EMRFD Message Archive 2376

Message Date From Subject
2376 2008-11-10 13:25:16 rcbuckiii Oscillator issue
I have built the VCO shown in fig 4.34 of EMRFD. I changed the
frequency of operation to run from 48-60 MHz. I replaced the 2.2K
source resistor of Q1 with a 1 uH choke. And the two 270 pF caps are
now 39 pF. And the varactors are different to handle the frequency that
I am convering.

The problem I have is the oscillator will not run with a J310 at the Q1
position. I know the J310 is good as I have a package of ten new ones.
I have tried several. If I replace the J310 with an MPF102 the
oscillator immediately works and covers the range I need it to.

I don't have a problem using the MPF102 other than apparently there is
no SMD version available. I looked at the data sheets and the J310 and
MPF102 are fairly close in specifications so I don't understand why it
won't work. I tried changing the 1 uH choke to a 2.2K resistor as a
test but the circuit still does not oscillate.

Does anyone have any suggestions as to why the J310 doesn't work?

2377 2008-11-10 14:54:02 Allison Parent Re: Oscillator issue
Two things come to mind..

one is the 78L05 may be going into over current shutdown due to the
J310 bias conditions.

two, the bias for the MPF102 (and it's Idss) is very much differnt.
Try adding 50-200 ophms in series with the 1uh choke and possibly
also a gate bias diode (1n4148 get to ground).

Idss (at Vgs=0) for typical MPF102 is in the 5-14mA range and the
J310 is in the 15-30mA range. I've never had a case there J310 didn't
work where MPF102 also worked though I've had cases where the 310 was
much better.

If you read the text around there they get into a case where power
and bias interacted and the oscillator would not work..


2379 2008-11-10 19:27:35 rcbuckiii Re: Oscillator issue

Thanks for the input. No, the 78L05 isn't going into shutdown. There
is 5.02 volts on the drain of Q1. I do have a 1N4148 on the gate of
Q1. Sorry, but I forgot to metion that. Other than that, the circuit
is identical to Fig 4.34. Apparently the circuit in the book will
work at 7 MHz but not at 50 MHz. It could be as you say, the biasing.
I was trying to design a circuit that could be duplicated without
having to tinker with it to make it work. It appears the MPF102 is
probably better in that regard. I guess this may be why all the ARRL
publications for the last 20 years have used the MPF102.

Also, if the J310 requires more current to run, wouldn't that mean it
was more prone to drift? This is not really a concern since the VCO
is being dropped into a PLL circuit. But, for a stand alone analog
VFO I could see that as being a problem.


--- In emrfd@yahoogroups.com, "Allis
2398 2008-11-12 13:09:57 timshoppa Re: Oscillator issue
2399 2008-11-13 08:11:03 rcbuckiii Re: Oscillator issue

Thanks for the tips. I'll play with it more when I have time. For now
the MPF102 is working well so I'll go with it. I like the idea of
being able to drive a 50 ohm load with the oscillator so I'll
experiment with that setup.