mb1504 control
Thanks strabush for your valuable suggestions.
I have recalculated the loop filter R and C values using the following specs.
Frequency range: 61.310 to 62.290 MHz
Channel spacing: 20 kHz
Maximum frequency change during a step (fstep): 800 kHz
The desired time for the carrier to step to a new frequency (ts): 10msec
The frequency of the carrier, within the desired time ts, after a step (fa): 1 kHz
Damping factor: 0.707
VCO sensitivity (calculated, not measured) (Kvco): 1.126 MHz/V
Charge Pump Current (Icp): 3mA
With these specifications and using the formulae given in fujitsu loop filter calculation document the value of R and C comes out to be:
C24 = 114nF (value used: 100nF)
R18 = 1.2k
C26 = 1.14uF (value used: 1uF tantalum)
C27 = 114nF (value used: 100nF)
R20 = 1.2k
Loop Bandwidth = 517 Hz
Now the output of PLL (input to VCO) is smooth and the receiver has tuned by adjusting L3 but the tuning is highly unstable. There are two problems: First with moving the transmitter away from the receiver the output becomes distorted and it seems that VCO is not tuning. The PPM output is fine by placing the transmitter next to the receiver. Secondly changing the channels by rotary dip switch, the output doesn't change with same transmitter channel (72.190MHz). It is desired that with RX channel change there should be no output until the TX is also changed to the same channel. I have checked the PIC16F84 output (data, clock, LE and LD) and its working perfectly fine by changing dip switch position, since it is in the plugable module form.
Could it be possible that VCO sensitivity is not properly adjusted or changing the values of C14 (68pF) can improve the tuning? L3 inductance is 150nH. Any suggestions...