VCO tug of war (A tale of two VCOs)

One vco and some sort of offset mixer might be easier. I've seen well-shielded crystal oscillators, on separate boards in a card cage, lock with enthusiasm, and precision xo's have been known to lock from being in the same room.

The waveform distortion is probably caused by the variations in lock gain around the phase range, sort of a wobbly lock.

John

Christiaan Huygens noticed this first in 1665, except with much slower oscillators. John Harrison solved it in his work by placing the 'oscillators' [1] in separate rooms.

Cheers, James Arthur

[1] Pendulum clocks. Exquisitely fine, in his case.

I think the dual DDS and frequency multipliers approach should work. I'm not an RF guy but I am aware of one application similar to yours where a DDS (AD9858) is followed by a single mixing stage and then cascaded multipliers.

Come to think of it, the 9858 has some mixers on it, too. You could probably build a board around two of them that does most of what you want, even without any external multipliers.

--Mac

I've seen offset PLLs where the second loop is driven by the first with a mixer and the offset frequency between. The design looked sufficiently braodband, what cannot be told about another method, the SSB. With SSB you can make a second frequency from the first with a fixed offset, but only if you have a

90 degree phase shifter over the band. The carrier could be expected 20 to 30 dB down. If the 90 degrees are not 90 degrees, the carrier and other sideband are supressed less.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

200mhz

tune

hapens,

this

suspect

the

gate

not

lm2434

loop

freq

yes that would be easier, and I wouldnt have the limit on resolution of the IF frequency by using 2 high freq PLL. However im not sure how to calculate what impact any residual of the original frequency and other sidebands would have, probably limit sensitivity.

Yes im inclined to agree, although the hytite mixer low frequency output is bandwidth down to DC so shldnt cuase any distortion, I havnt experienced using it in this range before.

Colin =^.^=

200mhz

tune

hapens,

this

suspect

the

VCO,

gate

not

lm2434

realy

resolution.

loop

freq

Im thinking that even if I just use one vco and generate the other frequency from it, it might still pull the VCO somehow, I think il try to make some duaghterboards for the vcos and see if that helps. theres a nice mixer from AD8346 wich does the 90' for you on the LO input and produces a ssb from I and Q inputs, it looked quite expensive when i first coinsiderd it but if it makes an easy job of this maybe worth it, but if it still pulls the first vco it would make it noisy or distorted.

I have thought of having 2 vcos of very diferent lower frequencies and multiplying them by diferent ratios to get what i need ie x2 and x3, hopefully they shouldnt be able to lock on to such widley spaced frequencies even if they have do harmonics that are close.

Colin =^.^=

What's the coupling mechanism with clocks? Just mechanical vibrations transmitted through their frames and then through the floorboards or some such?

Pheromones are supposedly the coupling mechanism in menstrual cycle oscillations "locking"...

Yes, that's it. Curiously, they're said to lock in anti-phase.

Yes, I've heard and observed it. Certainly true in cats, though the girlfriend, objecting to the idea of her enslavement to others' pheromones, cited a snow storm of contrary studies sufficient to steal my thunder. I yielded, lacking the inclination to sort through and evaluate, and, given the incentive: landing myself in the doghouse if I 'won.' :-)

Cheers, James Arthur

Nowadays, birth-control pills contribute a strong anti-locking effect.

John

Hello John,

Hmm, I have never had that happen. Maybe the oscillators I design are lacking communication skills :-)

So, is there a Yahoo Personals for oscillators?

--
Regards, Joerg

http://www.analogconsultants.com

Nah, your crystal oscillators just drift too much. The more accurate they are, the fewer nanovolts it takes to lock them.

John

Hello John,

I did have to run some close together at times. The closest ones were a two-tone test at 7.000MHz and 7.001MHz (pre-historic, before the advent of DDS). And they were clean like whistles. Checked it by dividing down and then listening to the harmonics beating with WWV.

--
Regards, Joerg

http://www.analogconsultants.com

...

Colin, It seems to me the route for coupling is most likely through your mixer that you're using to look at the beat frequency. Besides the 10 dB pads, an amplifier with a good isolation might help. Also, you could look at the beats using a hybrid combiner feeding a well matched detector, rather than using the mixer.

Paul Probert University of Wisconsin

1KHz is way too much. Try 1Hz.

The term is "injection locking". Young hams learn about this when they build their first regenerative receiver. It is difficult to get close to zero beat without the oscillator dropping into lock. Later when you build your first superhet, you have the same problem with the BFO for receiving Morse code.

It is also a major problem with precision pll's. Multivibrator type vco's can develop a large phase offset when crosstalk from the incoming signal interferes with the switching point in the vco cycle. Severe crosstalk can also force the vco into a limit cycle oscillation, which renders the pll useless. Part of the problem is the multivibrator needs to have widebandwidth in the comparator to give fast switching, so it responds to the narrow switching transients from nearby logic.

An LC oscillator can reduce this effect by several orders of magnitude by attenuating the crosstalk energy in the resonant tank. The difficulty is now to initialize the phase of the oscillator to the incoming data in order to reduce the lockup time. Ironically, injection locking can be used to accomplish this, but it takes many cycles of driving the oscillator directly with the incoming signal. An example of this is in US patent

3810234:

Regards,

Mike Monett

Hello Mike,

Yeah, probably.

Actually mine was rather tolerant there. Before it locked the bandwidth of that receiver was way below what is useful even for morse code. Wish I still had it but my better half said it was too ugly. Believe it or not, what brought it to grief was that the front panel rusted out (!).

When I was young and thus on a shoestring budget I made myself a "synthesizer" that wasn't a real one. A free running oscillator that would lock onto the harmonics of the 1MHz gate oscillator of my counter. That was I could build a stable HF receiver because I had cut the whole band into 1MHz slivers. Not pretty but cheap.

Regards, Joerg

You should have no problem receiving WWV, then:) A popular type now is Direct to DC receivers where the local oscillator sits on the carrier and all the gain is at AF. An example is the Tayloe mixer, and some excellent designs such as the SM5BSZ and R2 Direct Conversion receivers.

These need very low noise amplifiers, so much of the discussions here in s.e.d. are very useful.

Regards,

Mike Monett

Hello Mike,

Actually I never had a problem with that, no matter where in the world. Either 10MHz or 15MHz usually came through.

These have another nasty surprise in the bag. If just a wee bit of the oscillator power leeks out and hits a power line that gets modulated with all the crud on that line, courtesy of non-bypassed rectifiers in cheap power supplies. Then the audio contains that "rat-tat-tat" and you can't get rid of it.

On very leaky direct conversion sets you can get interesting and cool sounds when swinging a big metal plate in front of them.

--
Regards, Joerg

http://www.analogconsultants.com

===========================

It sounds as if there is not enough isolation between the two VCOs. Considering that a VCO is essentually a regenretive amplifier, if there is ANY amount of a second signal coupling into the VCO, assuming it is close to the same frequency as to VCO is tuned to, the VCO regenratively amplifies it and results in either a powerful spur, or possibly "pulling" the VCO to the other frequency.

You probably need better than 30 dB isolation between the two VCOs. I'd add attenuators and amplifiers to both VCOs to increase the isolation. You can't just buffer and isolate one of them, the other one will be just as happy to regenerate the spur as the first one.

Jim

--

19:15 Pacific Time Zone
May 26 2006

International Time
02:15 UTC
27.05.2006

Yes. A comb generator strong enough to injection lock the first LO would feed other harmonics directly to the mixer. At 10MHz, a frequency error of 1e-7 in the crystal would produce a 1Hz beat note, interfering with the wwv signal. Of course, it also gives an indication when the crystal is adjusted on frequency:)

Nah. First, there should be very little leakage from the LO for the reason described below. Second, power supply rectifiers are slooow. Even if some LO signal did get to them, they wouldn't even see it.

Sounds more like snap recovery diodes in the power supply making a 120 Hz buzz. This has been discussed here several times.

Self-Mixing due to LO feedthorugh. Produces a DC offset. This is a well-known and understood problem with DC receivers. It also affects the Binary Sampler when heterodyne mixing is used and the fequency offset is set to zero:

There are other problems with DC receivers as well, but they are fixable with clever design and good layout. Regards,

Mike Monett