recieve comparisions k2 vs ft 980
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hello all. Just wondering if anyone would comment on this. recently purchased a yaesu ft 987d, when compairing recieve sensitivity on the higher bands it seems the K2 suffers. If you turn on the k2's pre it seems to equal out what I'm hearing. There is a circuit in the ft 897 that turns off the front end preamp, this curcuit is turned on, cutting out the pre in the ft 897. Should the preamp in the k2 be turned on for normal recieve? Tom km4cu
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tom martin wrote:
> Should the preamp in the k2 be > turned on for normal recieve? I find that I usually have the preamp on for 14 Mhz and higher, while I turn it off on the lower bands (unless I'm using my low gain ferrite loop on 160). It will depend on the gain of your antennas and the amount of local noise you have. The preamp setting is saved per-band, so you don't have to change it a lot. -- 73, Vic, K2VCO Fresno CA http://www.qsl.net/k2vco _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: recieve comparisions k2 vs ft 980
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In reply to this post by tom-239
Tom, KM4CU asked:
hello all. Just wondering if anyone would comment on this. recently purchased a yaesu ft 987d, when compairing recieve sensitivity on the higher bands it seems the K2 suffers. If you turn on the k2's pre it seems to equal out what I'm hearing. There is a circuit in the ft 897 that turns off the front end preamp, this curcuit is turned on, cutting out the pre in the ft 897. Should the preamp in the k2 be turned on for normal recieve? --------------------------------------- Great question, Tom! The real measure of sensitivity is the S+N/N ratio. What you are comparing is the gain available. That's something quite different. The amount of gain is simply the total amplification available from the antenna to the speaker or phones. If those amplifiers are noisy, their own noise will cover up signals. What's usually considered really important is how weak of a signal can the receiver hear, down in the noise. To do that, it's important that the receiver not mask weak signals with its own internally-generated noise. Down on the lower-frequency bands, minimal band QRN, even in the quietest QTH's, is usually much stronger than what even a cheap receiver generates internally. But, up above about 7 to 10 MHz, the band QRN drops off dramatically, putting more demand on having a quiet receiver if we don't want its internal noise masking the weak signals. That's a never-ending quest and the frequencies go up right into the VHF, UHF and Microwave bands, but it start at about 7 to 10 MHz. The most difficult noise inside a receiver is going to be that developed in the "front end", since whatever comes out of the first stage(s) of a receiver will simply be amplified by all the subsequent stages. The K2, like most high-performance receivers, has a low-noise "preamplifier" that will increase the level of signals at the antenna input. In addition to the obvious effect of increasing the overall receiver gain, it is electrically quiet so it can improve the level of signals without masking them with noise. That helps the receiver hear quiet signals up on the higher frequency bands where the band noise doesn't cover them up. There's another issue with the ability of a receiver to hear weak signals: the receiver bandwidth. The bandwidth is usually defined by the I.F. filters. The wider the passband, the more band (and internal noise generated in the receiver 'front end') passes through to compete with the signal. That's why we routinely cut down the I.F. bandwidth (choose a narrower filter) to help us hear a weak signal. Even when there's no QRM present a narrower filter helps the signal "jump up" out of the noise. Since bandwidth affects the ability to hear weak signals, most Ham receivers are compared using an I.F. bandwidth of 500 Hz. That way we can make meaningful comparisons. So, the ability of a receiver to hear a weak signal is measured by injecting a weak signal and measuring the amount of audio output the receiver produces. Typically a 1 uV signal is injected into the antenna terminal. That gives you the Signal + Noise level, since we are hearing both the signal and any noise coming through with it. Then we turn off the signal and just listen to the noise. The ratio between the two, expressed in dB, is the S+N/N ratio. Another measurement that is often used is the "Minimum Discernable Signal" or MDS, expressed as a number of dB below a milliwatt, such as -130 dBm. My K2 shows a S+N/N ratio of about 27.2 dB on 80 and 40 meters with no preamp, and about 24.5 dB on 20 meters with no preamp. That translates to a MDS of about -134 dBm on 80 and 40 and -132 dBm on 20. On 20 the preamp adds useful Signal/Noise ratio be improving the signal level before it hits the mixer and I.F. circuits. With the preamp on, my K2 show a S+N/N ratio of 29.8 dB for an MDS of about -137 dBm. To compare this with another receiver you need to conduct similar S+N/N tests on the second receiver. A well-calibrated and shielded signal generator to make meaningful measurements can be very expensive. Elecraft makes a nifty little test oscillator (XG2) that provides a selectable 50 uV and 1 uV output on 80, 40 and 20 meters for making just such measurements. Although limited to three bands, it is quite reasonably priced considering what one has to pay for a commercial unit. (The 50 uV output is a popular level for calibration S-meters at S-9). Bottom line, it sounds like your K2 is functioning normally, but until you find a signal down near the noise level, using the same antenna on both receivers and the same I.F. bandwidth, etc., and see how they compare actually hearing the signal, you don't know much about how they compare in "sensitivity". The signal generator and calculations are just a way of quantifying that so you can easily compare receivers under controlled conditions and without having to look for weak signals <G>. If you want to see the numbers and don't have a well calibrated signal generator, I recommend getting the little XG2 from Elecraft and running the tests. Complete instructions for calculating the S+N/N and MDS are included in the manual. They require using logarithms, but most inexpensive hand-held calculators do that a the touch of a button these days (and I haven't missed my old log tables!). One of the things I like about what Wayne and Eric have done is that they both provide quality gear AND they provide ways for those who are curious to be able to understand just what their rig can do and how it compares to others. Can't make the usual disclaimer because I have the pleasure of being involved in some of the new Elecraft products, and I'm proud to have the opportunity. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: recieve comparisions k2 vs ft 980
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In reply to this post by tom-239
I was gonna quote Sherwood Engineering to you
(<http://www.sherweng.com/table.html>), but see that the FT-897 is not on their receiver test comparison table. But you can see the K2 is right up right near the top of the list in company with some very expensive and capable receivers. But apparently Elecraft has taken the time to list the FT-897 specs (and others) alongside the K2 themselves (<http://www.elecraft.com/K2_perf.htm>). David, K3TUE . --- tom martin <[hidden email]> wrote: > hello all. Just wondering if anyone would comment on this. recently purchased > a yaesu ft 987d, when compairing recieve sensitivity on the higher bands it > seems the K2 suffers. If you turn on the k2's pre it seems to equal out what > I'm hearing. There is a circuit in the ft 897 that turns off the front end > preamp, this curcuit is turned on, cutting out the pre in the ft 897. Should > the preamp in the k2 be turned on for normal recieve? > > Tom km4cu > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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