A spectrum analyzer can detect very weak signals and tell you their frequency. It is also very useful for other measurements. You can get an HP 8566B on eBay for around $1,500. It spans 100Hz to 22Ghz.
The problem with your detector is it doesn't specify the frequency range so you don't knowwhere it cuts off. You can make your own that goes from 1MHz to 10GHz with 55dB dynamic range. It is available for $7.37 at Arrow.
On a sunny day (Fri, 5 Oct 2018 23:41:03 -0700 (PDT)) it happened Steve Wilson wrote in :
Yes, I have an rtl-sdr:
Also build a mixer to extend the range
It is not hard to build mixers up to several GHz with modern 'tronincs.
sigh, what do you even mean with that? read the specs, it is good to 6 GHz
It is the user, not the tool, that often sets what can be done.
To detect 'if something is oscillating' that is the issue here, the ebay thing is world class, has adjustable threshold, and you can set it so sensitive that if 2 houses down the road a light switch is flipped it beeps.
That chip you refer to is no better, and something like that is maybe even in it :-) (I had it open, glued the beeper back in place after I dropped it, forgot to look up the detector, as it works OK) but now add the cost of housing, battery, potmeter, antenna, beeper .... plus the camera lens detection thing it has, magnetic compass, what not.
What RF detectors have you designed and build?
China has won again! LOL spectrum analyzer my foot
The signal doesn't have to be fast, and the bypass doesn't have to be slow. Its impedance only matters up near the instability.
And reset circuits and cap multipliers are very useful.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
If you put a resistor in series with the emitter, the reflected impedance at the base is a negative resistance. If that becomes larger than the extrinsic base resistance, the thing becomes unstable. The parasitic Ls and Cs help set the oscillation frequency, but even without them, the transistor becomes a relaxation oscillator.
It is precisely a cap multiplier. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
The key ingredient is negative input resistance.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
That term always bothered me. Negative input resistance to what? If you look a the base input impedance, it is always positive. When the oscillations get going, the transistor is cut off most of the time. Where is the negative input impedance then?
The key ingredient is positive feedback. Say the emitter generates a voltage that is fed into the tank. The capacitive divider generates a voltage on the base that is higher than the emitter. The emitter follower operation means the voltage fed to the capacitive divider is larger than the original. Positive feedback. Oscillations ensue.
No, it isn't. YCLIU.
Present during the conduction angle. The amplitude limits where the average gain around a cycle equals unity.
A Colpitts network is one way of generating a negative input resistance, but it can also happen inside the transistor die. The B-E capacitance and the 1/f dependence of beta put two lags in the R_E -> R_b transformation, so it comes out negative.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
My first (or second) cap multiplier oscillated and I added a F-bead and
100 ohm resistor on the base lead, through hole hair-pin. No problems since then.George H.
Right. cap multiplier is a high-side emitter follower. Thanks,
George H.
A simple LTspice measurement shows the base impedance is always positive.
Can you supply a simple LTspice analysis showing negative input resistance?
There are many sites using the term negative input resistance. None of them explain why adding a resistor in series with the tank kills the oscillations.
You still haven't defined negative input resistance with respect to any point in the circuit.
LTspice shows the input impedance of the base is positive at the peak of the cycle.
You have just confirmed my positive feedback argument. The loop gain goes to unity.
Oscillations cease increasing when they hit a limit, such as base voltage above VCC. It can't go any further.
You still haven't defined negative input resistance with respect to what?
You can add external capacitors to form the tank. The result is the same. The emitter follower signal is in phase with the signal on the base.
Consider a 74H04 Pierce oscillator with a LC tank feedback. The input impedance of the 74H04 is capacitive.
By the energy equation, E = (CV^2) / 2, the impedance is always positive.
Where is the negative input impedance? How do you measure it?
Beads are bad. They can crack or break. The 100 ohm resistor is all that is needed.
Negative input impedance from base/gate to GND. It is not positive and it sustains the oscillation. And it can be measured.
<This is S11 of a baseband amplifier with absurdly high gm to enforce low voltage noise with IF3602 JFETs. Everything that is outside the circle that goes through X=0 has a negative real part of the input impedance. The exact values at the 3 cursors are given in the picture.
The green line is outside of that circle for most of the intended frequency range, until the amplifier runs out of steam.
BTW I have found no way to cure that without compensating it with a positive resistance in the input or cutting the feedback. None of this is attractive in a low noise measurement amplifier. :-( :-( :-(
Those network analyzers are so unforgiving. If you just ignore it, you can happily measure around in circuits that have enough source resistance. Just sometimes, it won't work. There are lots of low noise amplifiers that share its fate.
BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from AOE3. The non-differential version, since my source is never centered around 0 Vdc. Works as promised. :-)
Yes, it could use even more 'lytics, but that was a test balloon for a chopper amplifier. At 500 KHz the caps are no more that critical.
<cheers, Gerhard
Gerhard, I have the utmost appreciation for your accomplishements and your skill in electronics.
However, a simple Colpitts oscillator in LTspice shows the input impedance at the base is positive. The emitter current is also in phase with the base current.
Another example is a simple 74HC04 Pierce oscillator with an LC tank as feedback.
The input impedance of the 74HC04 is capacitive. The impedance is
Xc = 1 / (2piFC)
There is no negative input resistance.
A further example is "A General Theory of Phase Noise in Electrical Oscillators, by Ali Hajimiri, Student Member, IEEE, and Thomas H. Lee, Member, IEEE"
"Consider as one example the Colpitts oscillator of Fig. 5(a). The collector voltage and the collector current of the transistor are shown in Fig. 13. Note that the collector current consists of a short period of large current followed by a quiet interval. The surge of current occurs at the minimum of the voltage across the tank where the ISF is small."
Clearly the collector current is a function of the base current, and is in phase with the base current. The input impedance at the base is positive, therefore there is no negative resistance term available.
Oscillators work on positive feedback. If the loop gain is positive and in phase, oscillations ensue.
FWIW it's not hard to demonstrate the existence of oscillators that rely purely on negative resistance. L, C, carbon arc. The v/i plot of the carbon arc shows positive large signal dc resistance, negative small signal ac resistance over a limited v swing.
NT
Tunnel diodes
Some pages of Randall W. Rhea: Discrete oscillator Design.
That won't stay there for long, but for the discussion, methinks it is fair use. The book is worth every penny.
<Sorry for the quick & dirty repro. 04:30 am here. Have a good night.
Gerhard
Steve Wilson
Steve I completely agree with your viewpoint. Just saying 'input has negative impedance' does not explain a thing, and then adding a resistor to make it positive again is fake close to string theory. Better is to understand the Cbe - CeGND step up, and that can be compensated for in many ways and is of no significance when driving the base with some circuits.
I have had exactly one case of an oscillating emitter follower in my life, and that was in a missile tracking system, replacing the transistor fixed it, not all trannies are the same ...
I do not use base resistors in my RF amps, that just decreases performance. And no those do not oscillate.
You oscillators are doing something basically wrong, layout?
;-)
hehe ;-)
Nice! Please post your complete schematic. We only show a common-emitter amplifying stage, with millions of paralleled transistors, and our measurements, pages 506-509.
--
Thanks,
- Win
Not always. Most positive feedback loops just latch in one state.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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An over-general claim. If the positive gain is between 1.0 and 2.0 it isn't even unstable and won't latch. Back in 1979, I put a positive gain of 1.00
3 into a circuit to raise the current through a platinum resistive sensor a s it got hotter, to compensate for the decline in the temperature coefficie nt of resistance with increasing temperature.The clown that took over the circuit claimed that this would make the circu it "unstable", and replaced it with a diode-break linear approximation. Som eone at Honeywell seems to have invented the same circuit as I had at much the same time, and it worked fine in the field.
Positive feedback loops can be designed for all sorts of jobs, and - if the y are designed by people who know what they are doing - they won't latch up .
-- Bill Sloman, Sydney
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