To: | cq-contest@contesting.com, topband@contesting.com |
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Subject: | Topband: Sept/Oct NCJ article on DX Prowess of Receivers |
From: | Bill Tippett <btippett@alum.mit.edu> |
Date: | Tue, 31 Aug 2004 13:51:36 -0400 |
List-post: | <mailto:topband@contesting.com> |
Here's a copy of a note I sent to SP7HT regarding his recent article in the Sept/Oct NCJ. 73, Bill W4ZV Hi Tadeusz, I just got my new NCJ and read your article. I was surprised by the following statement (next to last paragraph on page 4): "These statements mean for me that somebody unfamiliar with all measurement set-up and procedures nuance cannot use data derived "directly" from Swept BDR or IMD DR3 oscilloscope graphs close to the listening frequency instead of point results measured manually. Therefore, I have consequently used in my Table only manually measured test point data at 5 kHz and 20 kHz spacing." I had believed that ALL discrete points plotted on the ARRL graphs came from manual measurements, so I asked Mike KC1SX who does ARRL's measurements. I see that Mike copied you on his reply which did confirm that ALL discrete measurement points in the ETR swept graphs are done manually. Thus the IMD and BDR data down to 1 kHz spacings (always the innermost point in all ARRL ETR plots since 1997) are perfectly valid for comparison (although sometimes difficult to interpolate, and I notice Mike has now included both the 1 & 2 kHz IMD and BDR measurements in tabular form for the IC-7800 ETR). In general, I agree with much of what you said, but have some differences of opinion in the following areas, primarily from the viewpoint of a contester (since your article was also published in the NCJ in addition to the recent issue of QEX): 1. 5 kHz spacing (BDR and IMD) is far too wide for most contest interference in my opinion. I prefer ARRL's 1 kHz measurements as being much more realistic, especially for contesters. When do you last recall a contest with signals spaced only 5 kHz? In my opinion, signals spaced at ~500 Hz is much more common, especially in the major contests. 2. By only looking at 5 kHz spacing, several of the receivers you reviewed appear to be much better than at much closer spacings. A good example is the Elecraft K2. I have a K2 and like it very much for its intended use, but the receiver basically falls apart for close signal spacings. ARRL shows BDR drops to ~116 dB and IMD drops to ~67 dB at 1 kHz spacings. Compare this to Orion with BDR of ~119 dB and IMD of ~84 dB at 1 kHz spacings (ARRL data using the stock 1000 Hz roofing filter). 3. Given "proper design" (similar to your "not good" comment on page 11 about IMD DR2 and IP2), I would rank IMD DR3 as the most important parameter for contesters for the following reasons: a. Unless you have an extremely close neighbor, or unless your BDR is terrible, BDR is not likely to be as significant a problem as IMD DR3. The reason is very simply that BDR performance is normally MUCH higher than IMD DR3. Thus the latter becomes more critical since you will reach the IMD DR3 limit sooner. W8JI cites an example on 160 below where he sees his maximum dynamic range needed being around 95 dB, which most of the receivers in your list exceed at 5 kHz. As long as BDR is above this number, or unless you have a very close neighbor or are in a multi-multi operation, BDR of 95 dB at should be sufficient, although I would add that this performance should be maintained at 1 kHz or less instead of 5 kHz for the reasons cited in #1 above. http://www.w8ji.com/receiving.htm b. Phase noise also becomes critical at very close signal spacings. An example of poor phase noise performance is the IC-7800. At close spacings of 1 and 2 kHz, you can see in ARRL and RSGB results that Phase Noise overrides the receiver's IMD and BDR performance (note asterisks by measurements on pages 17 & 18 of ARRL's ETR). As KC1SX describes on page 34 of his August 2004 QST article, Phase Noise becomes effective BDR (or IMD) on a noise limited measurement. I fully agree with this definition since it makes no practical difference to the operator whether the source of the noise is due to BDR, IMD or Phase Noise. If Phase Noise performance is poor, then it becomes much more important than either BDR or IMD DR3 since it will over-ride those effects at close spacings. Assuming the receiver's Phase Noise is always below IMD and BDR performance, then it is not so important. c. Assuming Phase Noise is below both BDR and IMD DR3 effects, and assuming BDR is "adequate" for your operating conditions, I feel that very close spaced IMD DR3 (1 kHz or even less) becomes the critical parameter for contesters. This is mainly because it is not uncommon to have many strong signals spaced at 500 Hz (or even less) in a contest capable of creating IMD DR3 products. This was my primary motivation in working with Inrad to develop the 600 Hz #762 filter for Orion. At this BW, even 650 Hz spaced signals will fall just at the -30 dB BW of the #762, hence the resulting IMD products should be attenuated by ~90 dB, and we can even improve on that by using PBT (note that these estimates for IMD at spacings <1 kHz have not yet been confirmed by third-party measurements). http://users.vnet.net/btippett/inrad_.htm As a contester, I rank very close-spaced IMD DR3 performance as my #1 criteria, as both W8JI and Sherwood Engineering do. Granted, this assumes that both BDR and Phase Noise are "adequate" as described above. W8JI has some very practical advice below which relates to what levels of MDS, BDR and IMD DR3 are "good enough". http://www.w8ji.com/receiver_tests.htm I realize that contesters' needs are slightly different than DX'ers, which was the primary focus of your article, but even DX'ers sometimes experience DX stations that only listen 1 kHz away (or less!), which places a premium on receiver performance at spacings much closer than 5 kHz. 73, Bill W4ZV _______________________________________________ Topband mailing list Topband@contesting.com http://lists.contesting.com/mailman/listinfo/topband |
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