We seek to understand and document all radio transmissions, legal and otherwise, as part of the radio listening hobby. We do not encourage any radio operations contrary to regulations. Always consult with the appropriate authorities if you have questions concerning what is permissable in your locale.

Author Topic: Modifying the SW receiver for a lower noise figure  (Read 9607 times)

Capt. Radio

  • Guest
Modifying the SW receiver for a lower noise figure
« on: November 20, 2013, 1600 UTC »

I'm wondering if anyone on the forum has modified their SW receiver to have a lower noise "front end" for better reception at roughly 20 mhz to 30 mhz? it's well known (I think) that most gen coverage receivers could be improved at the higher end for better higher freq. sensitivity.

I'm looking at the schematic of my Yaesu FRG-100, and I see it uses two 2SK125 FET's in the "front end". As these devices establish the noise figure for the radio, I'm wondering if I could improve 11m-10 meter performance with better devices than the present FET's?

Anyone have any input on this?


Online redhat

  • DX Legend
  • ******
  • Posts: 1395
  • Karma: +32/-4
  • USA
  • Music is my drug.
    • View Profile
    • Email
Re: Modifying the SW receiver for a lower noise figure
« Reply #1 on: November 20, 2013, 2010 UTC »
I could be wrong, but I thought that at these frequencies, noise figure is of little merit, as the noise power present at the input to a receiver is generally higher than the self-induced noise figure of the receiver front end.  Usually when your worried about noise figure is north of 400 MHz, and into the C and KU ranges.

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Capt. Radio

  • Guest
Re: Modifying the SW receiver for a lower noise figure
« Reply #2 on: November 20, 2013, 2034 UTC »
You could be right redhat. Of course, 10 and 11 meters is getting close to the VHF range. I generally don't have much man-made noise bothering me at those freqs, so I wondered if there were better devices available that would make a noticeable improvement worth the time/cost/effort.

After all, the Yaesu "frog" isn't state of the art anymore.
« Last Edit: November 20, 2013, 2245 UTC by Capt. Radio »

Capt. Radio

  • Guest
Re: Modifying the SW receiver for a lower noise figure
« Reply #3 on: November 22, 2013, 0333 UTC »

Well, in the absence of replies from readers that have experience with this sort of mods on their receivers, I did a little searching on my own.

It seems the 'ole standby FET for this vintage receiver as well as many others, was the 2SK125. It has been discontinued, but I'm sure it can still be purchased.

Looking at my receiver, the FET's are in a balanced mixer configuration as the "front end". There is no RF preamp section ahead of the mixer... only bandpass filtering.

So... it seems that the J310 FET is the device of choice for updating the 2SK125's. The two 2SK125 FET's are on the underside of the main board, in a SOT-23 package... not much bigger in size than a surface mount chip cap or resistor.

It's hard to determine if there will be much in the way of improvement in performance. I would assume the main "gain" would be in the ability to handle larger sigs without "overload". I tried to compare noise figures but the specs used different freqs for their noise figures. I would hope to see a bit better performance on the 10-11 meter bands, which is what I'm looking for.

I understand that any "gains" might only be seen on a test bench, and not by casual tuning through the bands. The only "negative" I see is that I may have to rebalance the mixer after the swap-out. The devices are pretty cheap, so cost isn't a drawback.

I've never been one to "leave well enough alone" (or if it ain't broke don't fix it).  :D

I'll most likely give this a shot in a few weeks. Before I do, I should try and document some signal levels from LW to 10 meters. Unfortunately, I don't have the equipment to do a real check on stats such as image rejection, sensitivity above 11 meters, etc.

Until then...


Online redhat

  • DX Legend
  • ******
  • Posts: 1395
  • Karma: +32/-4
  • USA
  • Music is my drug.
    • View Profile
    • Email
Re: Modifying the SW receiver for a lower noise figure
« Reply #4 on: November 22, 2013, 1104 UTC »
Looking at the datasheet, the J310 has 3dB of noise figure at 450 MHz, the 2SK124 is spec'd at 1.8 at 100 MHz; Hard to say what will happen if you replace them.  It does appear plausible that you may pick up about 3-4 DB more power gain, but its hard to tell without modeling it.  Its kinda like tuning a bike without a dyno...

...Just my 2

+-RH
Somewhere under the stars...
WinRadio Excalibur/305 w/ a chi-town resonant loop, Kenwood KDC-U356 for mobile listening.
Please send QSL's and reception reports to xfmshortwave [at] gmail [d0t] com

Capt. Radio

  • Guest
Re: Modifying the SW receiver for a lower noise figure
« Reply #5 on: November 22, 2013, 1412 UTC »
Yep, your right that it's difficult to get a "handle" on the results by the spec sheets.

However, I guess this is what's the fun part of the hobby. Just getting the soldering iron out and giving it a try!

I believe the J310 is a better device all around, swapping out should be a "positive" instead of a negative change.

Anyway, the devices are really cheap to purchase, so it won't hurt the pocketbook to give it a try.

p.s. After more study, I've found a good test for reciever sensitivity. If you listen to the reciever noise before you connect the antenna, and compare the noise after you hook up the antenna, if the noise is greater after, then no increase in Rf amplification is helpful.
« Last Edit: November 22, 2013, 1515 UTC by Capt. Radio »

Offline ka1iic

  • Hero Member
  • *****
  • Posts: 902
  • Karma: +0/-0
  • Troy, Ohio
  • Troy, Ohio. 20m Vertical & low long wire E/W,
    • View Profile
    • Email
Re: Modifying the SW receiver for a lower noise figure
« Reply #6 on: November 23, 2013, 0009 UTC »
Isn't the noise factor more a function of the mixer stage and not the RF stages...  or maybe I don't understand your question.

Mixers are by their nature are quite noisy and the RF stages are such as to off set that fact.

At HF frequencies the noise level is dictated more by external conditions rather that the 'front end' of a receiver.  But 10 meters etc is a 'crossing point' for noise...  but still more a function of mixer stage and not the RF stage.

I had an old NC183 and I changed the 6SG7 (pentode) RF stages with 6DJ8 cascode duo-triodes, it  did help BUT... That was an early 1950's design.  The mixer noise continued tho... So it was a win-win/lose...  Disconnect the RF stages and listen to the noise... If the noise continues, it's the mixer and NOT the RF stages.  that's the only real way to find out what is causing the noise.

It's worth a try (only because I like to do mods) but save yourself some grief and install transistor sockets at the point of interest and you can switch in what you may have as you go... saves a lot of time.

Not sure if this will help you but here it is... :-)

73
Vince
KA1IIC


73 Vince
KA1IIC

"If you can't be anything, you can at least be annoying"

Troy, Ohio. 20m Vertical & low long wire E/W, Yaesu FT-187ND, SDRplay 2, Ratt Shack 2 meter rig, and other little bits of electronics I'm not talking about, homebrewed and otherwise... so there bleech!

Capt. Radio

  • Guest
Re: Modifying the SW receiver for a lower noise figure
« Reply #7 on: November 23, 2013, 0212 UTC »
Vince,

My particular reciever has no RF amp ahead of the balanced mixer stage. The only components ahead of the balanced mixer and after the antenna jack is the switchable bandpass filtering (frequency dependent components).



So, I would say this mixer sets the noise figure and other important receiver parameters. I'm thinking better devices in this first stage would be a "plus" in more ways than sensitivity.

The transistor sockets would be a good idea if the txstrs were the conventional type. The receiver uses surface mount technology, so the 2SK123 and J310 devices are SOT-23 design. They are so small they don't lend well to socketing. (I wish it were otherwise).

Offline ka1iic

  • Hero Member
  • *****
  • Posts: 902
  • Karma: +0/-0
  • Troy, Ohio
  • Troy, Ohio. 20m Vertical & low long wire E/W,
    • View Profile
    • Email
Re: Modifying the SW receiver for a lower noise figure
« Reply #8 on: November 23, 2013, 1752 UTC »
No RF stage ehhh?

Well in that case I would go with an external pre-selector first and see how that works...

But that's only me... I hate to work on surface mount stuff,,,, one sneeze and the part is lost to the carpet ;-) been there...

let me know what you got going....

73
Vince
ka1iic
73 Vince
KA1IIC

"If you can't be anything, you can at least be annoying"

Troy, Ohio. 20m Vertical & low long wire E/W, Yaesu FT-187ND, SDRplay 2, Ratt Shack 2 meter rig, and other little bits of electronics I'm not talking about, homebrewed and otherwise... so there bleech!

Capt. Radio

  • Guest
Re: Modifying the SW receiver for a lower noise figure
« Reply #9 on: November 28, 2013, 0207 UTC »
For now, I'm going with the J301's only. They're on their way, not sure when they'll arrive due to the holiday.

As stated, I'm going to take written and mental notes how the rcvr works presently... LW, BCB, low and Hi freq SW. S-meter signal levels, overload signal capability, etc. Wish I had access to some good test equipment to see things like image rejection, dynamic range, etc.

I know, I know... some will say "just buy a better receiver, you cheapskate"!  ;D

I'll post any difficulties and the final outcome... good or bad.

p.s. As far as an RF preselector goes, I am thinking of making a dual J301 FET RF amp for my small 19" tuned loop for 10 and 11 meters. I have a few J301's to use that are the normal size transistor configuration, so it will be easier to make the amp with those.

 
« Last Edit: November 28, 2013, 0209 UTC by Capt. Radio »