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In typical FM transmitter topology, the modulating signal directly modulate the VCO and the VCO are locked by a PLL. The DC component is 'absorbed' by the PLL. So it is always an AC-coupling FM. Any body know how to make a DC-coupling FM circuit? Or any chipset to do it?
Due to its low data rate, the signal BW is only around 5 to 10kHz. The sensitivity of a typical pager can comes to -130dBm. Meanwhile, since paging service is normally one-way, the transmitter can be very high in power level. They are typical around 100 to 500W and build on mountain high. So...
Maybe due to the IF filter is integrated and don't need external component. I found the most of Low-IF transceiver don't have external pin for the IF filter output. Maybe some secret test mode can access the output, but generally not mentioned in data sheet. It seems that only indirect method...
2.4ghz balun design
I use FR4 for 2.4GHz in long time. I believe the majority of 2.4GHz consumer product are using FR4 due to cost. Of course you can't expect too much.
Besides the curve at the beginning, the signal itself don't have constant average value. It is not suitable to use this slicer circuit. Imagine after a long '0' or a long '1', the output of the RC low-pass filter will becomes too low or too high to slice the coming signal. e.g...
In a Low-IF transceiver, the RX side IF bandpass filter is usually intergrated into the IC. Does anybody know how to measure the bandwidth of this IF bandpass filter?
vco,pulling
The VCO pulling is due to the poor isolation from VCO tank circuit to its external circuitry(conducted or radiated). If your problem is really due to pulling, you can solve it by improving its isolation. One simple thing you can try is to add a buffer amplifier between the VCO and...
My company uses a GTEM cell from Schaffner. Take a look to see if it is useful to you... http://www.schaffner.com/test_systems/en/product/tsyssecondelevel11.asp?level=3$2$12&language_id=12
To add two signals, you can use "Wilkinson power divider" to do it. But its input becomes output, vice versa. "Microwave Engineering" from Pozar mentioned its theory in detail.
About the harmoinc ?
yep..., the matching network may be changed. May be more components needed. Some calculation/simulation and on-board fine tuning seems to be unavoidable. Just a suggestion.
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