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 <k7...@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....@contesting.com> On Behalf Of > Dave Cuthbert > Sent: Monday, March 16, 2020 2:05 PM > To: n...@arrl.net > Cc: Topband <topband@contesting.com>; Guy Olinger K2AV <k2av....@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 <n2ica...@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 <n2ica...@gmail.com > > > <mailto:n2ica...@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 > > > _________________ > > > Searchable Archives: http://www.contesting.com/_topband - > > > Topband > > Reflector > > > > > > -- > > > Sent via Gmail Mobile on my iPhone > > _________________ > > Searchable Archives: http://www.contesting.com/_topband - Topband > > Reflector > > > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ > Searchable Archives: http://www.contesting.com/_topband - Topband > Reflector > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector