EMRFD Message Archive 8067
Message Date From Subject 8067 2013-01-07 18:45:06 RichardV NE602 Failure I am stumped.
I have build the DC receiver on pages 1.8-1.9 in EMRFD and loved it. Just fun. I decided to play around with an NE602 and get 'down-to-basics'.
I found a great Primer on the NE602 available here:
Inside I found a great schematic for a very simple VFO and I thought I would give it a try. I am using the "E" Schematic here:
In the Primer Article (pdf) there are a few formulas for calculating the values of C1 and C2. C1=100/SqRoot(F) and C2=1000/F. I have tried to apply those values and a 10nf cap, 1.16uH coil and 470pf cap.
I can't not get the circuit to oscillate.. at least in any frequency my general coverage receiver can pick tune.
What I do notice is that the values for my C1 and C2 work out to 38pf and 142pf for the voltage divider, however the EMRFD DC circuit has 680pf caps as the divider for a similar circuit. Why are mine so different. Is there some other formula I missed? I guess I am trying to validate if the formula I used is correct or just for crystals and note tuned LC type circuits.
Any help would be appreciated.
8070 2013-01-07 20:26:19 Ashhar Farhan Re: NE602 Failure rich,
how are you detecting the oscillations? i am not sure that NE602 has an
oscillator output. your method should be such that it lightly loads the
oscillator, the oscillations can very well stop, if, for example, you
connect a 50 ohms power meter across the tank.
at hf frequencies, the ratio is non-critical with the range of what EMRFD
describes and what you have calculated. i would suggest the following:
1. using the datasheet, find out the voltages expected and check that the
DC voltages are fine.
2. double check that the capacitors are not swapped around.
3. check that the voltages on the IC pins are the same as those on the
capacitor end (to rule out bad solder)
4. if you have an oscilloscope, use a 10x probe and check for the
oscillations at the emitter of the internal resistor (pin 7).
i would suspect that the problem is in an open/shorted connection.
a way to ensure a nice and clean performance of the NE602 is to try a
vacker variation. this ensures much higher voltage in the tank than the
NE602 is capable on it's own. it also pushes the oscillator into limiting
8072 2013-01-07 23:03:59 victor Re: NE602 Failure I'm not sure which is your 10nF capacitor C1 or C2.
Anyway, the ratio of these capacitors control the level of the positive feedback. If the feedback level is too low (C2>>C1) then the oscillator will not oscillate. If the feedback level is high (C1>C2) then the oscillator might develop parasitic oscillations. There is a wide range of C1/C2 ratio that can be used and the oscillator will work properly. I suggest you use C2 = 2 *C1 which means a 470pF for C1 and a 1nF (not 10nF) for C2. Use NPO (COG) ceramic or mica capacitors. You will have to bring the oscillator frequency to the wanted one using the capacitor across your inductor.
Victor - 4Z4ME.
8073 2013-01-08 07:25:20 RichardV Re: NE602 Failure I use an HF receiver (Yaesu FT-817). I utilize a a piece of wire that goes from the antenna port to a couple of loops around the board. You can easily hear oscillator when it's working. I'm not attaching anything to the NE602 that would load it to stop/offset the oscillator.
8074 2013-01-08 07:31:54 RichardV Re: NE602 Failure The 10nf capacitor is not used as C1 or C2. It's used as a DC block to keep PIN6 from grounding through the inductor on the tuned circuit.
Thanks for the insight
8075 2013-01-08 07:45:57 bobtbobbo Re: NE602 Failure Try this Rich: Add a permanent oscillator test point. Connect a 10 pf cap from 602 pin 7 to the gate of a J310 fet. Put a 100K from gate to ground. Put a 1K from the source to ground and a .01 cap out from the source to a test point. Put a .1ufd cap from the drain to ground and apply +10 volts to the drain. Put a 10X probe between the test point and ground and connect to a frequency counter. You should leave this in the circuit permanently for any further frequency checks you might want to make. It will not affect performance.
8076 2013-01-08 08:42:10 RichardV Re: NE602 Failure Thanks... I was wondering how to add a buffer circuit so I could check the oscillator without loading it.
8077 2013-01-08 11:11:15 victor Re: NE602 Failure One more try: it seems to me that the values that you probably use as c1 and C2 (39pF and 150pF) are too small in relati 8078 2013-01-08 12:08:13 RichardV Re: NE602 Failure T37-6 about 20t of 26g. 1.18uH or so on my L meter.
From your explanation below I think I am starting to get it. Too little loop gain makes oscillation no-go! I'll try your values and report back.
8079 2013-01-08 17:11:16 RichardV Re: NE602 Failure Victor,
Success! Using your values it oscillates just perfect!