darcyrandall2004
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uhf transmitter schematic
Hello All,
Out of interest and for the sake of my own learning, I have been trying to design a UHF transmitter.
The good news is, with my latest design I have achieved lock.
The bad news is that although I may have achieved lock, the output frequency is slighlty off the desired frequency and I am unable to view the demodulated intelligence signal on my Marconi test set.
To get to the stage of acheiving lock:
1. I first had to alter the components surrounding the resonator. The VCO would not oscillate at the frequencies I had simulated. I am guessing parasitic capacitances/inductances were responsible for this.
2. I had to remove the RF choke feeding the varactor and replace with a 10kohm resistor.
To help diagnose why the output frequency is slightly offset from the desired:
1. I have confirmed with a logic analyzer that the PLL is being setup with the correct data.
2. I modified the board, removing the 20MHz Fox oscillator and replaced with a 10MHz oscillator from CMAC -> no change.
I witnessed
1. if I program the PLL for an output of 440MHz with a 10MHz ref, ie R=1, FRAC=0, INT=44, MOD=400 then with my Marconi test I have been able to tune the reference osc so that I can achieve roughly 440MHz with an error of 100kHz(my channel spacings are supposed to be 25kHz).
2.If I program the PLL or an output of 450.325MHz with a 10MHz ref, ie R=1, FRAC =13, INT=45, MOD=400 I end up with a signal at roughly 448MHz.
I also think it is worth pointing out that the digital lock detect output is typically a value of 2.5V that occasionaly dips to 0.
Does anyone have any ideas as to why I am not achieving the expected output frequency?
Does anyone have any ideas as to different things a can try to diagnose the problem of both the wrong output frequency and the lack of the moduating intelligence signal?
I am thinking that the reference oscillator is not oscillating at the correct frequency, yet I cant understand why as it was purchased off the shelf.
There is no shielding on my board. Typically, should I be able to tune a UHF transmitter regardless of wether shielding is in place or not?
My schematics are here:
**broken link removed**
I will post a photo of the board I am working with in time. Perhaps the state of it will provide a hint as to why it is not working properly.
Thanks again
_________________
Regards, Darcy Randall, Perth, Western Australia
Hello All,
Out of interest and for the sake of my own learning, I have been trying to design a UHF transmitter.
The good news is, with my latest design I have achieved lock.
The bad news is that although I may have achieved lock, the output frequency is slighlty off the desired frequency and I am unable to view the demodulated intelligence signal on my Marconi test set.
To get to the stage of acheiving lock:
1. I first had to alter the components surrounding the resonator. The VCO would not oscillate at the frequencies I had simulated. I am guessing parasitic capacitances/inductances were responsible for this.
2. I had to remove the RF choke feeding the varactor and replace with a 10kohm resistor.
To help diagnose why the output frequency is slightly offset from the desired:
1. I have confirmed with a logic analyzer that the PLL is being setup with the correct data.
2. I modified the board, removing the 20MHz Fox oscillator and replaced with a 10MHz oscillator from CMAC -> no change.
I witnessed
1. if I program the PLL for an output of 440MHz with a 10MHz ref, ie R=1, FRAC=0, INT=44, MOD=400 then with my Marconi test I have been able to tune the reference osc so that I can achieve roughly 440MHz with an error of 100kHz(my channel spacings are supposed to be 25kHz).
2.If I program the PLL or an output of 450.325MHz with a 10MHz ref, ie R=1, FRAC =13, INT=45, MOD=400 I end up with a signal at roughly 448MHz.
I also think it is worth pointing out that the digital lock detect output is typically a value of 2.5V that occasionaly dips to 0.
Does anyone have any ideas as to why I am not achieving the expected output frequency?
Does anyone have any ideas as to different things a can try to diagnose the problem of both the wrong output frequency and the lack of the moduating intelligence signal?
I am thinking that the reference oscillator is not oscillating at the correct frequency, yet I cant understand why as it was purchased off the shelf.
There is no shielding on my board. Typically, should I be able to tune a UHF transmitter regardless of wether shielding is in place or not?
My schematics are here:
**broken link removed**
I will post a photo of the board I am working with in time. Perhaps the state of it will provide a hint as to why it is not working properly.
Thanks again
_________________
Regards, Darcy Randall, Perth, Western Australia