Craig F. Simmons' wrote: > Does anyone put credit in the Sherwood Engineering Inc receiver findings? The > reason I ask is in the past the K3 and then the KX3 was mentions as the > testing show they were among the top receivers. Will now that the Flex radio > 6700 is at the top now, does that say it is the best? Just wondering about > what the test measurements revile versus the real would use of the radios.
Hi Craig, Generally speaking, any of the radios near the top of the chart -- those having "Dynamic range narrow spaced" (2 kHz) in the high 90's or better -- will perform so well that you'd rarely notice the variation between them. You can then select based on other criteria, such as cost, size/weight, and features. If cost is your primary concern, I'm happy to say that you have two cost-effective choices in the top five from Elecraft. The other three radios in this group cost thousands more in their basic configurations. The Sherwood chart, by the way, is at: http://www.sherweng.com/table.html There's another of Sherwood's criteria which is notable and revealing: "100 kHz Blocking" (fifth column from the left in his table). Of the top 12 radios listed, only the K3 and the Hiberling hit the 140 dB threshold. What these two radios have in common is a down-conversion superhet receive architecture. Signals are first converted to a low intermediate frequency (8.2 MHz in the K3), where they are applied to one of several crystal filters. Downstream from these filters is an analog-to-digital converter, which drives the digital signal processor (DSP). The DSP does the work of demodulating signals and adding further amplification and signal processing, such as noise reduction. The advantage of the superhet architecture is that the A-to-D converter is protected from strong out-of-band signals by the crystal filters. This is of critical importance in multi-transmitter environments, such as big contesting stations, DXpeditions, Field Day, etc., as well as for home stations having high-gain antennas, or those that are in close proximity to huge interfering signals such as shortwave broadcast (or neighboring hams). Now back to your original question about the Flex 6700. It's certainly a heavyweight contender, and has some interesting characteristics such as the ability to look at multiple MHz of spectrum on its associated PC. (Of course it is a radio without a front panel, requiring the use of a PC keyboard and mouse for operation.) Two of the 6700's test results from Sherwood's table bear further scrutiny: 1. 100 kHz Blocking is shown as simply "A/D Limit" As I mentioned above, the K3 uses a superhet receiver with crystal filters that protect its A-to-D converter (ADC). This is not the case for the Flex, which has its ADC very close to the RF input of the receiver. The ADC is not protected by anything but very wide band-pass filters (multiple MHz), unless you turn on an attenuator. What this means is that the ADC becomes the limit on blocking dynamic range. ADCs do not handle overrange gracefully: as power builds up due to multiple signals appearing in the passband of the receiver, the ADC can start limiting, which generates a large number of spurious image responses ("ghost signals" that can be heard up and down the band). It is unclear just where this might start happening in the case of the 6700, since Rob did not publish any numbers. "A/D Limit" is a pretty ominous summary of what might be going on. I have heard that some testers will be evaluating front-end ADC limits using 3-tone testing, which can be very revealing. I'm looking forward to seeing those results. Meanwhile, I'm glad my K3 has crystal filters. 2. Noise floor with preamp off is only -118 dBm One of the nice features of the K3 is that even with the preamp off, the noise floor is -130 dBm (see Sherwood's chart). The Flex 6700 has a noise floor of -118 dBm with preamp off. What this means in practice is that on the higher bands, you'd be tempted to run with the preamp on at all times. There's also a footnote on the "Dynamic range narrow spaced" column that relates to the preamp. The 6700 is at the top of the chart only if its preamp is turned on (DR then = 108 dB). With its preamp off, DR is around 99 dB -- nearly identical to the K3 at 2 kHz, and worse than the K3 by 6 dB at 20 kHz. The reason the 6700's DR is better with the preamp on is probably because of the dramatic improvement in noise floor (from -118 to -135). But on the low bands where you run with preamp off, the DR will be 99, not 108 dB. On high bands with the preamp on, the ADC is subject to "A/D Limiting" 20 dB sooner. Time will tell what this means for operators. Regarding the KX3: The KX3 was optimized for small size and low current drain, and it's very low-cost relative to the other top-performning radios, so we were pleased to see that it performed so well in third-party testing. While it is not a superhet, it does have roofing filters as an option (in this case, analog filters based on precision op-amps). This is unusual for an SDR (software-defined radio). The K3 would be a better choice if you need excellent blocking performance close-in, again because of the crystal filters. But the KX3 is no slouch. Personally, I think it would be an excellent choice for a new General, but I may be biased :) 73, Wayne N6KR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
