## Re: [softrock40] Re: Noise Figure Question

Expand Messages
• With a 1:2 input transformer, the secondary should see 200 ohms as you say so that the input remains at 50 ohms for filters, etc. The lowest noise commonly
Message 1 of 15 , Mar 15, 2013
With a 1:2 input transformer, the secondary should see 200 ohms as you say so that the input remains at 50 ohms for filters, etc.

The lowest noise commonly available RF op-amp I'm familiar with is  the OPA847 from TI, with a noise input voltage of 0.85 nV/sqrt(Hz).  You have to be careful with it (3.9 GHz bandwidth and some restrictions on permitted stable gain) but it will deliver a NF in the 10 dB range and below.  Plugging its noise parameters into my spreadsheet shows 9.1 dB NF but I have not checked the default resistor values to see if they are permitted for the OPA847.

Jack

On 3/14/2013 11:20 PM, imitchel4you wrote:

Hi Jack

I found a problem. The spreadsheet uses fA not pA so I was out by 1000 for the current noise. The noise parameters should be:

I-Noise 1hz: 50,000 fA (ie 50nA)
I-Noise 1MHz: 32,0000 fA (ie 32nA)

This gives a NF of 14.6dB with the resistor values chosen. Much closer to your estimate.

With a 1:2 transformer I'm not sure what resistor values to use for Rs, R1 and R2. I assume from a circuit point of view R1 should be 200 so that any input filter sees 50 ohms. R2 400 to counter the 6db loss to match the output into 50 ohms. I put Rs at 100 and R1 at 100 and NF is 15.6 an overall NF of 9.6.

With Rs and R1 at 200 the NF is 15.7 - so seems about right.

--- In softrock40@yahoogroups.com, Jack Smith <Jack.Smith@...> wrote:
>
> No, I didn't use that calculator. I developed my own spreadsheet
> analysis for the non-inverting case from the equations in TI's
> Application Note at:
>
> http://www.ti.com/lit/an/slyt094/slyt094.pdf
>
> Another useful ApNote from TI is at http://tec.upc.es/ie/AOnoise.pdf
>
> In addition to matching the sample calculations in the ApNote, I've
> measured good agreement between the results in my spreadsheet and
> measured noise with an HP 8970A NF meter, so I'm reasonably confident
> that (a) the ApNote equations are correct and (b) I've properly
> implemented them in the spreadsheet. Reasonable confidence <> 100%
> certainty, of course.
>
>
> Inverting and non-inverting modes don't yield the same NF from what I
> can determine, with inverting being a bit better. And there is some
> opportunity for optimization in picking the various resistor values, so
> your 12 dB calculation is plausible. That also suggests a 1:2 input
> transformer might get the NF down to 6 dB or so, which is starting to
> look useful.
>
> Jack
>
>
> On 3/14/2013 7:49 PM, imitchel4you wrote:
> >
> >
> >
> > Hi Jack
> >
> > I found the TI calculator here: http://www.ti.com/tool/opamp-noisecalc
> >
> > Is this the one you are referring to?
> >
> > Anyway, I set up the parameters for the LT1229 as follows:
> > V-Noise 1Hz: 10
> > V-Noise 1MHz: 3.2
> > I-Noise 1hz: 50
> > I-Noise 1MHz: 32
> > UGBW: 100MHz
> > Input C: 3
> >
> > On the InvAmpNoise page I used the following parameters:
> > Rs: 50
> > R1: 50
> > R2: 200
> > R3: 0
> >
> > The result was a NF of about 12db. Does this seem ok to you? Did I do
> > something wrong?
> >
> > I note that R3 (connected to the opamp non-inverting input) makes a
> > significant difference to the NF which I've set to zero because it is
> > AC coupled to ground and only provides the bias voltage. Is that correct?
> >
> > Cheers, Ian
> >
> > --- In softrock40@yahoogroups.com
> > <mailto:softrock40%40yahoogroups.com>, Jack Smith <Jack.Smith@> wrote:
> > >
> > > I compute the NF of an LT1229 as 17.6 dB when configured for 16.4 dB
> > > gain. Methodology is as per a TI/National Semiconductor application
> > note
> > > that I then put into a spread sheet.
> > >
> > > 17.6 dB NF isn't all that great as a low noise preamp, in fact, it's
> > > pretty poor in that regard. The Norton amplifiers I build clock in
> > > around 2.5 dB (measured at 10 MHz with an HP noise figure meter). I
> > > have yet to see an op-amp that comes close to the NF a noiseless
> > > feedback amplifier built with discrete semiconductors can demonstrate.
> > >
> > > That being said, consider that a typical HF receiver will have a NF in
> > > the 10-15 dB range and that is considered to be acceptable. As an
> > > example, the ARRL measured an Elecraft K3's NF at 14 MHz as being 9 dB
> > > with the preamp on and 12 dB with the preamp off. The reason the preamp
> > > only improved the NF 3 dB, in my educated guess, is that a good part of
> > > the composite NF is due to input filter losses and the like ahead of
> > the
> > > receiver's first active stage.
> > >
> > > As Warren notes, gain in an HF receiver does not fall into the "if a
> > > little bit is good, then a whole lot must be even better" category.
> > >
> > > In particular cases, such as an ultra-low noise QTH or a very
> > > inefficient antenna, a low noise preamplifier can benefit a receiver,
> > > and there are some receivers that are on the deaf side where a bit of
> > > additional low noise gain can be beneficial.
> > >
> > > There are also ways of reducing the NF of an op-amp based preamp.
> > > Probably the easiest is implement is to take advantage of the op-amp's
> > > high impedance input and use a 1:2 or 1:3 turns ratio step up
> > > transformer on the input. This will give you 0 dB NF gain equal to the
> > > turns ratio (1:2 is 6 dB). Actually there will be a small NF increase
> > > over 0 due to transformer loss, but that's usually on the order of a
> > few
> > > tenths of a dB for a properly designed transformer. I've built and
> > > measured this improvement, for example improving an AD8007 current
> > > feedback amplifier's NF from 17 dB to 11 dB with a 1:2 input
> > > transformer. (I'm going by memory on these numbers and I could be off a
> > > dB or so.)
> > >
> > > Jack K8ZOA
> > >
> > > On 3/13/2013 7:21 PM, warrenallgyer wrote:
> > > >
> > > >
> > > > Ian
> > > >
> > > > I am in the same boat as to figuring noise figure so I cannot help
> > you
> > > > there.
> > > >
> > > > I am wondering however what it is you are trying to accomplish by
> > > > pre-selection tuning then there may be some benefit. If you are
> > > > it for system gain then I think you are probably going to be
> > > > disappointed in the results.
> > > >
> > > > Some of us are advocating reducing the gain of the basic Softrock by
> > > > 6-10 dB. There seems to be plenty of system gain in the base
> > circuitry
> > > > as witnessed by anywhere from 8-25 dB of antenna noise rise across
> > the
> > > > HF band. The limiting factor in dynamic range currently is the
> > > > setting and audio card) or the 6231 op amp. In most builds the op
> > amps
> > > > saturate with input signals in the -12 dBm range. If you add a preamp
> > > > you will move this overload point down by the gain of the preamp
> > > > leaving the receiver much more susceptible to overload.
> > > >
> > > > Because of the existing noise floor rise when the antenna is
> > connected
> > > > you can expect no improvement in weak signal performance by adding a
> > > > preamp. Finally, by adding active gain at this point you introduce
> > > > another choke point for potential overload and IMD3 responses.
> > > >
> > > > Maybe you already know all of this. But it seems to me a preamp will
> > > >
> > > > Best regards
> > > >
> > > > Warren Allgyer - W8TOD
> > > >
> > > > --- In softrock40@yahoogroups.com
> > <mailto:softrock40%40yahoogroups.com>
> > > > <mailto:softrock40%40yahoogroups.com>, "imitchel4you"
> > > > <imitchel4you@> wrote:
> > > > >
> > > > > Dear All
> > > > >
> > > > > I'm looking at using the LT1229 as an RF pre amp up to 30MHz (12db
> > > > gain). It has a voltage noise of 3.2 nv/rtHz and current noise into
> > > > the -ve input of 32 pa/rtHz. I'm unsure if this is sufficiently low
> > > > noise to be used as a pre amp. There seems to be complex maths
> > > > involved to determine the noise figure but even if I could I'm not
> > > > sure what is an appropriate noise figure.
> > > > >
> > > > > I know there are MMIC amps that would be appropriate. I would like
> > > > to know if the LT1229 would also work since it can be configured in
> > > > differential mode to drive a sampling detector.
> > > > >
> > > > > Cheers, Ian
> > > > >
> > > >
> > > >
> > >
> >
> >
>

• ... Subject: [softrock40] Re: Noise Figure Question ... I was just checking to see how much, or little, noise we may expect.
Message 2 of 15 , Mar 15, 2013
----- Original Message -----
Subject: [softrock40] Re: Noise Figure Question

> Alan and I have been having an offline exchange and we would both be interested in input from the group.
>
> Between us we have 3 RXTX builds for 10/12/15 meters. We would be interested to hear from others how much, if any, noise rise you
> see when the antenna is connected on 10 meters.
>

I was just checking to see how much, or little, noise we may expect.
http://www.ofcom.org.uk/static/archive/ra/topics/interference/documents/rsgb.pdf gives some idea. Now what does, for example, a
field strength of -20dB(uV/m) convert as dBm at the antenna terminals?

Is this right? Add about 2dB for antenna gain of a dipole = -18dBuV at the antenna terminals.
1dBuV = -107dBm into 50 Ohms. So -125dBm, 0.1uV at the antenna??

Now we have to know how this shows after FFT has done it's job.
Although could this figure could be compared with MDS measurements?

73 Alan G4ZFQ

> To see and measure this consistent with our measurements you would need to use HDSDR with the lower spectrum display set for RBW
> of 1.5 Hz. The lower left slider all the way to the right will expand the scale to 1 dB resolution. The second slider from the
> left will move the scale up and down so you can see the noise floor. If you set Avg to 128 the noise floor will smooth out after a
> few seconds and you can get a +- 2 dB measurement.
>
> We would both be interested to hear the difference between this number with antenna connected and without.
>
> Thanks so much for your input!
>
> Warren Allgyer - W8TOD
>
> --- In softrock40@yahoogroups.com, "Alan" <alan4alan@...> wrote:
>>
>>
>> ----- Original Message -----
>> Subject: [softrock40] Re: Noise Figure Question
>>
>> Warren,
>>
>> Take any absolute figures lightly, I think my -73dB is no worse than +/- a few dB.
>>
>> So 10m noise floor Delta 44 16 bit driver with HDSDR set with RBW 1.5Hz is:-
>> 50R -142dB
>> Antenna Cobwebb, dipole (equivalent) 10m up 40m RG213 -141dB
>> Antenna + 5dB preamp -137,-138dB
>>
>> Antenna 2 Inv V doublet fed @10m but immediately above bungalow. No preamp -134, -135dB
>>
>> RX subjective test between the two antennas. Found weak SSB signals, Cobwebb + preamp maybe 3 readability points better.
>> Without preamp, maybe 1 readability point better.
>>
>> The 40m of feeder is an unknown. Should be less than -2dB but it's been there a long time. Must try some TX comparisons rather
>> than
>> take it down to check. Maybe the preamp is just compensating for feeder loss.
>> From the noise figures it might seem that the Cobwebb/no preamp situation is about right but the preamp definitely helps.
>>
>> 73 Alan G4ZFQ
>>
>>
>> > Yes, if you do not get an antenna noise rise then I would expect a preamp would improve receiver performance. I am surprised
>> > though because both of my 10/12/15 builds exhibit 6-10 dB of antenna noise. You have mentioned before that you have a
>> > relatively
>> > quiet QTH and this may be the explanation.
>> >
>> > Another explanation could be a high noise floor on the audio interface you are using. If you want we can compare some absolute
>> > numbers I just took a look at one of my rigs on 10M using the internal Realtek interface. After S9 calibration at -73dBm I
>> > disconnected the antenna and read a noise floor of -129 dBm. A 5 meter piece of wire hung out the window gives me a 6 dB rise
>> > to -123 dBm.
>> >
>>
>> >
>> >> At the higher end of HF antenna noise is lower and is below my unmodified Softrock 6.3's noise level.
>> >> As a crude test I setup a weak remote source on 10m and received it with a better S/N with an 8dB Norton preamp.
>> >> This was prompted by a simultaneous WSPR RX check between the Softrock and a FT840 where without a preamp the Softrock was
>> >> inferior.
>>
>
>
>
>
> ------------------------------------
>
>
>
>
• There s a frequency factor that must be included when converting between field strength in uV/m and signal power. From
Message 3 of 15 , Mar 15, 2013
There's a frequency factor that must be included when converting between field strength in uV/m and signal power.

From http://www.softwright.com/faq/engineering/FIELD%20INTENSITY%20UNITS.html

E(dBµV) = E(dBµV/meter) + Gr(dBi) - 20log f(MHz) + 29.8

P(dBm) = E (dBµV) - 107

P(dBm) = E (dBµV/m) + Gr(dBi) - 20log F(MHz) - 77.2

where P(dBm) is received power into 50 ohms, in dBm
E is the received signal voltage in dBuV
Gr is the receiving antenna gain in dB with respect to isotropic
f is frequency in MHz
E is the field intensity in dBuV/meter

At 28 MHz, the -20 log f(MHz) almost exactly cancels the +29.8 so the received signal voltage equals the field strength plus antenna gain. But that's not the case generally.

On-line calculators relating these factors can be found at many places on the Internet, e.g., http://www.giangrandi.ch/electronics/anttool/field.html

Jack K8ZOA

• ... Subject: [softrock40] uV/m to dBm ... Thanks for giving me the relavant information Jack. I did actually find these two sites but my mathematics teacher
Message 4 of 15 , Mar 15, 2013
----- Original Message -----
Subject: [softrock40] uV/m to dBm

> There's a frequency factor that must be included when converting between
> field strength in uV/m and signal power.
>

Thanks for giving me the relavant information Jack.
I did actually find these two sites but my mathematics teacher never told me all those formulae could be useful and interesting so I
tend to run a mile when I see them! So I missed the frequency related factor.

Now I make sense of the calculator.

73 Alan G4ZFQ

> From
> http://www.softwright.com/faq/engineering/FIELD%20INTENSITY%20UNITS.html
>
> E(dBµV) = E(dBµV/meter) + Gr(dBi) - 20log f(MHz) + 29.8
>
> P(dBm) = E (dBµV) - 107
>
> P(dBm) = E(dBµV/m) + Gr(dBi) - 20log F(MHz) - 77.2
>
> where P(dBm) is received power into 50 ohms, in dBm
> E is the received signal voltage in dBuV
> Gr is the receiving antenna gain in dB with respect to isotropic
> f is frequency in MHz
> E is the field intensity in dBuV/meter
>
> At 28 MHz, the -20 log f(MHz) almost exactly cancels the +29.8 so the
> received signal voltage equals the field strength plus antenna gain. But
> that's not the case generally.
>
> On-line calculators relating these factors can be found at many places
> on the Internet, e.g.,
> http://www.giangrandi.ch/electronics/anttool/field.html
>
> Jack K8ZOA
>
>
Your message has been successfully submitted and would be delivered to recipients shortly.