Topband: Slightly OT - amplifier noise

Mon Mar 16 18:05:28 EDT 2020

Circuit gain is accounted for in my calculations. I ran a sim of the input
amp and could get it to work with gains from 0.5 to 2 using different
transformer ratios. So, I think the gain-of-2 I used is good enough.

On Mon, Mar 16, 2020 at 11:32 AM Lee STRAHAN <k7tjr at msn.com> wrote:

>   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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