Posted by
Elecraft mailing list on
Aug 16, 2017; 6:01pm
URL: http://elecraft.85.s1.nabble.com/K2-Sensitivity-Spec-tp7633423p7633445.html
Thanks for your comments, Don. When you test K2's, can I assume you have the signal generator connected directly to the K2 (or through an attenuator) with no antenna connected?
Normally, one might hope that a preamp would provide some improvement in receiver noise figure (and hence MDS, sensitivity, noise floor), or there isn't much point in having it. I've never run into an intermodulation or blocking problem on 10 GHz (sideband noise issues tend to dominate the *rare* strong signal scenario) so I expect to leave the K2 preamp on in an attempt to maximize the sensitivity
I agree that most often transverters have lots of gain but I noticed, with the particular combination I have, that the increase in receiver noise output resulting from turning on both the 10 GHz-to-144 MHz and the 144 MHz-to-28 MHz transverters was only about 7 dB (K2 preamp on, AGC off) compared to the K2 alone, rather less than usually seems to be the case, so I was afraid that the front end noise might not dominate the overall receiver noise. The intermediate transverter does have relatively low gain, by design. With a couple of tweaks and a minor repair in the transverters I've now increased this ratio by a couple of dB.
Since the 10 GHz transverter noise figure was measured by the manufacturer at 1.2 dB and the K2 is something closer to 10 dB the increase in noise may not have to be all that much for the system to be working right. I haven't felt up to doing all the necessary math this week to make sure, especially without a reliable number for the K2 noise figure. Most likely I'll try it out in the 10 GHz and Up contest this weekend and if it proves to be deaf compared to others' rigs, I'll have to delve deeper.
73,
Steve VE3SMA
--------------------------------------------
On Tue, 8/15/17, Don Wilhelm <
[hidden email]> wrote:
Subject: Re: [Elecraft] [K2] Sensitivity Spec
To: "Steve Kavanagh" <
[hidden email]>,
[hidden email]
Received: Tuesday, August 15, 2017, 8:29 PM
Steve,
Typically, the receiver is limited by the noise
floor, so the MDS will
not change much
whether the preamp is on or off. While the preamp will
increase the strength of the signals, it also
adds to the noise floor.
Generally a transverter will have a low noise
preamp with plenty of
gain, so there is
usually no advantage in running the transceiver with
the preamp on. Use your ears while listening
to a strong signal to see
if you can detect
overload of the receiver preamp - if so turn it off,
but if there is no overload distortion, you can
leave it on.
Typical MDS
measurements are made at 500 Hz.
I typically
do a quick test of the sensitivity of K2s that I repair. A
-130dBm signal should be detectable above
the receiver noise floor. For
that test,
I have observed that it makes little difference whether the
preamp is on or off. So I would suggest
you test whether strong signal
performance
is better with the preamp on or off. My wager is that you
will choose to leave it off unless your
transverter(s) have negative
gain on RX.
73,
Don
W3FPR
On 8/15/2017 4:10 PM, Steve
Kavanagh via Elecraft wrote:
> I'm
just trying to evaluate if I have the gain distribution
right for a K2 used as an IF for a 10 GHz transverter,
through a 2 m transverter mounted inside the QRP top. The
K2 manual gives a specification for the MDS but fails to
define the bandwidth at which it is measured.
Sherwood's receiver table lists some measured values at
500 Hz bandwidth but fails to note whether the preamp is on
or off for these measurements. So I am not quite sure what
the sensitivity should be.
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