image frequency- not the image frequency. The image frequency cannot be filtered out in the IF stages. It must be rejected by the RF stage. That is the reason for double conversion.
As I said before, it would be smart to keep the image frequency out of the band.
Old 04-04-2003, 02:35 AM
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Default Re: image frequency

Originally posted by dirtybird
You are talking about frequencies close to the image frequency- not the image frequency. The image frequency cannot be filtered out in the IF stages. It must be rejected by the RF stage. That is the reason for double conversion.
As I said before, it would be smart to keep the image frequency out of the band.
Isn't that what he said?

"it is because the image (of a possibly interfering rc channel which is about 2x455 KHz away) falls in between two channels "
Old 04-04-2003, 03:30 AM
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Default singal or dual conversion

Phil,

I stand corrected ... went and re-read your post for the n-th time, then got what you were saying the 1st time around Guess was my knee-jerk reaction, JR+"Wrong Shift" my bad.

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