(292) 12 Jan 92 23:22:00 By: Bill Cheek To: All Re: Icom R-1 Problems St: ------------------------------------------------------------------------------ @PID: RA 1.1 @MSGID: 1:202/719.2 4f0f6b3c TO: Owners of and others interested in the Icom R-1 ================================================================= From: Bill Cheek; PO BOX 262478; SAN DIEGO, CA 92196-2478 ================================================================= I have completed an intermediate technical evaluation of the ICOM R-1 DC-to-Daylight pocket scanner; a most awesome little package of dynamite! The focus of my investigations was on the alleged and much-talked-about weaknesses & deficiencies of this unit; not on its strengths and good points, which are numerous and well documented elsewhere. The bottom line of my analytical effort is that I will not be adding the R-1 to my arsenal of monitoring equipment at any time in the near future; not because of any new discoveries or terrible shortcomings but because the danged thing is so tiny and miniaturized that it is next to impossible to do anything with it, either to cure its few shortcomings or to add anything new and exciting. Here is the deal in a technical sense: the R-1 has two serious problems: (1) Very subject to strong signal overload, and (2) extremely susceptible to adjacent channel interference, mostly on the LF-MF-HF AM bands where selectivity is more than desirable; it's a necessity. Poor Adjacent Channel Rejection in this unit results from the wide 455 KHz IF filter on the DET-A board, which is used in common with both AM and NFM modes. The 15 KHz width of this filter is more or less satisfactory for VHF & UHF NFM requirements but much too wide for LF-MF-HF AM shortwave bands where channel separation can be as little as 3 KHz. Imagine then, up to five different signals on one channel. No thank you! So what about a remedy for inferior Adjacent Channel Rejection? It does seem that a company in England, (RayComm), is doing a mod that essentially replaces FL-1 on the DET-A board with a narrower much sharper IF filter. I am amazed that they dare do this, but more power to them. I won't attempt that approach at this time for several reasons. The problem as I see it, is two-fold: first there is the matter of space or available real estate in the R-1; zilch. All decent IF filters of which I am aware are somewhat larger than the stock FL-1, and therefore will not find an easy residence to replace FL-1. But even if one is found, and RayComm apparently knows of one, a serious problem still remains! A useful IF filter for LF-MF-HF AM operation, say with a 6 KHz bandwidth, will be too narrow for some NFM signals, especially at 800 MHz and up! A narrow IF filter will result in clipping of FM signals which are FM-deviated more than 3 KHz as is usually the case for 800 MHz & up. Therefore, the ideal remedy for excessive Adjacent Channel Rejection is the installation of a narrow IF filter that is auto-switched INTO the circuit whenever the AM mode is selected and OUT for NFM. An ideal approach that won't cause more problems than it resolves is not yet at hand because of space limitations and probable unavailability of a suitable miniature IF filter. I am uncomfortable at performing mods that can't be made rather professional as if they were designed there in the first place. Therefore, I will not be performing modification services to the R-1 in the near future, and at least not until more information and experience on the unit become available to me. At this time, I see a remedy of the poor Adjacent Channel Rejection as causing other problems where there were none before; i.e., limiting the utility and usefulness of the UHF-hi bands where the full bandwidth of the NFM IF filter is required. Then there is the matter of invading and hacking the R-1. After careful review of the innards of the R-1, I can only conclude that due to its compressed design and lack of working room, there is a substantial chance of serious error, mechanical and/or electrical, associated with an invasive effort. This makes a commercial hacking venture quite unfeasible and at best, very risky to the hacker. Not many pro's can afford to replace a mis-hacked R-1 at its current cost, you see. Neither can most hobbyists, therefore any amateurish invasion with the idea of correcting Adjacent Channel Rejection problems is ill-advised at this time. I do not say that it's "impossible" since further investigations, research and development may yet disclose a viable approach. Any such are not apparent right now. There is the matter of strong signal overload which pretty much eliminates the use of external antennas and preamplifiers for the R-1. It will be virtually impossible to remedy this serious deficiency. The singular reason for the overload problem is that the RF Front End of the R-1 does not employ AGC (Automatic Gain Control), therefore it can operate only over a limited range of amplification before it is driven into cutoff and saturation, either/both of which produce symptoms of overload including clipping, distortion, desensitization, spurious & pseudo signals across the band. The only practical remedy for this deficiency is to reduce the signal strength into the R-1, either by a less efficient antenna or by use of an attenuator between the antenna and the receiver. Neither remedy is what I call very appropriate for this serious of a receiver. One possibly viable approach to remedying the Adjacent Channel Rejection problem of the R-1 (for you adventuresome hackers, myself included), will be to intercept a signal trace in series with FL-1 on the DET-A board, either into or out of the FL-1 filter. A suitable point on this circuit trace will be broken and a narrow, sharper 455-KHz IF filter will be inserted in the cut so as to be in series with FL-1. To make this new filter switch in and out, depending on AM or NFM mode, the IN & OUT terminals of the new filter will also be connected to the IN & OUT terminals of a CMOS bilateral switch such as a 74HC4066, with the control terminal of the switch connected to the Collector of Q-7 or either Base of Q-5 on the DET-A board. This is a +5v control signal for the NFM/WFM modes (0-v for AM) which will enable the 4066 switch to bypass the new filter in the NFM/WFM modes, and to be inactive in the AM mode. Some refinement to this approach may be necessary for ultimate best results, but it sure seems the way to go at the moment for the researcher. For those who can't visualize this technique, imagine a simple SPST switch connected to the IN & OUT terminals of the new filter. When the switch is closed, the filter is effectively shorted out, thereby routing any signal around the new filter. When the switch is open, signals must pass through the filter. Ok, the Collector of Q-7 or either Base of Q-5 on the DET-A board have this Mode Control Signal available, which can be used to control a 4066 CMOS bilateral switch to bypass the new filter as desired; i.e., OUT for AM, and IN (shunted) for NFM & WFM. NOTE: A 4066 chip is rather large for the R-1, so a suitable alternative might be a TC4S66F (IC-10 in the PRO-2006) or a replacement TC4S66F (IC-2 on DET-A of the R-1). The TC4S66F is a very small single section SMT CMOS bilateral switch. In closing and summary, any improvement of the Adjacent Channel Rejection in the LF-MF-HF AM bands will degrade the performance of the NFM mode in the VHF-UHF bands. Depending on your interest, the tradeoff of one serious problem for another is not a good way to go. More research and analysis is required before we settleś on a solution. I keep hearing that RayComm of England has solved the problem, so I would like to hear from those who had this procedure done; specifically, I want to know how performance was affected on the Cellular Mobile Telephone bands where the FM deviation is in excess of 5 KHz. I would also like to scrutinize and test an R-1 that was modified by RayComm. At this point, I am concerned that in solving one problem, they may have created another, possibly worse one. This concludes my intermediate technical analysis of the R-1 and any pending solutions to its deficiencies. 73/ Bill Cheek --- Origin: Hertzian Intercept Point, San Diego, CA (1:202/719.2)