Topband: Slightly OT - amplifier noise
Michael Tope
W4EF at dellroy.com
Wed Mar 18 11:13:37 EDT 2020
You were spot on, Dave. Thanks for the suggestion. I rechecked the
numbers this morning and the AUX (i.e. sense antenna path) gain varies
about 2dB as a function of where the phase control is set. It appears I
had the phase control sitting in the worst-case minimum gain position
(apparently quite by accident) which seems to occur around "8" or "9".
The maximum gain occurs with the control fully clockwise at "10", so
there is a lot of variation between "7" and "10". Between a phase
control setting of "0" and "7" the change in the sense path gain is much
smaller. When you flip the phase polarity, there is a small amount of
gain change. The maximum gain numbers are pretty close to the main
antenna path numbers.
10 Meters: 2.8dB (0.5dB min)
15 Meters: 4.6dB (2.6dB min) [note: the 1.6dB number previously reported
may be an error]
Please take the decimal point with a grain of salt. These are not lab
grade measurements [think quick and dirty order-of-magnitude measurement
done with a fair amount of haste]. The cables and adapters I used should
probably be thrown away and replaced with better quality parts and it's
possible Nixon was still president the last time the 141T was calibrated
to NIST standards :-)
73, Mike W4EF.........
On 3/17/2020 9:56 AM, Dave Cuthbert wrote:
> Mike, thank and these are great data points you've provided.
>
> Questions:
> Was the MFJ-1026 PHASE control set near zero? That could account for
> the low AUX gain.
>
> *Mike's data*
> FT1000MP MK-V
>
> Main Antenna Path Gain:
> 10 Meters: 4dB
> 15 Meters: 3dB
>
> Sense Antenna Path Gain (sense-path pre-amp on and internal jumpers set
> for maximum gain):
> 10 Meters: 0.5dB
> 15 Meters: 1.6dB
>
> 12 to 14 dB increase in the noise floor on 10 meters when the MFJ unit
> was turned on
>
> *MFJ-1026 MAIN measured noise and calculations*
> Sherwood Engineering measured the FT-1000 MP MKV Field at -133 dBm in
> 500 Hz BW. This is -160 dBm in 1 Hz, or 2.2 nV/Hz^0.5.
>
> The 12 to 14 dB increase in the noise floor with the MFJ-1026
> activated tells us its output noise power is ~13 dB above -133 dBm, or
> -120 dBm in 500 Hz BW. This is -147 dBm, or *10 nV/H^0.5.* Given the
> measured AUX path gain of 0.5 dB this tells us the MFJ-1026 AUX
> input-referred noise is *~9 nV/Hz* on 10 meters. But wait, there's more.
>
> In my March 16 email I said "The MFJ-1025 [calculated] output noise
> is*9 nV/Hz^0.5."* But, given the MFJ-1026 measured AUX gain of 0.5 dB
> (10 meters) when I estimated 14 dB AUX gain (20 meters) is a huge
> discrepancy. This points to the phase shifter circuit attenuating the
> signal. Might the PHASE control knob be set to 0? This attenuates the
> AUX path.
>
> *Notes*
> The MFJ-1025 and MJF-1026 use the same circuit with the 1026 adding a
> whip antenna amplifier.
>
> *References*
> https://www.mfjenterprises.com/support.php?productid=MFJ-1026
>
> http://www.sherweng.com/table.html
>
> http://www.sherweng.com/documents/TermsExplainedSherwoodTableofReceiverPerformance-RevF.pdf
>
>
> Dave KH6AQ
>
> On Tue, Mar 17, 2020 at 4:07 AM Michael Tope <W4EF at dellroy.com
> <mailto:W4EF at dellroy.com>> wrote:
>
> I just happen to have a spare MFJ-1026 at home. I did a real quick
> test
> using my FT1000MP MK-V while powering the MFJ-1026 from an Astron
> supply. Similar to your results, Steve, I got about a 12 to 14 dB
> increase in the noise floor on 10 meters when the MFJ unit was
> turned on
> and connected to the MK-V's main antenna input versus when the MFJ
> unit
> was powered off. This suggests you would have to resort to having
> low-noise pre-amps ahead of each of the MFJ-102x two signal paths and
> the right amount of attenuation after the MFJ-102x's combined
> output to
> work in your particular very low noise situation.
>
> Since the main antenna path is normally bypassed for transmit, to
> put a
> pre-amp ahead of the main antenna path you would have to either move
> that MFJ-102x out of the transmit path by putting it in a
> receive-path
> breakout-loop that is common on many modern transceivers (e.g.
> between
> RX_OUT and RX_IN jacks), or you would have to have add a dedicated
> T/R
> bypass scheme for the external main path pre-amp.
>
> FWIW, I also made some gain measurement on my MFJ-1026 (both main and
> sense path gain controls fully clockwise):
>
> Main Antenna Path Gain:
> 10 Meters: 4dB
> 15 Meters: 3dB
>
> Sense Antenna Path Gain (sense-path pre-amp on and internal
> jumpers set
> for maximum gain):
> 10 Meters: 0.5dB
> 15 Meters: 1.6dB
>
> Difference between having Sense Path Pre-Amp On vs Off:
> 10 Meters: 8dB
> 15 Meters: 9dB
>
> I used an Elecraft XG3 as the signal source and an HP141T to measure
> amplitude.
>
> 73, Mike W4EF...................
>
>
>
>
> On 3/16/2020 9:47 AM, Steve London 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
> > _________________
> > Searchable Archives: http://www.contesting.com/_topband - Topband
> > Reflector
>
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