JRC_NRD-525_technical_review.pdf

《JRC_NRD-525_technical_review.pdf》由会员分享，可在线阅读，更多相关《JRC_NRD-525_technical_review.pdf（6页珍藏版）》请在收音机爱好者资料库上搜索。

1、NRD-525: A Technical Review Dallas Lankford, 15 XII 92 There have been a number of reviews of the NRD-525 in the past, but none of them answered certain technical questions I had regarding 525 performance, especially in the MW band. The 525 was discontinued last summer, and the remaining stock was o

2、ffered at a discount by several retailers. Not being able to resist a bargain, I bought one of these last 525s with a serial number in the high 51,000s. Line drawings of the 525 front panel and vacuum fluorescent display are given below. Previous reviews of the NRD-525 include Magnes RDI white paper

3、, Edition 2.0, 11 June 1987 (presumably there was an earlier review), the 1987 WRTH, pages 555-556, Rainer Lichtes review in his 1987 book More Radio Receiver Chance Or Choice, pages 41-48, and John Bryants Proceedings 1989 review, Wastegunner on a 525, pages R12.1-R12.9. Because these reviews cover

4、 most basic aspects of a 525, my discussion of basic features will be brief in places. For additional details you may consult those sources. The front panel and vacuum fluorescent display line drawings above tell most of the story about the basic 525 features. Some controls are conventional analog c

5、ontrols, while some are push button switches. Other reviewers have fussed about certain aspects of the controls, but I havent been seriously annoyed by any of these so-called defects. Yes, it would be nice to have an analog S- meter. But the main reason the S-meter jumps around so much in AM mode is

6、 because the AM attack times are not appropriate. My mod described in NRD-525 AM AGC mod 2 takes care of this annoying defect. And yes, it would be nice if the front of the 525 could be tilted upward to get a better view of the front panel and keep your hand from bumping the table top while twirling

7、 the main tuning knob. Nevertheless, the flywheel-weighted, dimpled main tuning knob turns smoothly and is easy to use. Mode changing is easy with two switches r-. which move the mode indication left or right through the following: RTTY CW USB LSB AM FM FAX. By contrst, with the R8 you have to cycle

8、 completely through all other modes to get back to the one you want. Similarly, bandwidth changing is easy with two switches which move the bandwidth indication left or right through NARR INTER WIDE AUX. Changing the AGC setting is not as convenient; a single switch cycles through OFF FAST SLOW. In

9、addition to tuning with the main tuning knob, a frequency can be entered either in KHz or MHz using the keypad, and the frequency can be changed with the UP and DOWN switches. Some operating aspects of the 525 can be changed by the user. The tuning rate of the main tuning knob and the step increment

10、 of the UP and DOWN switches can be changed between coarse and fine by pressing the RUN switch. In fine tuning mode, the main tuning knob rate is 2 KHz per revolution and the UP/DOWN increment is I KHz, while in coarse tuning mode the rate/increment is 20 KHz/10 KHz. The 10 Hz digit of the frequency

11、 display can be turned off and on by pressing MEMO and I switches simultaneously. And the frequency indication in USB and LSB modes can be changed between automatic and manual modes by pressing MEMO and 0 switches simultaneously. In automatic mode the frequency display does not change when changing

12、between USB and LSB (and you do not have to retune). The NRD-525 Instruction Manual gives the tuning range as 90 KHz to 34 MHz. The 525 can be tuned below 90 KHz, but internally generated noise begins to register on the S-meter at about 70 KHz, so the 525 is not useful much below 90 KHz. VLF DXers w

13、ill need to use a VLF converter. In the past, the merit of a receiver was often determined by the three Ss - sensitivity, selectivity, and stability. With synthesized solid state receivers like the 525, stability is seldom an issue. Consequently, a more appropriate modern version of the three Ss is

14、sensitivity, selectivity, and spurious responses. Nevertheless, for completeness, here is what has been published about 525 stability. The instruction manual specifies Frequency stability +/- 3 PPM. Lichte measured +/- 5 Hz/hr while Magne reported less than +/- 10 Hz at 10 MHz, excellent. I dont hav

15、e the equipment to make such precise drift measurements. Suffice it to say that I havent observed any drift with the 525s I have used, and I dont expect to observe any. Sensitivity should not be an issue for modern solid state communications receivers, but surprisingly it sometimes is. Some receiver

16、s, like the R-5000, are desensitized in the MW band by design. Others, like the R8, are not quite sensitive enough throughout the entire tuning range. Both Magne and Lichte stated that the 525 sensitivity is derated in the MW band. Lichte even provided values of 16 and 19 microvolts at 1000 and 500

17、KHz respectively for a 10 dB S+N/N using the WIDE bandwidth and AM mode, compared to 2 microvolts for the SW bands. In addition, the 525 instruction manual specifies 15 microvolts or better sensitivity for the 0.90-1.6 MHz frequency range, and 2 microvolts or better for the 1.6-34 MHz frequency rang

18、e using AM mode. However, I do not know of any 525 with worse sensitivity below 1.6 MHz than above. As a matter of fact, the sensitivity of my 525 is about 0.35 microvolts throughout the MW band for a 10 dB S+N/N using WIDE bandwidth and AM mode, with a 400 Hz modulated source at 50% modulation. The

19、 WIDE bandwidth AM sensitivity of my 525 on the SW bands is not quite as good, about 0.45 microvolts, but still excellent. Perhaps some early production 525s were desensitized by design in the MW band. But that appears not to be the case for most 525s. The 525 is one of the few solid state receivers

20、 with adequate MW band sensitivity at locations like mine with low levels of man-made noise. Selectivity should also not be an issue for modern solid state communications receivers, but again it sometimes is. Insufficient number of bandwidths, and inappropriate bandwidths are the common defects. The

21、 525 design is excellent with regard to available bandwidths because it has four available bandwidths which are selectable independent of mode. A 525 comes with three selectivities - AUX, WIDE, and INTER. A fourth bandwidth is available in the NARR position when an optional filter is installed. Unfo

22、rtunately, no suitable AM filter is currently available for the fourth position. So you have to make do with the three stock bandwidths. The instruction manual specifies the AUX, WIDE, and INTER 6 dB/60 dB bandwidths as 12 KHz or more/not specified, 4 KHz or more/ 10 KHz or less, and 2 KHz or more/6

23、 KHz or less respectively. Typically the WIDE and INTER filter bandwidths are better than specs. For example, mine are 12 KHz/24 KHz, 5.9 KHz/8.3 KHz, and 2.4 KHz/4.2 KHz respectively. These bandwidths are satisfactory for virtually all listening situations, though I would like a 3.0 KHz filter if o

24、ne was available. In Magnes review it was said that ultimate filter rejection was limited to about 65 dB due to cross coupling in the matching networks of the IF stage. However, in 525s I have tested, the ultimate rejection has been excellent, typically greater than 100 dB (the limit of my measuring

25、 method). Measurement of ultimate filter rejection was complicated by what seem to be synthesizer noise sidebands and blips apparently due to synthesizer spikes. The blips were about 86 dB down on my 525 at about +/- 10 KHz and +/- 100 KHz from the primary synthesizer frequency in the MW and, and al

26、so at +/- 200 KHz in the SW bands. Weaker blips greater than 94 dB down were observed at +/- 200, +/- 300 KHz, etc.in the MW band and at +/- 300 KHz, etc. in the SW bands. To conclusively demonstrate that the blips were not due to filter leakage, I injected a signal generator source at the junction

27、of L103 and C19 on the CFH-36 IF FILTER UNIT PC board. Throughout most of the WIDE filter stopband, ultimate rejection was greater than 100 dB (the limit of my test equipment). Only at one point, about 510 KHz, was the ultimate rejection measurable at about 100 dB down. These synthesizer blips and n

28、oise sidebands can produce unexpected spurious responses where there should be none. Here is how. Lets say you have a super local on 1520 KHz and you tune to 1620 KHz. The -100 KHz synthesizer blip mixes with the super local on 1520 KHz at the first mixer before any significant selectivity and produ

29、ces a phantom copy of your super local where it should not appear. I dont have a super local close enough to the edge of the MW band to observe this phenomenon on the 525. Spurious responses due to synthesizer blips are not an idle concern. For example, some of the spurious responses I have observed

30、 on a Drake R8 are due to synthesizer blips .Using a signal generator, I determined that when an R8 is tuned to 1490.00 KHz there are synthesizer blips about 85 dB down at 1649.0 and 1849.0 KHz, and a somewhat weaker blip at 1759.4 KHz. Sure enough, there was my super local KRUS in the noise at 1649

31、.0 and 1849.0 KHz. The 1759.4 KHz spur could not be observed because of a stronger navigation signal on about 1760 KHz. Previously I had overlooked the 1849.0 KRUS spur because of a 3rd order IMD product on 1850.0 KHz (KRUS+KWKH/2x1490-1130). And I hadnt noticed the 1649.0 KRUS spur, perhaps because

32、 man-made noise levels were not low enough on previous spur searches. I have not done a thorough study of the R8 synthesizer blips , but they appear to occur about +/- 50 KHz from the primary synthesizer frequency, and at approximately multiples of 100 KHz thereafter. For example, with a signal gene

33、rator set at 1100 KHz, R8 blips were observed at 1153.8, 1253.6, 1352.8, 1453.6, etc. and at 953.8, 853.6 KHz etc. To measure the levels of spurious responses due to synthesizer blips I used a calibrated signal generator with a precision attenuator. The receiver S-meter was first calibrated using th

34、e signal generator connected to the receiver antenna input terminal. Then the signal generator was set to a particular frequency, say 1490.00 KHz, and the signal generator output was set for maximum, say 100,000 microvolts. The receiver was tuned above and below the signal generator frequency, and t

35、he frequencies and levels of any blips were noted. Thus, when I said that blips about 85 dB down were noted at 1649.0 and 1849.0 KHz on the R8, what I meant was that S-meter readings equivalent to 85 dB below 100,000 microvolts were observed. The actual synthesizer blips themselves were not observed

36、, only their effect, namely the mixing of a strong nearby signal with a synthesizer blip to produce a phantom signal where there should be none. This method of measurement simulates precisely what happens in an actual listening situation with an antenna connected. The NRD-525 produces fewer spurs du

37、e to synthesizer blips than the R8 because the 525 uses double tuned circuits ahead of the first RF amp which automatically track the received frequency, while the R8 uses switched broadband front end filters. At MW frequencies the 525 is unlikely to produce observable spurs beyond the first pair of

38、 synthesizer blips +/- 100 KHz, and the spurs are unlikely to be observable except in the 1610-1700 KHz and 440-530 KHz frequency ranges. The R8 is also unlikely to produce observable spurs due to synthesizer blips within the MW band, but clearly does produce multiple observable spurs for l00s of KH

39、z above the MW band and perhaps below the MW band when used with a rather ordinary wire antenna. I dont know if SW signals levels are high enough to cause observable spurs due to synthesizer blips on the 525 or R8. If they are, then the 525 might produce such spurs at +/- 100, +/- 200, and +/- 300 K

40、Hz from a strong SW broadcaster, and the R8 might produce such spurs up to several MHz away from a strong SW broadcaster. Such spurs are much more difficult to identify at SW frequencies in actual DXing situations because of the transient nature of SW signals due to fading. But it wouldnt surprise m

41、e if many SW DXers have chased weak hets which are actually spurs due to synthesizer blips , especially on R8s and other broadband front end receivers, and perhaps even on 525s. In addition to spurious responses due to synthesizer blips which follow you as you tune around, a 525 also has fixed inter

42、nally generated spurs. These can be most easily observed by tuning the 525 in USB or LSB modes with no antenna connected. On my 525 there is an S-4 spur at 100 KHz, and weaker spurs about every 100 KHz throughout the entire tuning range. Many of these spurs can be detected with an antenna connected,

43、 at least.at my location (which has low levels of man-made noise) with.a noise reducing antenna (which further reduces man-made noise), provided no received signal is present on the frequency. However, none of these spurs have been observed as a het on a MW band signal, perhaps because these spurs a

44、re almost exact multiples of 100 KHz, and are quite weak. There are also some irregular spurs in the MW band on my 525: 548.6, 617, 958, 1026, 1229, 1520, 1523.3, 1524, 1534, 1539.3, and 1560 KHz. None of these irregular spurs have been observed as hets on MW signals, perhaps because they are so wea

45、k, or perhaps because I do not have the right combination of suitably weak MW signals on adjacent channels. Both regular and irregular SW spurs are observed when atmospheric and man-made noise are low. Besides these unconventional spurs which were virtually unheard of before synthesized receivers, t

46、he 525 can produce phantom signals via conventional images of the 455 KHz IF, at least in principle. Magne gave the measured 455 KHz image rejection of a typical 525 as 82 dB, while Lichte gave 72 dB. Both are correct. It depends on the frequency at which you measure the 455 KHz image rejection. For

47、 my 525, the 580 KHz image rejection of 1490 KHz is about 100 dB, the 1090 KHz image rejection of 2000 KHz is about 90 dB, the 9.090 MHz image rejection of 10.000 MHz is 86 dB, and the 19.090 MHz image rejection of 20.000 MHz is 74 dB. Since signal levels tend to decrease as frequency increases, the

48、 525 decreasing 455 KHz image rejection decreases in the right way. Overall, the 525455 KHz image rejection is excellent, though not quite as good as the image rejection of an R-390A (455 KHz image rejection in excess of 100 dB at all frequencies, 2-3 MHz variable IF image rejection varies from 76 d

49、B at 20 MHz to 90 dB at 10 MHz, and below 8 MHz all images are down in excess of 100 dB). It is unlikely that I will observe any images on a 525 in an actual listening situation. The final way that phantom signals can enter a 525 is as intermodulation distortion (IMD) products, both 2nd and 3rd order. When I measured the 3rd order intercept (ICP3) of my 525 at 1100 and 1120 KHz and got -4dBm for the WIDE bandwidth and AM mode, at first I was disappointed. But then I remembered that an R-390A ICP3 is typically -12 dBm, and yet I have never heard any 3rd order IMD products on an