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AM Radio Design Log

M

M. Hamed

Jan 1, 1970
0
PS, as mentioned on that site, there is also an other issue with at least some ceramic caps:

they are not perfect insulators, and cause noise by themselves.

In this project:

http://panteltje.com/panteltje/pic/mag_pic/

at one point I thought I was seeing the magnetic variations in the earth core...

(Those exists at very low level), but in this case it turned out

to be fluctuating DC current changes (say varying leakage) in those

brown 470nF caps...

Bummer...

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 :)
 
M

M. Hamed

Jan 1, 1970
0
OK, now things are sinking, you missed an opportunity
to CALCULATE the capaciatnce of your meter.
You had L, C, and frequency f.
You added the meter, and found now a lower frequency f2.
L has not changed, so now you can calculate C meter (plus hands etc).
No need to even measure it.

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 produced a capacitance of a few pico Farads.
You talked about where to put the tap on the coil,
and noticed it made little difference.
Do you realize that sets the gain?

Yes, I did hence I reversed the coil so the gain isn't so much.
I remember I had to calcualte those freaking things,
there is
http://en.wikipedia.org/wiki/Thévenin's_theorem
http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws


and a few more.

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

Glenn

Jan 1, 1970
0
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
http://9y4ar.tripod.com/tayloe_mixer.htm

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:
https://en.wikipedia.org/wiki/Ring_modulation

What you really need is an analog multiplier, but this is emulated by a
Gilbert cell - and maybe a diode ring mixer?:
https://en.wikipedia.org/wiki/Analog_multiplier

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 :) :
http://en.wikipedia.org/wiki/Harmonic_mixer

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.

-

Gilbert cell:
https://en.wikipedia.org/wiki/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.
http://www.analog-rf.com/mixer.shtml
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
 
G

Glenn

Jan 1, 1970
0
On a sunny day (Mon, 22 Jul 2013 20:34:58 +0200) it happened Glenn


Yes I have some, from here:
http://www.can-superconductors.com/levitation-bulk.html

This is my sterling cooler:
http://panteltje.com/pub/cryo/index.html

That is cool!

-

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:
http://www.sciencedaily.com/releases/2007/04/070402122514.htm
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
...."

-

Maybe you can detect its OAM modulation? ;-) :
https://en.wikipedia.org/wiki/Orbital_angular_momentum_of_light

Nov 1, 2012, physicsworld.com: Spooky action with twisted beams:
http://physicsworld.com/cws/article/news/2012/nov/01/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
 
T

Tauno Voipio

Jan 1, 1970
0
Is this mixer better than the Gilbert cell?

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).
The Tayloe Sampling Mixer - also called:
Tayloe Detector
IQ based tranceiver
Dirodyne
linear detector
Quadrature Sampling Detector QSD
http://9y4ar.tripod.com/tayloe_mixer.htm

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

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).
 
R

Robert Baer

Jan 1, 1970
0
Jan said:
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
* --------------------^ Stirling en.wikipedia.org/wiki/Stirling_engine
 
J

John S

Jan 1, 1970
0
From my readings, capacitor low Q is usually not an issue up to high frequencies. Before that the inductor Q is more important. I am compelled to ask at which frequencies was this a problem to you?

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

M. Hamed

Jan 1, 1970
0
#4
---

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

I tried a Colpitts variation of the Hartley oscillator I posted earlier with 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 range and C is in the low pF.

Couldn't get the thing to oscillate at all no matter how hard I tried. I tried too many things to keep track of. I was suspecting the inductance is solow 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 values until I reached a circuit similar to this:

https://www.dropbox.com/s/fp3dv13ahmu6pzh/Collpits.asc

My L is 3-5 turns on a 68-6 toroid. The value on the circuit is just a guess 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 Irealized it had to do with the amount of feedback. If you reverse C1 and C2 however, simulation shows the voltage swing is low. I am guessing it has to do with transistor gain. Increasing C1 should probably also mean decreasing 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 get oscillation.
- I kept on trying to reduce inductance to increase frequency but that camewith 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 affectfrequency and voltage swing but it seems I should strive for having higherL 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 my circuit but it's not very stable.
- Even with everything laid out on a perf-board and with no power supply bypass you still can see 67 MHz of oscillation that is clean looking on a scope. 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.
 
F

Fred Abse

Jan 1, 1970
0
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.

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.
 
M

M. Hamed

Jan 1, 1970
0
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.

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.
 
M

M. Hamed

Jan 1, 1970
0
1 MOHm ?????? WHERE?

At the gate of the JFET in parallel with a diode. Similar to the Hartley circuit I got from EMRFD and linked to earlier in the thread (dropbox).
You need the right kind of JFET, BF254 are around for a few cent on ebay and
go up to 700 MHz.

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

Interesting idea. I'll try running some simulations.
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,

I was suspecting that is the case. The T68-6 is recommended up to 40 MHz. Ihave 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 oscillate I thought my inductance must be too low to be usable. After all the iterations I went through I am going to go back and reexamine this. Thanks for the hint.
JFETS are cool, they make good low noise amps too.
try again.

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

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

Oops. Yes I was. Not the counter but I was using a scope. I figured out a few hours ago. More details in my next post. PS: I do have this wonderful pieces of engineering!
http://www.tubesandmore.com/products/C-V365


Thanks for the time you took responding!!
 
M

M. Hamed

Jan 1, 1970
0
#5
---

Went ahead with my plans of increasing L and reducing Cs. Started with a relatively larger inductance, and reduced the tank cap to 6pF and the other caps are 6, 10, 10 pF. I got around 40MHz then I progressively reduced the number of turns on the inductor. At some point slightly above 66 MHz, oscillations 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 inparallel that would shift frequency and hurt the L/C ratio, especially when my tank caps are 6 pF.

After this simple realization, I added a JFET source follower and connectedthe scope to the JFET source, and voila! My scope showed around 80 MHz. Itwas a matter of shaving off one extra turn from the inductor and there I got 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 without the JFET load!

At some point I got concerned that I could be generating interference and my little oscillator could be acting as a transmitter. Sometimes I cranked up my power supply voltage to 18V and with the supply showing less than 1 mAdrawn, 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 somewhereso 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 understand the importance of shielding.
 
M

M. Hamed

Jan 1, 1970
0
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 nowI understand the >importance of shielding.

I will probably invest in one of these little fellows:

http://micro.arocholl.com/index.php...ducing-rf-explorer&catid=40:article&Itemid=64
 
F

Fred Abse

Jan 1, 1970
0
So in essence it's the equivalent parallel
resistance of the inductor

No.

With high Q inductors, L/C dominates the resonant impedance.

R is equivalent *series*, not parallel resistance, BTW, ie. R=wL/Q


If you've found the previous thread, see my second posted Spice simulation
to that thread (the current-fed, constant Q, one).
That demonstrates dependence of Zr on L/C.
 
F

Fred Abse

Jan 1, 1970
0
For example, this is LTspice my run for the 25 MHz DVB-S transmitter
exciter filter at 25 MHz with a 220 pF capacitor and 184 nH coil:
http://panteltje.com/pub/220p.gif

And this with a 22 pF capacitor and 1.84 uH coil (factor 10 change, same
frequency)
http://panteltje.com/pub/22p.gif

Now look at the bandwidth of the first one, compared to the second, At 50
MHz (2 x f0) attenuation is better than 33 dB.

And then the second one:
at 50 MHz (2 x f0) attenuation is about -14 dB, not very usable as filter.

That is why I wrote in a previous post that bigger C is better.


That's misleading. The Q of each circuit, as drawn, is not the same; the
difference in bandwidth is due to Q, not L/C ratio.

The effective Q of the 220pF circuit is 38.
The effective Q of the 22pF circuit is 3.8.

That's a 10:1 bandwidth ratio, right there. Nothing to do with L/C ratio.

Assuming the same wire gage, a 1.84uH inductor will have sqrt(1000) times
the resistance of a 184nH coil. That's about thirty times. You used the
same resistance in both cases.

Your simulation doesn't prove your case, regarding "bigger C", which is
wrong, anyway.

Now go figure how to simulate L/C ratios properly...
 
J

josephkk

Jan 1, 1970
0
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 understand the importance of shielding.

Hooray. You are actually doing very well. Good enough to make jealous
of all the fun you are having. Keep going.

?-)
 
Y

YD

Jan 1, 1970
0
#5
---

Went ahead with my plans of increasing L and reducing Cs. Started with a relatively larger inductance, and reduced the tank cap to 6pF and the other caps are 6, 10, 10 pF. I got around 40MHz then I progressively reduced the number of turns on the inductor. At some point slightly above 66 MHz, oscillations 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 when 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 got 108 MHz! Goal achieved.

Congratulations, you've just discovered the buffer amplifier, to
isolate the oscillator from the load. From there you can take the
signal and do something with it without disturbing the frequency.
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 without the JFET load!

And now you know to take parasitics into account. Good work!
At some point I got concerned that I could be generating interference and my little oscillator could be acting as a transmitter. Sometimes I cranked up 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 understand the importance of shielding.

At that power it's probably detectable at some tens of metres at most.

- YD.
 
Y

YD

Jan 1, 1970
0
Late at night, by candle light, "M. Hamed" <[email protected]>
penned this immortal opus:

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

So recalculate for some more reasonable L, like 60 nH. You probably
already know, but when you have two of f, L or C you can calculate the
third by

f = 1/(SQRT(2 * pi * L * C))
L = 1/(4 * pi^2 * f^2 * C)
C = 1/(4 * pi^2 * f^2 * L)

For winding the coil, do a search on "inductor calculator", there's a
lot of them on-line.

- YD.
 
M

M. Hamed

Jan 1, 1970
0
If you are on-frequency and have a clean power suply, then your radio should become very quiet,

as it sees an 'unmodulated' (= stable in frequency) carrier.

It turned out connecting my cheap frequency counter is the source of this noise. It probably totally messes up the biasing. When I don't have that connected, I get exactly the effect that you mentioned. It's pretty cool when the frequency on my radio matches the frequency readout on my scope!
 
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