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Old 9th Feb 2012, 12:41 pm   #41
BobGreen
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Default Re: Receiver aerial input impedance: questions.

Post #39 - Of course, if the resistor is cold then it won't produce noise.Surely the resistor has to be at absolute zero (0 deg K) to be noise free?
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Old 9th Feb 2012, 3:08 pm   #42
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Default Re: Receiver aerial input impedance: questions.

Yes, I was employing the usual British understatement!

I have been doing a bit of reading (still more to do) about the Norton amplifier. An interesting article by Chris Trask gives ways to improve it still further.
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Old 9th Feb 2012, 3:35 pm   #43
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Default Re: Receiver aerial input impedance: questions.

Quote:
You can't 'manufacture' resistance from reactance,
Dave:

Taken literally, you're right, but that's not the question here: I think it's
"can you have a resistive input impedance (in this case, 75ohms), without having the thermal noise from /equivalent to a 75ohm resistor"

The answer is a definitive "yes", and the proliferation of low-noise amplifiers with noise figures of << 3dB vindicates the thesis.

kalee20 has supplied one answer as to how this can be achieved, and there are others. The "design" requirement is to have an input which, when an ac voltage is applied, takes current in-phase with said applied voltage. A pure resistor is one way to achieve this, another way is of direct relevance to high-frequency (common-emitter) transistor stages:
If you operate a transistor at a frequency well above it's Falpha, but below it's Ft, the collector current lags c. 90 degrees behind the applied base current.

Now, the emitter is grounded, but in the real world, there is some internal (to the package) and external inductance in the emitter lead.

The voltage across an inductor leads the applied current, so with a lagging collector current flowing into the emitter, the "feedback" voltage is in-phase with the base input current. Hence the input impedance of the transistor takes on a significant resistive element, generated entirely by a 90 deg. phase-shift and a reactive component.

A similar arm-waving analysis (over these frequencies) can be undertaken with an HF transistor when considering the impact of the Miller capacitance, which again leads to a resistive input term (rather than the expected capacitive).

That's why many UHF transistors (such as the BFR90) show almost pure resistive input impedances over certain UHF frequencies.

John

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Old 9th Feb 2012, 3:54 pm   #44
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Default Re: Receiver aerial input impedance: questions.

Quote:
Originally Posted by G8HQP Dave View Post
... You can't 'manufacture' resistance from reactance, but you can transform resistance....
But you can, at least in theory.
Consider an amplifier with a high input impedance and a phase shift between input and output of 90 degrees, if a pure reactance is connected from the output to the input then the current flowing through it will be in phase with the input voltage.
If the current is in phase with the voltage then the input must look resistive even though no resistive components have been used.
(A variant of this using series dreived series feedback is often used at UHF and above e.g. http://www.avagotech.com/docs/5988-6670EN)


It is also possible to achieve low noise figure (<3dB) and matching without transformers or reactive elements!
Take an inverting amplifier with a high input impedance and a voltage gain of -A.
Connect a resistor from output to input of value A.R, the input will look like a resistor of value R.
Since the noise power in a resistor (=kTB) is independent of its value, the noise voltage of the feedback resistor (A.R) will be sqrt(A) times that of a 'real' resistor R. Now the voltage across the feedback resistor is also (A+1) times Vin.
Thus the element of Vin caused by feedback resistor noise is sqrt(A)/(A+1) times that from a 'real' resistor of value R.


Jim

I see John has posted while I was composing this covering similar ground.
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Old 9th Feb 2012, 5:29 pm   #45
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Default Re: Receiver aerial input impedance: questions.

I'm not doubting that it is possible, I'm just puzzled how its done. Making a resistance from reactance requires a 90 deg phase shift, but to make a 90 deg phase shift you need resistance! You can't make a 90 deg phase shift from reactance alone - even a 90 deg transmission line would need a resistive termination. OK, you can then hide the resistance by using active devices.

I think what is happening with Norton is that the input resistance is actually the load applied at the output - it isn't a resistance in the amplifier itself. (Did someone already say this in an earlier post - if so I apologise for not taking sufficient notice). As it doesn't exist in the amplifier it doesn't generate any thermal noise in the amplifier (although it still generates noise where it actually exists in the load). You could argue that a Norton amp is basically a transformer (and hence noiseless), but augmented so signal power out can exceed signal power in.
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Old 9th Feb 2012, 6:14 pm   #46
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Default Re: Receiver aerial input impedance: questions.

Dave:

Quote:
but to make a 90 deg phase shift you need resistance
I think you've got yourself into a mode of analysis which is not helping you!

Have another read of what Jim & I have tried to explain by example. Does it not make any sense?

Also, consider the following "resistance-less" example:

Take a perfect ac voltage source (zero output impedance) and drive a series L followed by a shunt C. Measure the voltage across the shunt C using a "perfect" scope probe (infinite input impedance). When the frequency is above the resonant frequency of the L-C combo, the overall voltage phase-shift will be -180 degrees.


If you were to replace the shunt C with a transconductance amplifier (I in to V out) you'd have the phase-shift due to the inductor alone (-90)

John
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Old 9th Feb 2012, 7:16 pm   #47
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Default Re: Receiver aerial input impedance: questions.

But a transconductance amplifier is, in effect, a resistance. OK, the current goes in at one port and the voltage comes out at another. Anyway, we are getting rather a long way away from the original topic so perhaps we should stop trying the mods' patience! Thanks for an interesting discussion.
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Old 13th Feb 2012, 7:01 pm   #48
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Default Re: Receiver aerial input impedance: questions.

Having "dipped my toe" in to this thread early on, I've just been looking through it to see what useful conclusions it has come to, which is to say, what does it tell us about how best to use a vintage comms Rx? Can someone tell me if it did reach any useful conclusions, and if so, what they were ?
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Old 13th Feb 2012, 7:27 pm   #49
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Default Re: Receiver aerial input impedance: questions.

From my viewpoint, Baz4CQJ, this thread is not yet complete. I have further 'input' that I intend to make, which will hopefully be soon, now that the weather had moderated. At that point, perhaps a few conclusions can then be drawn.

Al.
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Old 15th Feb 2012, 10:43 am   #50
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Default Re: Receiver aerial input impedance: questions.

Interesting thread, Boltzmann comes to mind.

In response to Bazz4CQJ - if your vintage comms Rx exhibits an increase in background noise level (excluding manmade noise) on the HF bands when the aerial is connected then you have nothing to worry about, improving noise factor or aerial matching will be a theoretical exercise that will not improve s/n of a recovered signal.

Regards, Bob
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Old 15th Feb 2012, 6:20 pm   #51
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Arrow Re: Receiver aerial input impedance: questions.

That there will be no benefit in the SNR, Bob, under the conditions that you state is true. However, if that matching is obtained by the use of various forms of tuned circuits, then although the SNR may not benefit, the reduction in the level of unwanted signals reaching the receiver will be improved by virtue of those additional tuned circuit(s) that are between the aerial and the receiver. There is more to optimising receiver / aerial performance than by only obtaining optimum SNR.
It is that very fact that has caused me to carry out some investigations & measurements which, in turn, has generated this thread.

Al.
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Old 16th Feb 2012, 4:29 pm   #52
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Arrow Re: Receiver aerial input impedance: questions.

To continue my investigation, I have carried out some measurements and calculations on my Eddystone 888A Amateur Bands only double-conversion superhet comms. receiver. I selected the 80 m. band for those measurements and calculations: 3.5 to 4.0 MHz.

The input cct. is quite conventional: untuned transformer primary; tuned secondary (fixed capacitors and variable capacitor), low-reactance capacitor-coupling to the control grid of the R.F. amplifier.

For the sake of brevity, I shall avoid going into depth here about my measurements and calculations. I calculated the dynamic impedance of the tuned secondary winding to be about 16,200 ohms at 3.7 MHz. I measured the transformation ratio of the R.F. input transformer at about 15:1. That ratio transforms the dynamic resistance figure to the primary of 16,200 ÷ 15² which is about 72 ohms. At each end of the band (3.5 and 4.0 MHz), since only the capacitance changes value, the dynamic resistance changes value also, so the input impedance changes too: -15% at 3.5 MHz and +15% at 4.0 MHz.

Interesting points arising.
(1). The set features an aerial trimmer of about 5 - 25 pF. A quick calculation shows that provided the set is originally aligned with that trimmer in its mid-capacitance position, some reduction in that input impedance deviation can be reduced by adjusting that trimmer - even when a purely 75-ohm source is presented to the aerial terminals - at the two band edges. Unfortunately, in their infinite wisdom, Eddystone chose to place the control knob for that trimmer inside the case of the receiver: only accessible by lifting up the hinged top cover!
(2) I made the measurements and calculations only on one band: 3.5 - 4.0 MHz; 500 kHz wide. All the other bands are also about this width (except Topband: 1.8 - 2.0 MHz), but the 10m. band is 2 MHz wide (28 - 30 MHz), leading me to suspect that over that band the aerial input impedance will vary over quite a wide range - which may be somewhat compensatable by the aerial trimmer.

So to conclude: when a receiver states that it has an aerial input impedance of xx ohms, should that not be an aerial input resistance of xx ohms? And the answer seems to be . . . . maybe! I say 'maybe' since it hinges on your understanding and interpretation of the two words 'impedance' and 'resistance'. If your understanding of 'resistance' is limited to d.c. resistance only, then 'impedance' seems appropriate. However, if your understanding of 'resistance' includes 'dynamic resistance', then the word 'resistance' seems correct. {N.B. In all the text books I have consulted, the term 'dynamic resistance' was the only term I found for the expression L/CR. The term 'dynamic impedance' was never found}. And for me, that answers my original, opening questions, q.v.

Overall, an interesting exercise: I'm sure that what I have written above will not be the end of this thread!

Al.
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Old 16th Feb 2012, 6:30 pm   #53
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Default Re: Receiver aerial input impedance: questions.

Quote:
Originally Posted by Skywave View Post
So to conclude: when a receiver states that it has an aerial input impedance of xx ohms, should that not be an aerial input resistance of xx ohms? And the answer seems to be . . . . maybe! I say 'maybe' since it hinges on your understanding and interpretation of the two words 'impedance' and 'resistance'. If your understanding of 'resistance' is limited to d.c. resistance only, then 'impedance' seems appropriate. However, if your understanding of 'resistance' includes 'dynamic resistance', then the word 'resistance' seems correct.
I tend to use the word 'resistance' for 'impedance-where-the-phase-angle-is-zeroish', ie not necessarily the same as DC resistance, but non-reactive nevertheless. And this would include Skywave's case of dynamic impedance.

Oscilloscopes usually specify their input impedance a bit more precisely - commonly '1MΩ in parallel with 20pF' (where the 1MΩ is understood to be both resistive and frequency invariant).

It might also be, reference noise optimisation etc, that the input impedance isn't 75Ω, and it's not resistive at the frequency that the radio's tuned to, but nevertheless it's been the best compromise that the designers could achieve - in which case it might be more accurate (but pedantic) to label the input socket 'For 75Ω aerial'. And as other have pointed out, mismatch and reflections with feeder cable may be of academic interest only!
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Old 16th Feb 2012, 6:52 pm   #54
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Default Re: Receiver aerial input impedance: questions.

Since Z is a combination of inductive and capacitive reactance and resistance then to describe the input of a receiver in resistance terms only would imply that there are no reactive components involved in the input circuitry. Certainly when XL and XC are equal and opposite, only R will be left and the attached antenna system will transfer maximum power at that one frequency. I would suggest that the input circuit resistance will remain constant in DC terms but the values of XL and XC will be variable (assuming a multi frequency receiver) therefore the term used for maximum power transfer from the antenna system to the receiver should be expressed in AC terms i.e. Impedance.
However since the input impedance can never be a fixed value for all input frequencies, its figure should be stated with wide tolerances and bandwidth in mind to say the least.
The antenna system will present a wide range of loads to the receiver, with change of frequency, so the input circuitry must be adjusted accordingly. Broadly described as “matching”. How that is done is irrelevant in this discussion but the fact remains that the receiver input circuitry must be changed in order to effect maximum power transfer and must be done with reactive components.

Quoted values of input impedance are just starting values for the antenna designer to aim at. After all it would be unreasonable to expect a random length of wire to be “matched” to the receiver across a 30MHz spectrum without some form of help.

So, my answer to your question would be. Yes, the units should be Impedance but their value should be stated along with acceptable tolerances.

I’m with Barrie in #2 on this one.

Cheers

John
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Old 16th Feb 2012, 8:03 pm   #55
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Arrow Re: Receiver aerial input impedance: questions.

Quote:
Originally Posted by Bazz4CQJ View Post
It is really the aerial and the feeder which determines what the impedance of the system is and that is why an aerial tuning unit is often used to match the receiver to the aerial.
Barrie
If you replace which determines what the impedance of the system is with which determines the impedance that the aerial input circuit 'sees', then I would agree with you.

Al.
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Old 16th Feb 2012, 10:11 pm   #56
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Default Re: Receiver aerial input impedance: questions.

Al

I think it's a case of which comes first, the chicken or the egg.

If I had an aerial system that I wanted to use over a broad band of frequencies, I would use open wire feeders. The impedance presented at the RX (or TX) end of the feeder would be variable over those frequencies. To get the best out of this aerial, my RX (or TX) would have to deal with those variables, therefore I would design it with a variable impedance input (output) circuit. In other words I would build in an Aerial System Matching stage. I.E. variable Z input (output) stage.

On the other hand, if I were stuck with a receiver/transmitter with a, shall we say, loosely fixed input/output impedance (which these days is the norm),the aerial system would have to include an external variable matching device to utilize the full potential of that receiver/transmitter.

Note that I have not used the term "aerial tuning unit" or "aerial matching unit" but "aerial system matching" device.The first two would have to be situated at the aerial feed point (physically difficult) whereas the latter includes the feeder system and is located directly before the receiver.
Also note that I have not mentioned specific impedance values.

But I will now.

The figure of 600 ohms is "generally" associated these days with open wire feeders, and 75 or 50 ohms with coaxial feeders.

In transmitter situations (slightly off topic but connected).

Open wire or balanced feeders are more tolerant of high standing waves which might occur on them due to mismatch because of operation outside the operating frequency of the aerial system. 75 or 50 ohm coaxial feeders are "generally" used to feed single frequency aerial systems.Outside the aerial resonant frequency various problems on the feeder system can occur due to high reflected power because of impedance mismatch.

On the receive side the above is still true but we can discount most of the problems (because of the very low voltages involved) except impedance mismatch. For maximum signal transfer we have to match the aerial system impedance (the RX end of the feeder) to the input impedance of the receiver stages.

I mention these points for the benefit of those who may be new to this aspect of the hobby.

It's because we are always referring to variable frequencies that we must refer to the aerial terminations of a receiver having a certain (albeit variable) input impedance.

John
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Old 16th Feb 2012, 11:50 pm   #57
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Default Re: Receiver aerial input impedance: questions.

I agree with everything you've written above John, except for the last sentence: "It's because we are always referring to variable frequencies that we must refer to the aerial terminations of a receiver having a certain (albeit variable) input impedance."
It does not logically follow that because the input freqs. to a receiver are variable that the aerial input current 'opposition' must be termed an 'impedance'. Moreover, a receiver could be designed where the input aerial current's 'opposition' is resistive at all frequencies - for example, input direct to the grid of an untuned-input R.F. amplifier, e.g. a grounded grid, and the signal frequency tuning performed at a subsequent stage.

You also say "For maximum signal transfer we have to match the aerial system impedance (the RX end of the feeder) to the input impedance of the receiver stages", which isn't exactly correct either. If you had said "We have to make a conjugate match between the aerial system impedance and the input impedance of the receiver and where, in so doing, the resistive components are equal", then I would agree with you.

I've come to the conclusion that the term "aerial input impedance" has come into use simply because the receivers that actually use that term are generally designed to be fed with a transmission line, and transmission lines are always quoted as having a 'characteristic impedance'. Now whether that should be more accurately referred to as 'characteristic resistance' is another topic for another day!

Al.
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Old 18th Feb 2012, 12:42 pm   #58
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Default Re: Receiver aerial input impedance: questions.

Points noted Al but I still think that the applied AC voltages from the antenna system would "react" with the stray L and C of the broadband resistive front end.

We're getting into realms analogous to the isotropic radiator here.

Cheers

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Old 18th Feb 2012, 4:01 pm   #59
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Arrow Re: Receiver aerial input impedance: questions.

Yes John, I will concede that in practice it is almost impossible to build a 'front end' that will be purely resistive at all freqs.: stray C and L will always be present, but hopefully in a design stated as 'purely resistive', these quantities will be insignificant at the design freqs.

I've had further thoughts on this, following my comment above about 'characteristic impedance' and 'characteristic resistance' of transmission lines, and I'd like to submit them here for comments and further consideration.

Apart from my much earlier comments about what definitions you choose for 'resistance' compared to 'impedance' and under what set of circumstances which one you consider as an appropriate term to use, the field of electronics does seem to have acquired a vague and confusing set of definitions of the units of resistance & impedance. For example, there are the definitions of dynamic resistance (tuned circuits) and characteristic impedance (transmission lines). However, each contain reactive elements as well as resistance. So on that basis, it seems to me that the term 'dynamic resistance' is inappropriate and should be 'dynamic impedance'. And then there are the units of 'current opposition': ohms. This unit is effectively regarded as a scalar unit of measurement when it is also used for a vector quantity, as in impedance: clearly another inconsistency. On the other hand, however, a sharp discrimination is made when we consider 'power' in a.c. circuits: we have watts, volt-amps (VA) and volt-amps reactive (VAr): no inconsistency; no confusion arises. On that basis, it seems to me that the units of 'opposition to the flow of electric current' need to be re-defined: something like r-ohms (real ohms) for pure resistance, x-ohms for reactance and z-ohms for impedance. This problem of sloppy units and definitions extends to even elementary concepts: we have 'd.c. voltage' and 'd.c. current' - the latter translating to 'direct current current'! [I notice that M.G. Scroggie picks up on this in his use of the term 'z.f.' for 'zero frequency' to avoid such an anomaly: an excellent alternative term. ]

The point that I am trying to make here is that it is these contradictions and loose definitions that have contributed to some of the confusion over 'aerial input impedance' and 'aerial input resistance'. However, based on my measurements and calculations, (q.v. previous posts) I am now satisfied that I now know why the aerial 'input impedance' of a receiver is given the figure that it is.

And finally, to respond further to the Q. raised by Bazz4CQJ in post #48, one of the reasons behind this thread is to help me design the optimum impedance matching unit (an external device) to fit between my receivers and my "T" 'longwire' aerial. Again, that could generate another Thread, so to avoid this particular Thread from going off-topic, perhaps it will be for the best if that topic is not discussed here.

Al.

Last edited by Skywave; 18th Feb 2012 at 4:22 pm.
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Old 18th Feb 2012, 4:44 pm   #60
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Default Re: Receiver aerial input impedance: questions.

Yes, we seem to live in a "short cut" world where saving time seems to be more important than the job in hand resulting in a drop in standards.
I must confess that the term "dynamic resistance" with regard to tuned circuits, has either passed me by or it's something that I have carefully forgotten.However I found this interesting web page http://www.circuit-fantasia.com/circ...ance.htm#step2.

It wasn't long before I felt the need for beer and to get back to sorting out my spares.Strangely enough it's the resistor box today.

Better leave it there before we get too far off topic.
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