[Elecraft] K2 receive sensitivity

[Paul Del Negro]

Ron D'Eau Claire wrote:

>Hi Paul: 
>
>The test on one band isn't valid on other bands because the band QRN changes
>a great deal with frequency. The fact that you hear little or not change in
>background QRN when you remove the antenna on 10 meters with the
>preamplifier off is because the K2 is optimized for maximum sensitivity and
>greatest dynamic range across the HF spectrum. 
>
>The sensitivity of a receiver - its ability to hear weak signals - is a
>function of the bandwidth of the receiver, the overall gain of the receiver
>and the amount of internal noise generated in the receiver. Most receivers,
>including the K2, have more than enough overall gain, so the things to look
>at are the bandwidth and internal noise generated in the receiver. 
>
>Narrower bandwidths make a band sound "quieter" because they filter out
>noise as well as signals on adjacent frequencies. The most rigorous test is
>to check the receiver at the narrowest bandwidth you'll use. (That's why a
>weak CW signal is easier to copy than an equally weak SSB signal. The SSB
>signal requires a wider bandwidth at the receiver, so more noise comes
>through that reduces the signal-to-noise ratio.) 
>
>The internal noise in a receiver is a concern only when it competes with a
>weak signal being received. Of course, in order to hear the weak signal on
>any receiver the signal must be strong enough to be heard over the band
>noise - the background QRN - being received along with the signal. If, when
>you connect the antenna, you hear the background noise increase you can be
>sure your ability to hear signals is limited by the unavoidable background
>QRN and not the receiver. 
>
>On the higher-frequency bands, typically 14 MHz and up, the background QRN
>may be low enough that the internal noise in the receiver might mask weaker
>signals. You sense when that's happening when you disconnect the antenna
>while listening to the background noise and the noise level does not change!
>The K2's preamplifier is a low-noise amplifier that boosts incoming signals
>so they easily over-ride the internal noise. In order to do this, the
>preamplifier must be placed at the input to the receiver, ahead of the I.F.
>filters, so it amplifies not only the desired signal but signals over a wide
>range of frequencies around it as well.  
>
>On the lower frequencies, too much "front end" gain (ahead of the I.F.
>filters) such as provided by the preamplifier can lead to overloading of the
>receiver by extremely strong signals outside of the passband. So the K2's
>preamplifier can be turned off when it's not needed. That results in a
>receiver that has the low noise figure needed for the higher frequencies and
>optimum strong signal handling capability on the lower frequencies. 
>
>While your test is a good quick check to see if the receiver is limiting
>your ability to hear weak signals, a more meaningful test for comparing
>receivers is to measure their signal-to-noise plus noise and minimum
>discernable signal (MDS) levels. Normally such measurements require some
>rather costly test equipment. However, Elecraft created a simple test
>oscillator that you can use to measure the signal-to-noise plus noise ratio
>at 1 uV and, based on that, you can estimate the MDS with good accuracy. It
>works with any receiver covering 80, 40 or 20 meters. The oscillator
>produces accurate crystal-controlled outputs on the three bands.
>
>It can also be used to calibrate your K2's S-meter and do other tests. Check
>out the XG2 Three Band Receiver Test Oscillator/S-Meter Calibrator on the
>Elecraft web site at:
>
>http://www.elecraft.com/mini_module_kits/mini_modules.htm
>
>It's the second-to-last item on the page.
>
>Ron AC7AC
>
>
>
>-----Original Message-----
>From: elecraft-bounces at mailman.qth.net
>[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul Del Negro
>Sent: Monday, January 15, 2007 7:11 AM
>To: elecraft at mailman.qth.net
>Subject: [Elecraft] K2 receive sensitivity
>
>
>Hi to all on the reflector.  I'd like to put something out here and see 
>if others experience the same thing or if I possibly have a problem with 
>my K2 sn5834.  Someone (I don't remember who) mentioned that a great way 
>to test receive sensitivity is to simply check for an increase in noise 
>level when switching from a dummy load to a resonant antenna on a dead 
>band (like 10m is lately), and I am a great believer in that method.  I 
>have used it for years with various commercial and homebrew rigs. So here's
>the deal.  K2 set for 10 meters, preamp off, antenna is 
>resonant wire (and also used a beam). When switching between dummy load 
>and antenna, there is no (or barely perceptible) change in receiver 
>noise output.  With the preamp on, the change is noticeable, as the 
>noise increases both with dummy load and antenna connected.  What I'm 
>concerned about is the preamp off situation, as all other receivers I 
>have tested do show a marked change in receiver noise when doing this test.
>I have already set the agc threshold so that there is no reduction in 
>band noise when turning agc on and off.  Band pass filter alignment was 
>done with a minimum signal from a signal generator and IF and filter 
>alignments were done with  Spectrogram and everything seemed to check 
>out OK.  BTW, the I used two antennas for this test, a 3 el tribander at 
>40 ft and a OCF dipole cut for 160 and tuned with ATU. 
>Do others have the same or similar results with this test?  I'd really 
>like to know if my K2 is behaving normally or I have a problem.  BTW, 
>the results improve as you lower the frequency, but the first band that 
>shows a really marked increase in noise level with this test is 17m. 
>Any and all comments are appreciated.
>73, Paul, N2PD
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Thank you for your response, Ron. (and also Don).
I am aware that the test I described is only a quick check to see if the 
ability to hear weak signals is being masked by internal receiver 
noise.   However, the K2 is the only  receiver (that I have owned) that 
has ever "failed" this test on 10 meters (meaning that more gain was 
needed so the preamp should be turned on) and I am wondering if others 
experience the same thing or there is something not right with my 
particular K2.  So far, I have not been able to get an answer to that 
question.
I do own the XG2 and have performed MDS measurements on my K2 using the 
XG2 and also a milspec signal generator I have.  The numbers agree very 
closely with published specs ON 20 METERS.  On ten meters I measured an 
MDS of -124 dbm (preamp off) and -130dbm (preamp on).  IMHO, an MDS less 
than -130 dbm is not acceptable for weak signal reception.  Which means 
to me that on 10 meters the preamp is absolutely required.  Knowing that 
most amateurs do not have the equipment to make MDS measurements on 10 
meters, I didn't think I could get my question answered on the reflector 
(the question being, 'Is my K2 receive sensitivity the same as everyone 
else's or do I have a problem with mine') by stating my measured MDS on 
10 meters. So I opted to put the 'quick-check' method out there instead, 
hoping that I would get answers like "yep, mine is the same" or " nope, 
u must have a problem".  It appears that the purpose of my original post 
has been misunderstood by many.  I just want to know if my K2 is healthy 
or not.
I think the K2 is a masterpiece of 'elegant simplicity' and my intent is 
not to knock the rig.  I've had more fun since the my kit arrived than 
I've had since the heathkit days. 
73, Paul, N2PD