TheRadioBoard

One of my ideas for my general-coverage superhet project is to use a wide-range (spanning ten or maybe even twenty MHz) varactor-tuned VFO with two ten-turn pots (100k and 100 ohm) connected in series as the bandset and bandspread tuning controls. To test the idea I lashed up a varactor-tuned Hartley oscillator and monitored its signal on my regen.

I'm having a problem getting a clean tone from the varactor-tuned oscillator. It sounds warbling as if it's being frequency-modulated, perhaps by 60 Hz hum. As soon as I replace the varactor with a variable cap the tone quality improves immensely, so obviously something is modulating the varactor control voltage.

The circuit is as follows:


The warbling is present regardless of if I use a switched mode power supply or a battery. Warbling gets worse when I touch the pot shaft or body. The connecting lead between the pot wiper and the varactor circuitry is about 5 cm long. Could it be that the pot shaft and the wiper lead are acting like an antennas and picking up stray 60 Hz modulation? Or could this be some variant of tunable hum where the oscillator signal radiates into surrounding AC wiring, gets modulated, and "somehow" gets back into the oscillator again? The capacitors on the input and output of the voltage regulator have long leads, but all other leads are short.

Physical layout:


Any suggestions how to fix the warbling? Is it realistic to expect adequate control voltage stability in a varactor-tuned VFO operating at HF that spans 10 or more MHz?

I really would like to tune my superhet with two ten-turn pots (they're more compact than Vernier-driven variable caps), but if adequate varactor control voltage stability can't be obtained I may have to scrap the idea.

Are you switching inductors or does this thing really cover that much frequency? Mechanical instability in the pots would be a first guess. To cover a wide range, the number of degrees of shaft rotation per hertz is vanishingly small. The traditional way to cover wide ranges was to use an array of crystal-controlled oscillators and a VFO that tuned a range of from 500-1000 kHz.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

No, it doesn't yet cover that much range, but it should eventually be able to if I can squeeze the full theoretical 30-530 pF out of my varactors. I think this might be physically manageable because of the 1000:1 ratio I am planning between between bandset (100k) and bandspread (100 ohm) pots. If the bandset covers 20 MHz in ten turns of the ten-turn pot, then the 1/1000th-sized bandspread pot will cover 20 kHz in ten turns, or 2 kHz per turn - reasonable, isn't it?

Anyway: currently I'm just testing the Hartley oscillator, and it just covers 1.67 MHz to about 3.5 MHz, a very conservative range. Currently I'm only using a 500k single-turn pot for testing. Yet even over this small tuning range it still warbles badly with the varactor in place - warbling which basically disappears when I use the variable capacitor. I can try replacing the pot with a higher-quality ten-turn pot to see if it's a mechanical issue.

However my gut feeling is that this is somehow powerline related, because the warbling doesn't seem to be related to or change in response to any mechanical vibration of the pot. I could build extensive shielding and add tons more decoupling and make all leads very short, but without knowing the cause of the problem, it might be wasted effort. Any ideas on how to troubleshoot this to know where to direct my efforts?

Turn off the power to your house and run the circuit off batteries? Do you have fluorescent lights in your workshop? The 1N4148 diode is probably in a clear package, and its junction could be modulated by the light. Then again, it's in the circuit even with a mechanical variable cap... Can you move the circuit to another room? You might try isolating the supply to the varactor from the FET supply, or additional bypassing. Such as running the varactor off a separate battery, or 100 uF from the junction of R3 and POT1 to ground. What sounds like warbling could be a LF "motorboating" caused by insufficient bypassing.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

I noticed that you used only one varicap diode. I usually use 2 in a head-to-head configuration. This does cut the capacitance in half, but prevents the RF modulation and forward biasing, if you have a large signal swing.



http://www.radio-electronics.com/info/d ... rcuits.php

Rich

_________________
Homebrew Radio ex-Silicon Valley

Hi,

You say that it happens even when powered by a battery?



I suspect something quite similar to what jim said.

Bias/operating point Jumping!

Where is your source resistor Bias for the Jfet? You can not really use "gate leak" bias with a J-fet.

73
kb0lxy

Thanks KR1S and Ham-er for the good advice.


Actually I may have been mistaken here. The oscillator was powered by a battery (a weak one - the only one I have on hand right now) and my monitoring regen was powered by the switching power supply. I think the warbling when the oscillator is battery-powered may be coming from the regen itself, not the oscillator. I need to get some more batteries to figure out what's really going on. It's tricky to test a DUT (my oscillator) when the measuring equipment itself (my regen) may be introducing distortion!


Yikes! There is none! I did this design rather quickly but obviously missed an important point. Thanks.

OK, I think I got the warbling figured out. It's definitely the switching power supply causing the warbling. I have three separate and separately-powered modules for this setup: the varactor-tuned Hartley oscillator (not connected to any other module), a regenerative receiver, and an LM386 audio amp connected to the regen. I have a switching power supply with a multi-way adapter so I can run power lines to up to 4 appliances.

When I power *any* of the three modules with the switching power supply, the Hartley oscillator warbles. Even if I power the regen and the Hartley oscillator with battery power (from the same battery), and only run the audio amp off of the switching power supply, the varactor-tuned Hartley (and maybe, to a lesser extent, the regen - which has a mechanical variable capacitor for bandset and a varactor for bandspread only) still warbles. Apparently some power line ripple is able to work its way through the amp back over the audio connection (between the regen and the audio amp) which somehow influences the control voltage on the varactor even though the varactor is separately powered by the battery (that also powers the regen).

I tried some additional bypassing in the form of 1000 uF from the junction of R3 and POT1 to ground, and also 1000 uF at the output and the input of the voltage regulator, all with no effect.

I noticed that if I touch the ground plane with my finger the warble decreases but is still noticeable. Unfortunately the switching power supply (like most appliances in Japan) is not grounded, which is probably also a factor in the power supply instability.

Looks like the easiest solution at this point is to just give up on the switching power supply and use batteries.

Even with the warbling fixed with the use of battery power, it still remains to be seen if the temperature-related drift will be acceptable. In my current 1.6-3.5 MHz prototype I am getting noticeable drift, which doesn't bode well for a super-wide-range higher-frequency version. Things might settle down if I construct the VFO properly in a shielded enclosure with short and rigid wiring.

I now see why some builders recommend building the VFO in its own separate and separately-powered enclosure, connected through shielded cable to the receiver proper. That technique allows the most physically demanding part of the receiver to be optimized separately.

On the topic of the Hartley itself, Ham-er questioned the lack of source bias. Is it really needed actually? See for example this circuit:

http://www.electronics-tutorials.com/os ... llator.htm

I suppose the dilemma I'm facing is the following. A varactor with two ten-turn pots in series offers good mechanical stability (control wires carry DC only and the ten-turn pots are easy to physically mount with stable and smooth rotation), but has poor temperature stability. On the other hand, a variable capacitor is more difficult to get mechanically stable (requiring a separate vernier drive, and short rigid control wiring that is placement-sensitive because it carries RF), but is more temperature-stable.

So the question is, for wide-range VFOs, which is easier to get stable, both mechanically and thermally? Any opinions?

Direct digital synthesis.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

I question your assumption that something from the switching supply is somehow getting back through the audio connection to influence the VFO control voltage. You could simply be hearing IM distortion from the audio amp.

Regens are known to be hypersensitive to supply noise; you should probably banish any switching supply from your bench while developing... and I don't know why you'd even consider one for this sort of radio project.

Re your requirement for stability - what's the final goal? You're discovering firsthand why most of these analog designs have been retired. As per KR1S - digital synthesis.

I suppose you could control this VFO by making it the oscillator in a phase-locked loop (PLL) arrangement...

Touché. While it's fun to do things the old-fashioned way, perhaps the time and labor spent perfecting an analog VFO might be better spent working on other parts of the receiver. Part of me resists the idea of using something I can't build myself, but another part of me is starting to accept it.

Any suggestions on good DDS units?

You've come a long way since you first showed up here. There's a lot to be learned by repeating the mistakes of the past, but at some point you have to move on.


http://www.pongrance.com/super-dds.html

http://midnightdesignsolutions.com/dds60/index.html

I think the N3ZI unit is easier to put into service. The AMQRP design is more appropriate for experimenting.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Just throwing out an idea... Another approach would be to make the best analog VFO you can, and include a frequency counter.

I'm currently building this one here, which will accommodate adding/subtracting IF frequency so that it will display the received frequency. You can apparently use it on any regen by just nudging the receiver into oscillation.

Hm, that's a good point. Come to think of it, the standalone amp unit doesn't have any voltage regulation. So in addition to IMD the audio amp supply voltage instability may be causing the warble.


Well, I have one around because I bought it to power my yet-unbuilt QRP CW transmitter kit (VXO), as the manual specifically showed a switching power supply being used in the construction photos. I was hoping I could use it for my receivers as well but am starting to understand the difficulties.


Two goals:

1) To determine if I need a superhet for on-air CW work, given my location and antenna. My finished receivers until now have been regens. One thing I read a while ago here on TheRadioBoard has been gnawing at the back of my mind though: the fact that a good DC or superhet receiver can hear signals inaudible on a regen due to the regen's higher noise. That bugs me, that a better receiver might hear more than my regens. So one goal is to build a superhet with reasonably modern specs for IMD, sensitivity, noise, etc., then to see if at my location and with my antenna I can actually hear any more signals with a good superhet than with my regens.

2) If the superhet is actually better, then to use it for on-air CW work. This mandates crystal-filter selectivity (both to reduce audible interference as well as to protect the sensitive IF strip) and sufficient VFO stability to prevent signals drifting out of the passband.

A third goal is, having gone through all the trouble of building a good superhet, I'd want to use the set for general coverage SW listening.

Your suggestion about a frequency counter is a good one for simplicity: Just live with VFO drift, and use the brain-locked-loop to bring it back on frequency.