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Author Topic: BFY90 VHF VCO  (Read 9870 times)

Posts: 2

« on: November 27, 2005, 02:46:44 PM »

Hi All,

I built this VCO circuit to test my PLL pcb.
The PLL software is working fine, i tested it with another VCO and i could get that very stable.
But somehow the transistor in that first VCO fried after several hours of operation. So i searched for another circuit and i found n built the one i posted above. First i put in a BC547 (don't laugh) cause i was running out of bfy90's hehehe. It oscillated nicely till about 90Mhz. Mind u the PLL could get it stable and locked from 70 to 90 Mhz.
I need 125-136 Mhz or something in that area, so i replaced the BC547 with the last bfy90. Also had to replace the coil with another one to boost up da frequency. Now i can get it to oscillate from 117 till 140 Mhz. With all values like in the schematic i posted. Only the variable capacitor i m not sure of the value, i put in one of them yellow ones. I m not sure i thought it was up to 55 pF Huh  But now with the BFY and the higher frequency i can't get the PLL to lock anymore. its always wobbling around the desired frequency. I tried adding capacitors at the input (before the 27K to the ground) , i tried .047 uF , 1 uF and 1,5 uF. Doesn't seem to help. How can i get it stable? or maybe can I somehow reduce the impact of the varicap a bit?? I m not really sure what to try more... As the BC547 locked fine up to 90 Mhz. Above 90 Mhz it stopped oscillating. Anyone , anything Huh

Posts: 17352

« Reply #1 on: November 28, 2005, 01:08:20 PM »

Some suggestions:

First, make sure that the ground lead of the TO-72 case
of the BFY is grounded.  

Make sure that the sense of the PLL output is correct -
that is, when the PLL senses the frequency is too low, it
needs to increase the output drive voltage.  (Also make
sure that there is DC continuity through the coil for
the diode bias voltage - might be better to reverse the
varactor and C1 so the DC return doesn't go through the

Measure the voltage out of the VCO - that will help to
tell what is happening.  It may be that the PLL is driving
the voltage out of the range of the varactor - it should
have at least 0.5V across it to keep it from conducting,
and I prefer to allow 1V in most circuits.  This decreases
the available capacitance range, but helps to keep the
varactor from becoming forward biases by the RF in the
tank circuit.  (Using two diodes in series, with the
tuning voltage applied to the junction, would be better.)

Make sure that the first stages of your PLL (divider
and counters) can operate up to 150 MHz.  You can test
the general operation by disconnecting the output of
the PLL from the varactor and using a potiometer to
vary the tuning voltage.  Monitor the PLL output with
a voltmeter to see if it responds as expected.

I presume you have some sort of buffer stages between
the output of the VFO and the inputs to the counter/rest
of the receiver.  If the load impedance here is too low,
you may have problems getting reliable oscillation.

Do remember that the BFY30 is only rated for 15 volts
and 25mA.  With +10V on the circuit, you might run
into problems if the RF voltage exceeds 5V peak.

Good luck! - Dale WB6BYU

Posts: 2

« Reply #2 on: November 28, 2005, 01:15:44 PM »

Thanks for the reply.
I replaced the 1 nF capacitor that was in series with the varicap with another bb405 varicap, to reduce the influence on the tuned circuit. Now i can get it to lock again. About the PLL i m slightly outta spec Wink) but i ve seen many designs that go up to 150 with that same old SAA1057. Anyway replacing that 1nF with another BB405 solved it. I get the VCO to lock n stay locked between 122.300 and 135.300Mhz
Next is to build a double balanced mixer to hook it to.....
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