Hi,
I am doing analysis of Transystem inc. Downconverter model AiDC+1746D with RF=2216-2300MHz to IF=470-554MHz. The issue that I am unable to find frequency synthesizer and mixer, only 6.8MHz logic oscillator and component marked 1507 that seems to be NJG1507R SPDT switch, while LO should be 1746MHz to get required IF. The other side of PCB is empty only XTAL soldered.
I can't understand how downconversion done so precise and temperature stable that this is suitable and used for digital TV reception. Please find the photo attached.
From my point of view having precise LO is a must for telecomunications, but it seems that here some solutions were used to reduce price. Can anybody help to explain how is that possible?
Thank you in advance
Regards,
Audrius Merfeldas
Thanks Alan, but it looks like I have found that there is transistor oscillator at the top left corner, temperature stabilized with diode close by. The issue how is it possible to ensure frequency stability (this downconverter is used for microwave tv (2.2GHz) downconversion to 470MHz (DVB-T). I believe that for DVB-T with OFDM frequency is a key issue. What do you think?
I would then look for any possibility for a PLL or a FLL (frequency lock loop) involved. I free running LO at at ~ 1700 MHz is possible depending on the demodulation bandwidth involved and any over laying correction routine that might be applied. Q2 has very little RF passives wrapped around it and the device at the RF input appears to be the preamp. Where is the LO active device... upper left corner?
Yes LO made from single transistor (marked in photo) and active bias as I assume from the diode next to it. But I still can't understand what NXP74HC86D with 6.82MHz oscillator do. As I checked it is generating 6.82MHz and 13.64MHz square waveforms, that are somehow connected to RF switch (not sure but marked "1507").Maybe this is somehow working as pll?
Yes sir... The 1507 is a PLL, see attached. Please find MB1507 by Fujitsu.
Ok, thank you very much, thats explains everything. I was confused by misidentified this component. Thanks a lot for your help.
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