Topband: Slightly OT - amplifier noise

Mon Mar 16 17:32:00 EDT 2020

  Don’t forget there is gain in the signal paths. That first AUX input stage is a Norton configuration amp with gain controlled by the unknown transformer ratios. The two J-310's that sum the signals also appear to have a few dB gain. A full analysis of the circuit is necessary to derive what the insertion gain of this device would be. Or a measurement might be in order with gain pots at max. It could be amplified input device noise at the output.
Lee   K7TJR

-----Original Message-----
From: Topband <topband-bounces+k7tjr=msn.com at contesting.com> On Behalf Of Dave Cuthbert
Sent: Monday, March 16, 2020 2:05 PM
To: n2ic at arrl.net
Cc: Topband <topband at contesting.com>; Guy Olinger K2AV <k2av.guy at gmail.com>
Subject: Re: Topband: Slightly OT - amplifier noise

Steve,

I performed a hand calculation of the MFJ-1025 14 MHz noise and it's (only)
9 nV/Hz^0.5, or 0.2 uV in 500 Hz. This is -120 dBm, or S-1. But you measure
S-4 noise. What is going on?

Questions to determine what is going on:

    Question 1) What model is the radio?
    Question 2: What bandwidth is the noise measured with?
    Question 3: What band?
    Question 4: What does the radio  measure connected to the MFJ-1025 but with the MFJ powered OFF?

*MFJ-1025 termination *
With the two MFJ-1025 antenna terminals left open the two JFET inputs are terminated in 0-78 ohms depending on the gain pot settings. Note that gain pots reduce the signal but not JFET noise and cutting the gain degrades the S/N ratio. With the gain pots at maximum the unterminated input noise from the resistors is (only) 1.1 nV/Hz^0.5.

*MFJ-1025 noise calculations*
Let's add up all the noise sources and derive the 14 MHz noise.

*Q4 output buffer*
   Q5 voltage gain, 1
   1 nV/Hz^0.5 transistor (my estimate; I can calculate it but it's low enough to just call it 1 nV/Hz^0.5)
   1.9 nV/Hz^0.5 for R8 (220 ohms)
   Total input noise 2.2 nV/Hz^0.5 (the transistor noise is my estimate)
   Total output noise 2.2 nV/Hz^0.5

*Q5, Q6 active combiner*
   Q5 voltage gain, 1.1
   R9||R26, 1.1 nV/Hz^0.5
   Q5 gate, 0.6 nV/Hz^0.5
   Input noise, 1.3 nV/Hz^0.5
   Output noise, 1.4 nV/Hz^0.5

   Q6 voltage gain, 2.3
   R13, 1.9 nV/Hz^0.5
   Q6 gate, 0.6 nV/Hz^0.5
   Input noise, 2.0 nV/Hz^0.5
   Output noise, 4.6 nV/Hz^0.5

   Q5 + Q6 noise
   Output noise, 4.8 nV/Hz^0.5

   *Q7 phase splitter*
   Q7 voltage gain, 1
   Q7 gate, 0.6 nV/Hz^0.5
   R13, 1.9 nV/Hz^0.5
   Output noise, 2.0 nV/Hz

   *Q8 AUX amp *
   Q8 voltage gain, 2 (that's a guess)
   R20||R27, 1.1 nV/Hz^0.5
   Q8 gate, 0.6 nV/Hz^0.5
   Input noise, 1.3 nV/Hz^0.5
   Output noise, 2.6 nV/Hz^0.5

*Adding it up*
The noise into Q6 is the AUX amp (2.6 nV/Hz^0.5) + the phase splitter (2.0
nV/Hz) = 3.3 nV/Hz^0.5

The noise out of Q6 is X2.3 the input noise = 7.5 nV/Hz The noise out of Q5 is 4.8 nV/Hz^0.5 Adding two these together the noise into Q4 is 8.9 nV/Hz Add 1 nV/Hz^0.5  for Q1 and we have 9.0 nV/Hz

The MFJ-1025 *output noise is 9 nV/Hz^0.5*

In a 500 Hz BW this is 200 nV, or 48 dB below S-9. *This is S-1*.
The AUX-referred noise is ~1.7 nV/Hz^0.5 The MAIN-referred noise is ~8 nV/Hz^0.5

*12V Power is well decoupled*
The 12V power is well decoupled with 14 MHz PS rejection ratio of 110 dB.
This is calculated for bypass cap C7 having 10 nH lead inductance. So, I don't see that noise on the 12V source makes a difference.

*LTSpice J310 noise model issue*
I ran sims of a J310 JFET in a simple source-follower circuit and LTSpice reports noise that is so far off it's unusable. Instead I use a claimed noise figure of 3 dB. If I use the specified 100 Hz noise of 10 nV/Hz^0.5 the output noise rises enough to make S-4 noise. But, I believe the 100 Hz noise is below the J310 1/f frequency and is much higher than the HF noise.
I think I can calculate the 1/f frequency to make sure. About 10 years ago I designed a test setup to measure MOSFET noise and ran thru the calculations for 1/f and noise.

*References*
MFJ-1025 manual with schematic:
https://www.mfjenterprises.com/support.php?productid=MFJ-1025

J310 datasheet:
http://pdf.datasheetcatalog.com/datasheet_pdf/intersil/J308_to_J310.pdf

J310 RF amp noise figure: http://receiverforjupiter.tripod.com/rfamp.htm

                Dave KH6AQ

On Mon, Mar 16, 2020 at 8:33 AM Steve London <n2icarrl at gmail.com> wrote:

> I did that - terminated in 50 ohms.
>
> 73,
> Steve, N2IC
>
> On 03/16/2020 12:27 PM, Guy Olinger K2AV wrote:
> > Depending on the exact circuit and device, to measure noise from the
> device
> > itself, don’t you have to terminate the inputs to put the normal
> impedance on
> > them? The circuits are often designed with the coax Z zero in mind.
> >
> > 73, Guy K2AV
> >
> > On Mon, Mar 16, 2020 at 12:47 PM Steve London <n2icarrl at gmail.com 
> > <mailto:n2icarrl at gmail.com>> wrote:
> >
> >     This has been a very interesting thread - Thanks for all the input.
> >
> >     Perhaps I have set my expectations too high.
> >
> >     A typical application is on 15 meters, late in the opening, 
> > working
> JA's from
> >     here in SW New Mexico. Absent any local QRN, the band is very quiet.
> Any local
> >     noise covers the bottom layer of 5 watt JA's calling me.
> >
> >     I did some more testing with the MFJ-1025. With no antennas
> connected, the box
> >     raises the noise floor about 10 dB, irrespective of the Aux 
> > Antenna
> Gain or the
> >     Main Antenna Gain. I haven't yet tried powering from a battery, 
> > to
> make sure
> >     the
> >     power supply isn't the source. Assuming the J310's are quiet, 
> > that
> leaves the
> >     2N5109 emitter follower, or the back-to-back protection diodes.
> Might also try
> >     disconnecting the RF sense circuit.
> >
> >     73,
> >     Steve, N2IC
> >     _________________
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> >
> > --
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