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ism transceiver with better TX channel filtering?

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And the jammer suppression was a little better too, but oddly it could handle a jammer very well on the high side, but did very poorly on the low side.
It's due to the IF filter performance. You should improve the IF filter.
I wonder if I replaced the cheap onboard xtal reference oscillator with one with good phase noise, if my "jammer suppression" would improve?
That's no use.
 

Good points. What they DO pack into these tiny chips is truly amazing. I just wish someone would dedicate two pins on the package to IF out and IF in, so that one could add a 1 MHz filter with steep roll off, to augment or replace the internal dsp filtering.

I had not thought about the phase noise of the LO--Normally 2FSK systems can tolerate a boat load of phase noise.....I wonder if I replaced the cheap onboard xtal reference oscillator with one with good phase noise, if my "jammer suppression" would improve?

Most of the new chips are direct conversion or at least very low I.F., like 500-700 kHz to put one of the images at zero frequency with I/Q output channels. The direct conversion receiver are biCMOS where BJT's can be used. The very low IF are CMOS where the very low IF is used to improve performance that would suffer due to poorer flicker noise of CMOS mixers over BJT designs.

I believe the original question is about out of modulation bandwidth noise reduction where transmitter broadband noise floor desenses another receiver at a close by channel. To achieve the GSM spec for +/-400 kHz offset, the transmitter PA input exciter noise floor must be better then -165 dbC/Hz. Achieving this level without RF filtering (which cost money and real estate on PCB) is challenging and requires very low noise synthesizer design. The purpose of this spec is to ensure a cellphone near a base station cell site does not desense the basestation receivers on other nearby channels. There is also specs for mobile Rx band to avoid desensing a cellphone near you but this is easier spec due to the greater frequency separation between Rx and Tx of phone.
 

Good points. What they DO pack into these tiny chips is truly amazing. I just wish someone would dedicate two pins on the package to IF out and IF in, so that one could add a 1 MHz filter with steep roll off, to augment or replace the internal dsp filtering.

I had not thought about the phase noise of the LO--Normally 2FSK systems can tolerate a boat load of phase noise.....I wonder if I replaced the cheap onboard xtal reference oscillator with one with good phase noise, if my "jammer suppression" would improve?

Biff

to add an external filter they (or we I should say) will need to add 8 pin (usually the IF section is differential with I/Q signals) with a big risk to add imbalance due to external filtering. Not easy way.

About using a good reference oscillator, I don't think so, except if is not forecasted by design of the IC. Usually the phase noise limitation in these sigma-delta PLLs comes from PFD & charge pump in the 10kHz-300kHz offset range, only below 10kHz the XTAL is dominating and above 300kHz the VCO is the dominating one.

RCinFLA said: "The best filterless design produces -165 dbC/Hz at 400 kHz offset". Are you able to give me the P/N?

Mazz
 

Well, yes, I was not saying send out the digitized IF signal....that would not be that useful.

I was thinking more like they could send out an analog low IF signal, where it could be filtered, and then return it to the chip for digitization. Something like the (fixed frequency) Melaxis 71112 receiver.

THEN we would have the benefits of a modern receiver design, but with the flexibility to use in in demanding applications.
 

Biff

I was referring to analog IF, indeed. As the IF signal is differential it would need 2 output and 2 input pins. And for low IF there are in phase and quadrature signals, both differential, so the pin count goes to 8.

The Melexis architecture is an old (but still valid) superhet with (a lot of) external filtering. New design, for cost and space reasons, goes to direct conversion (low IF or zero IF) schemes.

Mazz
 

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