Voltage controlled oscillator - where is this hideous harmonic coming from?

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
The attached voltage controlled oscillator circuit uses a Hartley configuration and a NE85633 VHF NPN transistor. The tank circuit consists of two simple air-core coils of 33nH each and a 5-20pF varicap (load inductor L5 is 100nH not nF as shown)2000 copy.jpg uses

What I get is the horrible waveform shown! I am aiming at the 100MHz range and, sure.it's in there, but where-oh-where is that ~7.7Mz monstrosity coming from please? I can't see which components could be resonating down there!
 

KL7AJ

Joined Nov 4, 2008
2,225
That's not a harmonic...it's a parasitic....probably due to power line coupling. Try some different values of VCC decoupling capacitors.
 

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
I thought I had it decoupled to kingdom come with that big choke and 10uF and 1nF capacitors thrown in! But I will definitely play around with the decoupling and report back either way - thanks!
 

KL7AJ

Joined Nov 4, 2008
2,225
If you don't get anywhere with decoupling, you can decrease the emitter bypass capacitor, reducing its gain at lower frequencies.
 

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
I just did that - there is NOTHING getting past the capacitors but there is a gorgeous and perfect little 100MHz signal on the regulator side of the big choke - I shall put a capacitor before the choke which I think I should have done in the first place - thanks again! Will report.
 

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
I suspect you can drop the emitter resistor to 10 pF or so.
I realize that in the above circuit I have left no path to ground following the 100nH load but instead the effective load has 68uH in series with 100nH!
Tomorrow I shall pop a cap in to give a ground after the proper load - thanks for the suggestions, they forced me to review my decoupling and if the problem persists I shall reduce that emitter bypass capacitor as you suggest although I hate to drop the gain.
 

KL7AJ

Joined Nov 4, 2008
2,225
I realize that in the above circuit I have left no path to ground following the 100nH load but instead the effective load has 68uH in series with 100nH!
Tomorrow I shall pop a cap in to give a ground after the proper load - thanks for the suggestions, they forced me to review my decoupling and if the problem persists I shall reduce that emitter bypass capacitor as you suggest although I hate to drop the gain.
By the way, what kind of oscilloscope are you using that you can see 100 MHz directly?
 

KL7AJ

Joined Nov 4, 2008
2,225
It's a Hantek DSO4102B rated 100MHz but I have seen strong 200MHz on it - presumably with big attenuation? Thanks for your interest - I will report the outcome.
I presume that the bottom end of the emitter resistor is grounded, along with that entire lower bus. It's shown floating on your schematic.....be sure that really IS ground.
 

panic mode

Joined Oct 10, 2011
1,837
can you post photo of how the circuit is actually built?
where exactly is power connected? and L5 is interesting, where can you buy 100nF inductor?
 

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
can you post photo of how the circuit is actually built?
where exactly is power connected? and L5 is interesting, where can you buy 100nF inductor?
Thanks for lending a hand - sometimes I overlook really obvious things but in fact that bottom rail is grounded. I agree a 100nF inductor would be a pretty useless item for most purposes - luckily it is actually marked 100nH is an air-cored coil and comes from Digikey. The power comes in at the regulator terminal marked "TP1"(!) and I've confirmed the regulated voltage is right. I should be more careful when I post my schematics.
 
Last edited:

SLK001

Joined Nov 29, 2011
1,548
I would say that your transistor has too much gain for the frequency.

Also, your layout is hideous. Is this a two sided board?
 

alfacliff

Joined Dec 13, 2013
2,458
you might try a lower frequency rated transistor, those micorwave transistors have termendous gain at lower frequency and can be a beaar to stabilise paracitics out.
 

KL7AJ

Joined Nov 4, 2008
2,225
you might try a lower frequency rated transistor, those micorwave transistors have termendous gain at lower frequency and can be a beaar to stabilise paracitics out.
That's why I suggested reducing the emitter bypass capacitor, to reduce the LF gain. Increasing the emitter resistance too, might help
 

Thread Starter

Spottymaldoon

Joined Dec 4, 2015
63
Gentlemen .. First of all, many thanks for your various and stimulating inputs.

KL7AJ, blessings be upon you: your very first suggestion (which you edited away?) was to add some capacitance to ground from the base and this, combined with your second suggestion to reduce the gain (I changed the emitter bypass capacitor to make it 10x) has given me a brilliant sinusoidal output, with the detested parasitic ground into the dust, and adjustable between 75 and 135 MHz with remarkably uniform amplitude.
You were all right too much gain.

Nothing happened however (not even parasitic) until I added a 12pF base-to-ground capacitor (alternatively my finger, which was a clue) but HOW does that work exactly?

This circuit was a quick feasibility thing and I apologize for offending you with the ugly board which was, in fact, single sided with an accessible ground plane behind - be assured the final version will be more politically correct.

Also, Bertus, thanks for the referral to the RF circuit design pdf.

I now plan to re-do this same circuit coupled (God help me) to a PD5700 hoping to boost the power to a quarter Watt or so and from that to a 5W power stage, likely using a 2N3375. You may see me here again, cap in hand - please continue to drop in a few gems!

After that I hope to make a graceful exit from VHF power design and resume my normal life - happy to have met you all!

Kind regards and thanks again
Dick
 
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