vfo noise?

[ssbothwell KJ6RSG]

sorry to make so many posts recently. i'm at this point with my transmitter project where i have most all the major components assembled and in final stages of individual testing before i put everything together.

i am using a hartley VFO design i found in the ARRL handbook. the design was originally intended as a reference oscillator for receiver testing but i assume it will be fine for a transmitter VFO.

it is stable down to the 10hz range and i have it setup to to output a little over 0dBm.

i was looking at the signal on my scope just now and i noticed that the peaks have some weird noise. i'm not sure if this is is a problem with the display on my scope or if it is a problem with the VFO, or if it is even a problem at all.

anyway, i'm attaching a photo and was hoping one of you guys could give me some insight. in the attached photo the scope is set to the 20mV/division setting.

[WD5JKO]

Solomon,

    Maybe turn your scope horizontal time base to about 1 ms / division. Here you will see any audible ripple (AM) on the carrier. Maybe that is all you have from what might be power supply ripple. To go much further you need a spectrum analyzer to look at the VFO output in frequency domain. Some of the SDR receivers might be capable of this.

Jim
WD5JKO

[w1vtp]

Yup - should have some hummage on it if you listen to it with a RX

[W4AMV]

put your vfo on a battery, see if the problem goes away.

[KB2WIG]

   " sorry to make so many posts. "


Why? Thats the nice thing about AmFone.... there is a cornucopia of knowledge on the board... not board, community, that makes it  valuable.
 
Your questions make me remember stuff I spent a lot of time learning and then forgot. Keep it up.

klc

[ssbothwell KJ6RSG]

Your questions make me remember stuff I spent a lot of time learning and then forgot. Keep it up.

awww, thanks for the kind words. i am really excited about building and understanding rf devices and its really amazing to so many experts.


i am powering the vfo with a nice HP lab variable supply, but i'll try using a battery and see if it makes a difference.

assuming it is coming from the power supply, is there anything i can do to clean up the voltage input? would running the voltage input through a 7812 regulator do anything?

[W4AMV]

there is a very clever circuit which provides power supply noise canceling and is useful to apply to a VFO or VCO supply line input. However, HP supplies are quite good and for most VFO circuits for HF you should not need to add this circuit unless you have significant supply line sensitivity. So first try the battery to rule out the supply and if the problem still exists, I would suspect there is an issue in your design. Vary your supply line Vcc, does the AM noise (?) change? Your oscillator may be squegging. For a latter discussion. 

[KM1H]

In the various labs Ive worked in an external 'lytic was just about mandatory on any supply, including the high and mighty HP.

[W4AMV]

Only if the VCO sensitivity either to the supply line (or in the case of a VCO) to the tune line is high. Sure, drop in a FARAD or TWO. Or just go get a battery. The noise canceling circuit as well as regulators supplied by Linear Tech, Hittite, and Maxim are available which will drive the noise voltage down to insane levels. The best I have seen is ~ 1nV/root Hz out to 30 MHz, and will beat your CAP hung across an HP supply. However, I don't think that is the battle he is fighting... just yet.

[ssbothwell KJ6RSG]

hi guys, just letting you all know i havent gotten around to testing the VFO with a battery. i'll edit this post with the results if i get a chance to test it today.

i suspect the noise is not coming from the power supply, i have run other experimental VFOs off this power supply without this problem.

[W4AMV]

I agree, I think you have an issue in your oscillator. Tweak your Supply V around, see if it goes away. Post a SCH. Happy New Year. 

[KM1H]

Only if the VCO sensitivity either to the supply line (or in the case of a VCO) to the tune line is high. Sure, drop in a FARAD or TWO. Or just go get a battery. The noise canceling circuit as well as regulators supplied by Linear Tech, Hittite, and Maxim are available which will drive the noise voltage down to insane levels. The best I have seen is ~ 1nV/root Hz out to 30 MHz, and will beat your CAP hung across an HP supply. However, I don't think that is the battle he is fighting... just yet.

It seems that the PS inventory was the last thing to be upgraded, if it worked mgmt wouldnt buy new ones. But we usually had the latest and greatest VNA's, SA's, sweepers, etc. since it seemed that every week we moved up a GHz in R&D.