AM Radio Design Log

Some pesky ceramics are superconductors:

Gives me an idea of a new FM modulator element, the capacitor modulator. Changes in audio voltage produces equivalent changes in capacitance and frequency changes accordingly :)

Actually I did both. But measurement showed about 100 nF capacitance which is strange because that would have changed the frequency by much much more than the observed change. I also tried the calculation method which produce d a capacitance of a few pico Farads.

Yes, I did hence I reversed the coil so the gain isn't so much.

I was trying to calculate the output impedance of the circuit but failed.

On a sunny day (Mon, 22 Jul 2013 20:34:58 +0200) it happened Glenn wrote in :

Yes I have some, from here:

This is my sterling cooler:

This is how I measure temperature:

More info on this type of cooler:

Mine is from a superconducting filter from cellphone towers. When the material starts super-conductiong the Q goes way up (no R), and that makes it possible to make increadble steep filters.

So I also have a super filter somehere... ;-)

Beware some types of high temperature superconductors contain Thallium, and can kill.

And those Sterling coolers are very very expensive normally ( >10,000 $ ), got it cheap on ebay. Seller seems to have figured out the real value by now, last time I looked his price was way up.

Now how to make your own rocket fuel...

Is this mixer better than the Gilbert cell?

The Tayloe Sampling Mixer - also called: Tayloe Detector IQ based tranceiver Dirodyne linear detector Quadrature Sampling Detector QSD

Actually it is a "bang-bang"/digital mixing that ought to produce a lot of aliasing products.

It seems to have something in common with the diode ring mixer:

What you really need is an analog multiplier, but this is emulated by a Gilbert cell - and maybe a diode ring mixer?:

But then how do you theoretically optimize the diode ring mixer for low IP3 and other "false" mixing products? "Of cause" you can not suppress the signal mirror unless IQ-mixing is used.

Actually another techniques is used with e.g. a harmonic diode mixer - or you are just "tuning" for the "false" product :-) :

I happened to find the harmonic diode mixer techniques in the HP 141T's

8555A input mixer - and it can be a "bad" techniques because of aliasing, that is why an external preselector is needed in front of the harmonic mixer.

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Gilbert cell:

Historical:

James Long, Ph.D., P.E. Analog and RF Consulting Engineer History of Mixer Technology (And Other Stories of Misplaced Credit) Things are seldom as they seem. Skimmed milk masquerades as cream.

Quote: "... All of mixer technology in use today was mathematically known several hundred years ago and reduced to practice by 1935 when the DPDT vibrator modulator was used in low frequency amplifiers. By 1949 this function was performed by the 7360 vacuum tube in radio circuits and six vacuum tubes in rocket telemetry circuits. ..."

Glenn

On a sunny day (Mon, 22 Jul 2013 21:21:43 +0200) it happened Glenn wrote in :

These days you can get cheap SAW filers with a precise bandpass curve, other frequencies will be surpressed by a lot of deebees, So I dunno how much it matters, you need to see the whole setup to make a choice.

Dual conversion, greater offset of the mirrors, it is a design choice.

...

That is cool!

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Somebody has used a superconductor cavity to trap and detect a single microwave photon - they say:

Centre National De La Recherche Scientifique (2007, April 2). Life And Death Of A Photon 'Filmed' For The First Time. ScienceDaily. Retrieved March 22, 2008:

Quote: "... A photon is an elementary particle of light. In general it can only be observed when it disappears ... The end result is that the atom changes to state 1 if the cavity contains a photon and remains at state 0 if it is empty, as in the standard method. However, this time the energy absorbed by the atom is taken from the auxiliary field and not from that of the cavity. As a result, the photon is still there after having been seen, and is ready to be measured again ... Suddenly the atoms appear in state 1, showing that a photon has been trapped between the mirrors. The photon comes from the residual thermal radiation which surrounds the cavity ... The moments at which the photons appear and disappear reveal the quantum jumps of light, which occur at random ..."

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Maybe you can detect its OAM modulation? ;-) :

Nov 1, 2012, physicsworld.com: Spooky action with twisted beams:

Quote: "... Using this technique, Zeilinger and co-workers found they could obtain differences in quantum number as high as 600 (in other words l = +300 on one photon and l = ?300 on the other). Lapkiewicz points out that there is, in theory, no upper limit to a photon's l value, which suggests that a photon ? a quantum object ? could acquire as much OAM as a macroscopic object, leading to what he calls a "tension between the quantum and classical worlds" ..."

Glenn

The Gilbert cell is a four-quadrant analog multiplier, which should be the best mixer, but due to the imperfections in the multiplication gives often worse results than a raw bang-bang switching mixer (diode bridges, rings, 7360 beam- deflection tube, CMOS analog switches).

It does, but it it very linear for the signal channel.

There are two things in the mixing which should be considered separately:

1. How to get an effective analog multiplication and linearity, at least for the signal input;
2. How to suppress unwanted mixing responses.

In practice, point 1 is difficult to achieve, especially for a large input amplitude range. For many practical purposes it is sufficient to multiply with sign of the oscillator signal, which is much easier to implement linearly for the signal channel.

The quadrature methods (phasing, IQ) attack question 2, by implementing a full complex number multiplication (called analytic signal in DSP theory).

--

-Tauno Voipio

On a sunny day (Mon, 22 Jul 2013 21:41:35 +0200) it happened Glenn wrote in :

Yes I read physics world newsletter. As to 'kwantum' and 'photon', I think the quantization of light is just the exchange with our every day matter (kick an electron in an higher orbit, or wave disturbance in aether when electron changes orbit), The whole 'kwaantuum computah' is in my view complete nonsense and will never bring anything as they are trying to make an analog computer, as light is NOT a stream of particles, at least not at that gross a level as our matter, You see indeed the whole kwaantumm fund raising jive is limited by the error rate, the same as we had with analog computahs, and the same as we have with multilevel FLASH memories. Not a paper is published these days that does not end with: 'And we think this will bring the quantum computer so much closer, given X years more funding blah blah, WOW we dunnit!' and then just like fusion power at ITER ever and ever more into the future.,

The practical thing I may want to use my cooler for is perhaps one day cool some amplifier to get noise down (SETI). Unfortunately the Sterling cooler has a lot of vibration, but long time ago somebody already cooled webcams, and got better night pictures. Condensation is a problem with cameras... Some RF preamp cooled should give better noise figure...

Depends how you define "better".

Some of the more recent high-performance amateur-radio transceivers are using the Tayloe-type H-mode mixers. The biggest benefit I've seen mentioned is that these mixers can have a very high dynamic range and very high IP3 / overload level. Their ability to reject/ignore strong, close-in interfering signals can be very good indeed.

I believe the radios using this design are becoming very much favored for use in Field Day and DXpedition operation, where there can be multiple transceivers operating in very close physical proximity.

The Gilbert Cell mixers usually have significant conversion gain, while the Tayloe mixers are more like diode-ring mixers in that they have conversion loss... you need to put your desired gain elsewhere in the signal chjain.

Like most diode-ring mixer setups it will also "detect" signals on the odd harmonics. It's usual to include a low-pass filter prior to the mixer so that only the fundamental is fed in.

I believe that the Tayloe mixer has an advantage over the diode-ring, in that the switching signal (LO) and the incoming signal (RF) are coming through different types of paths... the RF signal isn't getting into the "switch control" pathway at all. For this reason, different signal components in the RF aren't going to mix with one another (as can happen in a diode-ring mixer) and there will be fewer spurious mixing products than with a diode-ring arrangement.

As I understand it, the strong-signal robustness of a diode-ring mixer requires that the LO signal (which is in effect "switching" the direction of the ring) be sufficiently stronger than the strongest RF signal being received.

Commercial diode-ring mixers are available with several different design points for the LO signal. 50-ohm mixers seem to start out at about +7 dBm and go up from there in 3- or 6-dBm increments. I believe that the ones requiring the "hotter" LO signals are build with several diodes in series in each leg of the ring.

You could think of a Tayloe mixer's behavior as being sort of an ultimate-limiting case of a diode ring mixer, perhaps... the LO signal is "so incredibly strong" that it's in effectively complete control of the mixer's switching behavior, and the incoming RF signal can't perturb the switch timing at all. As a result, the only sidebands created by mixing between different RF components, will be due to the residual nonlinearity in the switched-signal pathway.

At least, that's how I understand it. Corrections welcomed!

• --------------------^ Stirling en.wikipedia.org/wiki/Stirling_engine

On a sunny day (Mon, 22 Jul 2013 18:01:08 -0800) it happened Robert Baer wrote in :

Yea, was probably still thinking about the price and thought:

:-)

Does it get colder now?

1.8 MHz. I changed to C0G (NP0) and it cured the problem.

#4

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I realized how much fun oscillators are and how Sine waves are so beautiful so I decided I'm going to up it a notch and take a shot at a 100 MHz oscil lator.

I tried a Colpitts variation of the Hartley oscillator I posted earlier wit h the same MPF102 JFET, a diode, 1 MOhm Res, the tank, and its surrounding capacitors. Inductors and Caps had to get much smaller. So L is in the nH r ange and C is in the low pF.

Couldn't get the thing to oscillate at all no matter how hard I tried. I tr ied too many things to keep track of. I was suspecting the inductance is so low and the inductor Q is inadequate. I tried different types of air-core wire turns. Eventually I moved to the Toroid version but no luck.

Next, I decided a JFET isn't the thing for this and decided to move to BJT. I used a 2N3904. I played a lot with the number of turns and capacitor val ues until I reached a circuit similar to this:

My L is 3-5 turns on a 68-6 toroid. The value on the circuit is just a gues s based on the frequency I'm getting. I had the following observations:

- I had a big 300p cap for C2 this killed the ability to oscillate (later I realized it had to do with the amount of feedback. If you reverse C1 and C

2 however, simulation shows the voltage swing is low. I am guessing it has to do with transistor gain. Increasing C1 should probably also mean decreas ing R3. I am taking shots in the dark here, I will try to do some analysis later).

- Even with C2=300p, increasing C3 to 66p restored oscillation.

- After reducing C3 back to 33p, I was able to reduce C3 back to 33p and ge t oscillation.

- I kept on trying to reduce inductance to increase frequency but that came with a price of reducing the voltage swing at the tank.

- I still do not have a unified theory of how the values of L and Cs affect frequency and voltage swing but it seems I should strive for having higher L for better stability and voltage swing. This means I need to reduce the C's to as low as I can get.

- In the end I was able to reach 67 MHz as the max I could squeeze out of m y circuit but it's not very stable.

- Even with everything laid out on a perf-board and with no power supply by pass you still can see 67 MHz of oscillation that is clean looking on a sco pe. Stability is another matter.

- Soldering and de-soldering is a pain when you have try different things.

- a plain vanilla 2N3904 can go long ways!

- I don't like JFETs. I love BJTs.

------------------------------------------------- Next steps:

- Add more turns to my toroid

- Try a better a toroid core, may be a 2 or 6 Mix

- Reduce caps to the minimum possible.

- Hope for 100 MHz.

You've stumbled upon "resonant impedance". There was some discussion about that, here, a while ago.

To a first approximation, the impedance seen across an LC circuit, with loss only in the inductor, at resonance is:

L/CR, where R is the inductor's series loss resistance (higher than at DC because of skin effect).

Simply put, the higher the L/C ratio, the higher the voltage across the oscillator tank.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

On a sunny day (Sat, 27 Jul 2013 13:36:50 -0700 (PDT)) it happened "M. Hamed" wrote in :

1 MOHm ?????? WHERE? + 12 |------- |--- d | ----------------------->| BF245 === | | |--- | 100n 5 turns === 47p | /// 8mm diam. |------------| 1 cm length === [ ] 470 Ohm to about 1k | | 100p | /// /// ///

You need the right kind of JFET, BF254 are around for a few cent on ebay and go up to 700 MHz.

Would it not be simpler to DC bias from the cold side of the coil? + | |-------- |/ | --------------------| === | | |>\ | 100n ( === 47p | /// ( L |------------| + ( === [ ] 1k | | | 100p | 33k [ ] | /// /// |------| | | 15k [ ] === 10n | | /// /// Now L is no longer damped by 33k paralel with 15 k, only by the transistor Zi, and that is about beta x 1k (Re).

At many MHz it is unusual to use toroids, those have a high Al, you want low. Toroids can have huge losses too at higher frequencies, depends on the material,

Confusing circuit, C3, IF you must have it, should be on the other side, connected to R1, R2, and a much higher value (10nF), as it purpose is ONLY decoupling, It should not be part of the tuning parameters. C2 C3 should then be connected to the coil.

Use an air coil, as drawn in the above diagram, for 100MHz and up, like in this picture:

JFETS are cool, they make good low noise amps too. I once did a cascode JFET input amp that beat the better BJTs at that time. It also depends on what make MPF120 you have, one datasheet here shows a graph that its gain is flat to 700 MHz. Remember that when using a source follower as gain element the VOLTAGE gain of the JFET stage itself is always a bit less than 1. So to get > 1 feedback you need to step up with either a transformer (tap on coil or extra turn), or as in this case with the right ratio of capacitors, AT LEAST 2 to 1, or even higher. try again.

No toroids!!!!

Air core!!! Use a trimmer capacitor, or a small coil former with a core you can screw in and out for adjustment.,

No, no, that will only create instability, Around 100 pF should be fine.

100 MHz is nothing, how do you measure frequency? Not loading that LC with a counter I hope? Use an FM radio tuned to 100 MHz, and a 50 to 250 pF (or there about) antique air spaced variable capacitor, of if not in possesion of such a wonderful piece on engineering, bend the air coil, and turn or bend until you hear 'flop flop' sounds from that radio. And make sure that is not just an harmonic...

I went back to the thread. So in essence it's the equivalent parallel resistance of the inductor and we want to keep that high I presume by increasing the unloaded Q of the inductor which can be achieved by increasing inductance.

At the gate of the JFET in parallel with a diode. Similar to the Hartley ci rcuit I got from EMRFD and linked to earlier in the thread (dropbox).

and

At the moment I have a few MPF102 JFETs. The datasheet labels it "VHF ampli fier" so I assumed it's adequate.

r Zi, and that is about beta x 1k (Re).

Interesting idea. I'll try running some simulations.

low.

terial,

I was suspecting that is the case. The T68-6 is recommended up to 40 MHz. I have a few toroids on order that are recommended for higher frequency and have lower AL (mix 10). I started with an air form but when it didn't oscil late I thought my inductance must be too low to be usable. After all the it erations I went through I am going to go back and reexamine this. Thanks fo r the hint.

Will do!

Here is the dilemma. For 100 pF @100 MHz, L is around 25 nH. This is very s mall. How is that more stable when it's susceptible to parasitic lead indu ctances and also am I not supposed to keep L/C ratio high?

ique

piece on engineering,

m that radio.

Oops. Yes I was. Not the counter but I was using a scope. I figured out a f ew hours ago. More details in my next post. PS: I do have this wonderful pi eces of engineering!

Thanks for the time you took responding!!

#5

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Went ahead with my plans of increasing L and reducing Cs. Started with a re latively larger inductance, and reduced the tank cap to 6pF and the other c aps are 6, 10, 10 pF. I got around 40MHz then I progressively reduced the n umber of turns on the inductor. At some point slightly above 66 MHz, oscill ations seized.

Then it occurred to me what I should have known better. All this time I've been probing the tank with my oscope probe. Effectively I'm adding a cap in parallel that would shift frequency and hurt the L/C ratio, especially whe n my tank caps are 6 pF.

After this simple realization, I added a JFET source follower and connected the scope to the JFET source, and voila! My scope showed around 80 MHz. It was a matter of shaving off one extra turn from the inductor and there I g ot 108 MHz! Goal achieved.

The JFET datasheet lists input capacitance as 7pF @ 1 MHz and 15VDC. So my guess is that the tank resonant frequency is probably around 200 MHz withou t the JFET load!

At some point I got concerned that I could be generating interference and m y little oscillator could be acting as a transmitter. Sometimes I cranked u p my power supply voltage to 18V and with the supply showing less than 1 mA drawn, that's 18mW which I thought should be nothing.

To test the theory I tuned my FM radio to 108 and I got some ugly sounding noise. That got me scared a bit that I could be jamming something somewhere so I immediately turned off my supply. With such low power and at 100 MHz I don't expect my interference would travel very far. But at least now I un derstand the importance of shielding.