Phase or frequency modulation?

Could I ask you to explain this a bit more please. Where is the 'spring' that is variable?

The spring is the mechanical stiffness of the quartz, plus the piezoelectric force caused by the interaction with the external circuit. The mass is the mass. ;)

Dorking the load capacitance slows down or speeds up the piezo restoring force.

Looking at it this way, i.e. as a simple harmonic oscillator, introduces some small error, because there is a small acoustic delay involved. It’s tiny compared with a SAW resonator, however.

I had this forcefully brought to my attention long ago, when my old colleague Doug Smith invented a super fast atomic force microscope by turning the vibrating cantilever into an oscillator.

Mine was way more sensitive, but his was, like, 200 times faster. Really a smart guy.

Cheers

Phil Hobbs

I suppose that's still true for a shear-mode overtone crystal. Just harder to think about.

Thanks, I should have seen that.

Less so, because the acoustic delay increases with the order of the overtone, and the coupling decreases.

The piezo response goes like the applied E field, which is pretty much uniform throughout the crystal. In a third order XO, the piezo effect is therefore only about 1/3 as strong, because at any given instant the piezo force is out of phase with a third of the mass motion, which cancels out another third.

Cheers

Phil Hobbs

If you push the signal into clipping, the very act of clipping is going to create a whole bunch of nasty high-frequency harmonics and IM that sound terrible. In that situation, applying further emphasis to this grunge is probably the opposite of what you want to do. Rather, let the THD/IM be rolled off by the receiver's deemphasis filter, and consider this an act of corporal mercy to the listener's ears :-)

If you put the emphasis after the clipper, then you end up having to set your deviation limit based on a worst-case prediction of high frequency content in the signal, and this may force you to use a lower deviation level than you'd otherwise prefer.

So, my vote is "emphasize, then clip".

Frequency and phase modulation should come out of a transmitter almost identical. As they start off different in the first stage of the transmitter the audio frequency going into the modulastor stage must be modified so the end results is the same for FM and PM.

Sure it does. You can only move the frequency a small amount. FM and PM is usually generated at 8 to 12 MHz and multiplied up to 50 to 450 MHz. This is for the old crystal controled ham and pulic servce radios.

It limits the frequency deviation, but not strictly the modulation bandwidth. The modulation frequency is the frequency of the modulation, i.e. what goes into the modulator or comes out the demodulator.

You can do PM at a 1 MHz modulation frequency and a 1-Ht deviation, no problem. You get weak PM sidebands at 1 MHz frequency offset.

The problems arise when the modulation frequency or the frequency deviation becomes comparable to the center frequency. At that point you get sideband folding, which loses information unless you have an I/Q system.

Cheers

Phil Hobbs

I've now altered the circuit to do it that way:

As the audio frequency rises towards 3 Kc/s, the 100nf cathode bypass starts to take effect, increasing the gain of the clipper and simultaneously reducing the audio signal on the cathode. This has the effect of bringing the 220pf capacitor into action to contribute another pole to the cutoff of frequencies above 3 Kc/s. In this respect, it acts a bit like a Sallen & Key filter with a sharper corner than the two RC circuits would have had if they didn't interact.

The clipper is 'soft' to some extent, so it doesn't generate such strong and wide-ranging spurious component as a hard clipper would. The 220pf capacitor across the output level control deals with some of the worst harmonics. The second stage of filtering is in the grid circuit of the reactance valve.

Yes - but it would appear that PM is the de-facto standard for commercially-built 'amateur' 2-metre transceivers despite all the literature calling it FM.

Interesting, I hadn't come across that before. Presumably the effect would be the same if the controlling reactance were an inductor or a tuned circuit? In my case, the 'variable reactance' is an amplitude-modulated 90-degree phase-shifted current injected into the controlling inductor.

The Q of a good crystal is high enough that it’s really just the phase angle of the load at resonance that matters. How you make that load is less important, though it still matters.

A bad circuit can produce multiple frequencies, but in a fundamental-mode oscillator that takes some talent.

Any given resonance of a crystal looks electrically like a series RLC in parallel with a capacitor. If the parallel cap is small enough, the impedance of the crystal goes inductive in a narrow frequency range, so it can be made into a tank circuit, e. g. in a Colpitts oscillator. The overall circuit resonates when the load reactance is minus the crystal reactance, and that happens at just one frequency in this case.

If the parallel cap is too large, the impedance is capacitive everywhere, so you need an inductive load. Unfortunately, due to the crystal’s impedance peak, there will be two closely spaced resonances, one on each side of the peak, and probably one or more LC peaks elsewhere, depending on the details.

It’s often useful to make one half of the Colpitts divider out of a series LC, to make sure the phase is wrong at the higher overtones, and keep the gain down to avoid LC oscillations.

(And then there’s startup behavior, but that’s another topic.)

Cheers

Phil Hobbs

Yes, your sentiment seems "on the money."

This thread apparently advanced from an IC-706 to a DIY receiver. If your schematic indeed pertains to your DIY receiver - excellent work showcased in an excellent thread! (It taught me a few things, thank you.)

For what it's worth, the IC-706 probably uses the 75 micro-second pre- emphasis standard prevalent in the Americas and Japan. Meanwhile, Europe favors a 50 micro-second standard.

Although Phase Modulation is popular in Asia:

it's doubtful the IC-706 was developed for an Asian market.

Danke,

It is based on an idea that has been in the back of my mind for many years. The change from an exchange-powered to a mains-dependent telephone service in the UK was the final factor that made me decide that I neded an alternative communications system. I thought that, while I was making one, it might be a good idea if it was EMP-proof, so I decided it would have no semiconductors. It has turned into an interesting project.

Those are the standards for wide-band broadcast FM, but they are outside the range of permitted modulating frequencies for narrow-band amateur work. The time constant for the Icom appears to be around 2000 microseconds.

Apparently it was. There is a Japanese version with a slightly different specification from the European one. You may have hit upon the real explanation, I haven't checked to see if mine is a Japanese one which has been sold in the UK by accident.

Dave Platt's "it's not a bug it's a feature" also makes a lot of sense. I got on the wrong track in regards to standards after an attempt to de-cipher repeater builder's cryptic warning about "Just watch the pre-emphasis and de-emphasis!"

IC-706 ... This is the HF radio that also does 6 meters. When coupled to a high-end repeater controller that understands what a HF remote base is (like an NHRC-10, an Arcom RC210 or similar capability controller) this makes a dandy HF remote base. Just watch the pre-emphasis and de-emphasis! ...

IC-706MKII ... This is the later 706 HF radio that also does 6 meters and 2 meters. The same firmware bug that affects the CTCSS frequency applies; use the same procedure.

Danke,

I'd put it this way - _compatibility_ with PM is the de-facto standard. It can be implemented using a PM transmitter, or an FM transmitter which applies the necessary pre-emphasis... to the entity at the receiving end, there's no difference.

As I understand it, one reason for this PM-oriented approach is that PM was favored for engineering reasons in early transmitters. My old FM-for-hams book points out that with PM, the phase modulation doesn't have to be done in the oscillator... it can be done in a separate stage after the oscillator. The oscillator can be made as stable as you want to make it, with no "bending" required. Although this sort of design isn't the popular one these days (except perhaps for crystal-controlled repeaters) the expection-of-compatibility remains.

From what I've seen of the design of ham VHF transmitters these days, they are usually PLL-based, based on a single crystal, and I believe the modulation is implemented as FM by injecting the signal into the PLL VCO control loop. This would require applying pre-emphasis to the injected audio, and the couple of components needed to do this do show up in the schematics I've looked at.

Some ham VHF radios these days are probably implementing their local oscillator using a DDS these days, with the DDS being fed a new frequency at a rate of a few kHz or a few tens of kHz. For this sort of design, the audio signal would be pre-amphasized (usually digitally) before it goes into the math calculation which tells the DDS what frequency to emit. I used an approach like this to make an FM-stereo test signal generator... FPGA spits out "change frequency!" commands to an Analog Devices DDS module at a rate of around 150 kHz, a modulated 10.7 kHz IF signal comes out, and it's then mixed up to the FM broadcast band.