K3 Buffer Mod

[Don Rasmussen]

Bill, just to clarify:

- I assume you can handle the SMT without any trouble
or worries.

- I also assume that you have a good reason to want
the extra 17.7db, so go mod, be happy.

But for anyone else that may not be comfortable with
SMT changes, I am suggesting that before they partake
in that, they understand what the mod does and how
it would help them in their application.

My suggestion is, that for someone that's using the
standard LP-PAN + Power-SDR + E-MU 0202 setup, there
is no benefit to doing the mod, as the Y coordinates
in PowerSDR are adjustable (as discussed on the Yahoo
LP-PAN list).

Works good - lasts a long time. ;-)

That's my story and I'm sticking to it.

Respectfully,
Don

-- And sorry Tony I think you were more interested in
the discrete vs. SMT packages but I wanted to clarify
on this point – think you got your answer. !!!


[Elecraft] K3 Buffer Mod
Bill W4ZV btippett at alum.mit.edu
Fri Aug 22 08:16:40 EDT 2008

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--------------------------------------------------------------------------------



Don Rasmussen wrote:
>
>
> I have the LP-Pan with recommended Creative Labs
E-MU
> 0202 and find this to be optimal, I mean no reason
to
> make a mod of any type to K3. If someone told you
that
> this must be done, I'd want to see why with my own
> eyes before doing this job.
>

17.7 dB transfer loss from RF IN to IF OUT is why...

http://www.cliftonlaboratories.com/elecraft_k3_and_panadapters.htm#What_then_is_the_transfer_gain_of_the_K3_



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[Bill W4ZV]

Don Rasmussen wrote

Bill, just to clarify:

My suggestion is, that for someone that's using the
standard LP-PAN + Power-SDR + E-MU 0202 setup, there
is no benefit to doing the mod, as the Y coordinates
in PowerSDR are adjustable (as discussed on the Yahoo
LP-PAN list).

The Y coordinates of PowerSDR have nothing to do with this.  Before PowerSDR ever detects a signal, there is effectively a 17.7 dB attenuator ahead of it and PowerSDR will never see any signals that are less than 17.7 dB above the noise floor.  For the first week I had CW Skimmer, I thought it was terrible at decoding weak signals.  Then I discovered the actual reason it wasn't had nothing to do with the Skimmer software but with the K3's transfer loss.

Thinks of it this way...if you're trying to listen to an extremely weak signal, would you like a 17.7 dB attenuator between it and your ears?  I don't think so.

73,  Bill

[Julian, G4ILO]

Bill W4ZV wrote

Thinks of it this way...if you're trying to listen to an extremely weak signal, would you like a 17.7 dB attenuator between it and your ears?  I don't think so.

But this is not a 17.7db attenuator in the front end. The signal will have been amplified by the time it gets to this point. So the noise will be attenuated as well as the signal. Perhaps it won't be as sensitive, but the loss won't be as much as 17dB. I find it hard to accept your implication that Elecraft designed an IF output that is effectively useless.

[Bill W4ZV]

Bill W4ZV wrote

Think of it this way...if you're trying to listen to an extremely weak signal, would you like a 17.7 dB attenuator between it and your ears?  I don't think so.

Julian, G4ILO wrote

But this is not a 17.7db attenuator in the front end. The signal will have been amplified by the time it gets to this point. So the noise will be attenuated as well as the signal. Perhaps it won't be as sensitive, but the loss won't be as much as 17dB. I find it hard to accept your implication that Elecraft designed an IF output that is effectively useless.

Who said it's "effectively useless"?  Please read the following from N8LP:

http://n2.nabble.com/K3-Buffer-Mod-tt741709.html#a760307

In particular, his last sentence below:

"The reason I came up with the mod is because a couple users were trying
to use CW Skimmer for weak signal detection with LP-PAN feeding it. The
mod improves the NF of the IF output by up to 10dB (K3 preamp off,
somewhat less with the K3 preamp ON). Even though the NF of LP-PAN
itself is quite good, it can't do anything to improve the system NF
since it's downstream of the loss."

If you're using the K3's IF OUT & LP-PAN as a simple panadaptor, it's probably OK as is.  If you're trying to use a waterfall in PowerSDR, Winrad or CW Skimmer to detect noise floor signals, it's not.  Even a $15 Softrock will do a better job of the latter unless the mod is installed.  

I'm quite happy with N8LP's mod, but let's not bury our heads in the sand and say it's unnecessary.  That reminds me of what I'd expect from Yaesu or Icom...

73,  Bill

[N8LP]

I added a section to the instructions for the mod this morning that echo Jack's suggestion to check the dissipation of Q10. Because of the great variability of specs with the J310, it is a good idea to check it.

Jack's example is not representative of any actual measurements, but rather just an example of how to calculate the dissipation.  In my case, I found a calculated dissipation of about 125mW... a quite safe value. My original SPICE simulation predicted 180mW. The device is rated at 350mW, and my goal was to keep dissipation to half that value.

Larry N8LP



Message: 24
Date: Fri, 22 Aug 2008 09:13:40 -0700 (PDT)
From: Bill W4ZV <[hidden email]>
Subject: Re: [Elecraft] K3 Buffer Mod
To: [hidden email]
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=us-ascii




Lyle Johnson wrote:

> >
>  
>> >> Elecraft is well aware of N8LP's mod, Wayne saw no problems with it at
>> >> first
>> >> glance and is now considering whether to make it a production mod.  Both
>> >> N8LP and K8ZOA seem to feel it is not "damaging the radio, etc".
>>    
> >
> > Remember to make the measurements Jack points out on his web page if you
> > do the mod.  This is to be sure the buffer transistor does not exceed
> > its thermal ratings.
> >
>  

Yes, here are the details:

http://cliftonlaboratories.com/elecraft_k3_and_panadapters.htm#Alternative_to_increasing_K3_IF_sample_port_output

**********************************************************
"If you make this modification, I suggest measuring the voltage across the
new R9. Q10's drain current is set by a combination of the voltage drop
across R9 and the individual characteristics of the J310 part in your K3.
FETs have a notoriously wide part-to-part spread in parameters and it's
possible that the J310 device in your K3 has parameters sufficiently far
from the mean that Q10's power dissipation limits will be reached or
exceeded. The surface mount J310 has a maximum power dissipation rating of
350 mw, and for reliability a safe operating value is 200 mw or so. Q10's
current can be easily determined by measuring the voltage drop across R9. If
changed to 49.9 ohms, Q10's drain current Id is 1000*Vs/50 (in milliamperes)
where Vs is  the voltage measured from ground to Q10's source pin. To
calculate the power dissipated in Q10, measure its drain voltage. The power
is then (Vd-Vs)*Id in milliwatts, where Id is in milliamperes.

An example. After replacing R9 with a 49.9 ohm resistor, the following
voltage readings are found: Vs = 2 volts, Vd = 12 volts. The idle current
through Q10 is thus 2 / 49.9 = 40 mA. The power dissipated in Q10 is (12-2)
* 40 = 400 milliwatts. This exceeds the J310's maximum permissible power
dissipation and would not be a good design practice."
**********************************************************

In my unit the voltage across R9 was 460 mV, which resulted in 0.46/51 = 9
mA and power of 90 mW, well below the maximum rating of 350 mW and below
Jack's recommendation of 200 mW.

73,  Bill


>  
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[N8LP]

The gain can easily be made up, and is, but NF can not be made up in later stages once it is set in an earlier one. Anything connected to the IF port can only increase NF, no matter how quiet it is or how much gain it has. The easiest way to improve composite NF would be to add a strong, low noise preamp with adequate gain AHEAD of the K3, but that would compromise IMD.

There would be no reason to un-modify a K3 to work with any kind of panadapter connected to the IF port.

73,
Larry N8LP



Message: 37
Date: Fri, 22 Aug 2008 16:50:46 -0400
From: Don Wilhelm <[hidden email]>
Subject: Re: [Elecraft] K3 Buffer Mod
To: Jerry Flanders <[hidden email]>
Cc: [hidden email]
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Jerry,

I do agree with you.  There is a requirement for an isolation amplifier
anyhow, so I do not understand why the -17.7 dB cannot be made up in the
isolation amplifier if the device to be driven needs  input at the same
level as the antenna signal.

Should Elecraft ever come out with a device (Panadapter?) that is
designed to connect to the IF out at the level it currently is designed
for, any K3 that has been modified will require that the modification be
  removed.

73,
Don W3FPR

>  
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