B81 mod to improve LF sensitivity

[K H]

Hello,

I'm still testing about LF and MF sensitivity.

Last week I've replaced T2 from the original to the bigger BN73-202 and now I can get 3 or 5 dB improvement of SNR on 630m band WSPR.

Next I try to add the 1uF to 10uF ceramic capacitor parallel to the input coupling capacitor of B81.

But I can NOT find B81 on the PCB because of my poor eyesight even though using the loupe. Is there big photo of the PCB or does anyone teach me the location of B81?

And my additional question is that also the B83 and B84 (input of the ADC) should be replaced with bigger value or not?

JA9MAT Hidehiko.

[Steve Haynal]

Hi Hidehiko,

B81 is on the back side of the PCB and shown circled in yellow in the attached picture.

B81, B83 and B84 are all DC blocking capacitors. It will be interesting to hear if increasing the value provides better performance for lower frequencies. You should make sure to use NPO, XR5 or XR7 capacitors that still have good performance at higher frequencies. There are some details on B83 and B84 on page 33 of:

https://www.analog.com/media/en/technical-documentation/data-sheets/AD9866.pdf

73,

Steve

kf7o

[K H]

Hi Steve,

OK, Thanks for your support again!

I will order a few of chip capacitor and try.

I'm enjoying the improvements of HL2 for LF radio.

73, JA9MAT Hidehiko.

[K H]

Hi Steve,

I think It's a better procedure to add 1uF, 2uF and 10uF parallel with B81 sequencially.

But I do not want to damage to precision PCB. I think my chance is a one time. So from the first I put the 10uF25V "MURATA" ceramic capacitor parallel with B81. This has low ESR / ESL and good high frequency characteristics, and the temperature characteristics are X7S.

I've tried to put my modification photos with pdf file but does not allow (maybe) by the security policy. So I report in text as follows.

In the daytime the reception on 17m and 20m band is no problem and I've tested on 2200m and 630m band last night.

 

The result is good on 2200m band and also fairly good on 630m band. But not decoded DX until now.

The results of the measurement of sensitivity limit by hp8656B Signal Generator are follows;

 

1) On 2200m band, after the B81 mod, the limit sensitivity is from 117dBm to -122dBm, 5dBm up.

2) On 630m band, the limiti sensitivity is -127dBm the same with before modification. But the signal can be heard stronger than before.

3) The other all band covers -127dBm of limiti sensitivity.

I think its fairly good and no longer to replace B83 and B84.

This is information for LF/MF enthusiast.

73, JA9MAT Hidehiko.

[Steve Haynal]

Hi Hidehiko,

Thanks for your report! I agree that putting an extra capacitor "piggy back" on the existing B81 and maybe B83 an B84 is a good way to go for LF/MF enthusiasts.

Maybe the picture was too large for attachment? There are not security settings which will prohibit photos.

73,

Steve

kf7o

[K H]

Hello Steve,

I found the file attach issue is caused by the browser. I'm using "BRAVE" but
 it is not allowed to attach files to this groups.

Well I change the browser to "Firefox" and attach pdf file about my modification.

73 and thanks for your kindness support.

JA9MAT Hidehiko.

[Bob Tenty]

Hi Hidehiko,

I wonder if you also measured the sensitivity between 10 and 136 KHz with your modification?

73,

Bob VE3TOK

[Brad Brooks]

Hidehiko-san, thank you for posting the details of your experiments. It looks like the modification is worth consideration for improved overall sensitivity. 73 Brad WF7T

[K H]

Hello Bob,

My SG is out of spec. below 100kHz but anyway I've just measured them as an attached file.

I feel the sensitivity (audible CW tone) is gently down to 10kHz.

Do you receive SAQ or another VLF utility? Here time standard station of JJY at 40kHz and 60kHz are heard anytime with HL2+Miniwhip antenna. I tried to catch SAQ but not yet achieved. Next chance I will try with HL2.

73, JA9MAT Hidehiko.

[K H]

Hello Brad,

Yes, I also surprised about that.

IN3OTD, Claudio measured the BN73-2402 frequency response before.
According to his result It shows better frequency response overall the HF frequency better than the original T2.

My modification is based on his information and Steve's kindness support.

I thank them.

73, JA9MAT Hidehiko.

[Bob Tenty]

Hello Hidehiko,

Thank you very much for the measurements. I can receive a lot of radio beacons and other utility stations here but never received SAQ as yet.

73,

Bob VE3TOK