[Elecraft] IF Output Buffer Gain Mod

[Duncan Carter]

Don Nelson wrote:
> Ken K3IU wrote:
>   
>> G'morning:
>>
>> Tomorrow I am going to make this mod on the K3 of a friend. To try to 
>> see what difference it makes, I though I'd look at the IF signal on a 
>> scope before and after. I have looked at the output on a modified K3 
>> (mine) and it looks like about 2.5 mV (P-P).
>>
>> Does that look about the right order of magnitude???
>>
>> Thanks...
>>
>> Ken K3IU
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>>   
>>     
> Ken,
>
> I have already made this modification to my K3. But I have not tested 
> it. So your email triggered a curiosity to check it. Since the test gear 
> is in the basement and the K3 is here in the shack, I thought of 
> connecting my FT817 and listening to the IF output. The IF center 
> frequency is 8.215 MHz more or less and so you can listen above the IF 
> and below the IF to what stations are transmitting. The FT817 was chosen 
> because it is highly portable and tunes to 8.215 MHz easily. You could 
> do an A/B test on signal level. There were a lot of signals on 20m and 
> so I had a lot to hear. If you put the K3 on say 14.350 MHz, for 
> example, then a signal at 14.325 MHz is going to be 25 KHz above the 
> 8.215 MHz IF or 8.240 MHz out of the IF output. This is because the IF 
> signal is inverted and goes up in frequency as you go down, in this case 
> down from14.350 MHz. The signals are going to be LSB for the same 
> reason. You could tune for WWV at 10 MHz to have a stable signal for the 
> before/after test.
>
> By the way, this is an odd and awkward way to add a sub receiver to the K3.
>
> Don, N0YE
>   
.I see about the the same on my new K3 that had the mod factory 
installed.  I had expected to be able to look at IF noise with my 
oscilloscope like I do on my FT-101E but the level is too low and 
there's some pickup of other signals away  from the IF so you need some 
gain and some selectivity ahead of the scope.  I suppose I'll get an 8 
MHz crystal for the 101 because I'm really curious about how well the K3 
noise blanker works on different noises that I'm used to seeing on the 101.

I did a bit of blanker experimenting last night and sent this note to a 
friend W5JAW, who is very experienced in blanker design, as I am.

"I'm accustomed to looking at the IF noise on my FT-101E with an 
oscilloscope but the IF out level on the K3 is at such a low level that 
it's hard to see if you don't have big signals.  I've seen a few such 
big signals but only with the blankers off.  There one that even my 
FT-101E can handle at any gain level, a series of five pulses that may 
be a control signal sent over the AC lines every few minutes, that I 
don't see on either radio if the blankers are on. The FT-101 can handle 
other noises but some are really level sensitive and it doesn't have an 
external control except for the 20 dB attenuator and my trusty alignment 
tool.

I just did a check on 30 meters which is usually my worst band for 
noise; tonight so far, the noise is not too bad.  To test, I set the K3 
in AM and looked at the audio out spectrum on a spectrum analyzer.   
With the blanker off, I see a harmonic series of 120 Hz.  On the 
oscilloscope, I don't see any sharp spikes so I suspect this noise may 
be generated by neighborhood TV sets.  If my neighbor who moved to 
Boston six weeks ago were still here, I'd have a severe example for 
testing but he and his nasty tv set are gone.  With the dsp blanking, at 
the extreme end of the IF WIDE range, I can lower the higher order 
harmonics, 4th order and higher.  If I add the IF DSP at the more 
extreme end of it's ranges, E3-7, the 120 Hz harmonic series completely 
disappears.  I haven't tried this with any big signals in the ham bands; 
apparently my neighbors who are phone men are too discouraged by the 
lousy conditions.  The only big signals that I've heard are on AM 
broadcast.  If I add the Noise Reduction, the interference really goes 
away.  With the limited or no gain ahead of the k3 blanker, the IF 
blanker may need fairly large pulses to trigger blanking. "  or not.

73, Dunc, W5DC