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Using the Icom R7000 as a spectrum analyser The Icom-R7000 & similar receivers can be readily interfaced to a personal computer to form a simple, inexpensive spectrum analyser. The resulting analyser has 100dB of dynamic range & is capable of examining almost any section of the spectrum between 25MHz & 2GHz. Several different resolutions can be selected, from 2.8kHz to 150kHz. By JAMES LLOYD & JOHN STOREY* What makes this possible is that the Icom, like many modern receivers, incorporates a CPU-controlled PLL synthesiser. This same CPU controls the other functions of the receiver and can itself be controlled from the RS-232 port of a personal comput­er. The only modification required to the receiver is to tap into the AGC (automatic gain control) line to meas*School of Physics, UNSW, Kensington. 70  Silicon Chip ure the signal strength. Once this is done, all that is required is software to scan the receiver through the range of frequencies of interest, to record the signal strength at each frequency step, and to display the result. Receiver interface The interface between the PC and receiver is the “Icom Communication Interface V” (CI-V). The first thing needed is a small piece of hardware to convert the CI-V voltages to and from RS-232 levels. This little box is avail- able from Icom but in fact, it contains little more than a MAX-232 integrated circuit and would be easy to build. The Icom CI-V is a serial, half-duplex bus which has the advantage of being able to control several receivers. This actu­ally presents a few difficulties in interfacing to a PC. Firstly, the RS-232 standard is full-duplex (having a separate transmit and receive line) and has hardware hand­shaking facilities. This hardware hand­shaking allows the transmitter and receiver to agree when they are both ready to exchange data. The CI-V bus, however, consists of just a single data wire plus ground. The problem of bus arbitration (or agreeing who can talk and when) is tackled with the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol. This simply means that any device may try to use the bus at any time. Any collisions (more than one party attempting to transmit simultaneously) are detected and resolved by one of them waiting for the other to complete. This allows multiple devices on the one bus and is generally an efficient protocol if the bus is not over-committed. However, it does present some difficulties in implementation on a PC, since there is no readily available “carrier-sense” signal. We solved this problem by allowing all the communications between the receiver and the computer to be handled by a share­ware communications library. This tests for “line busy” by moni­toring the traffic in and out of the data buffers, thus emulating the “carrier sense” signal, equating carrier to transmission or reception of data. When the receiver detects a collision, it transmits a series of jammer codes which cause the software to stop sending. At the end of a command string the receiver transmits “acknowl­ edge” signals which are monitored to ensure reliable operation. The general command structure is a stream of bytes, con­sisting of two preamble bytes (to signify the start of the com­mand, each being the value FE hex), a destination or “to” address (08 hex is the default for the R7000), a sender or “from” address (default E0 hex for the controller), one or two bytes specifying the command number and an optional subcommand number, a variable number of data bytes and an-end-of-command byte (FD hex). The frequency data is sent as five BCD (Binary Coded Deci­mal) bytes, in least significant to most significant order. Binary coded decimal is a scheme whereby each decimal digit is encoded as a 4-bit nibble and thus a 2-digit number can be encoded in one byte. For example, the frequency 123.456789MHz would be sent as the bytes 89 67 45 23 01 (BCD). Since the tuning step is 100Hz, the first byte (1’s and 10’s of Hz) is ignored. The GHz digit is also ignored, since the 1-2GHz range of the receiver is set by manually pressing a button on the control panel. To “set frequency” the command number is 05, with no sub­command. To tune the receiver to the frequency 123.4567MHz, the command string would be FE FE 08 E0 05 00 67 45 23 01 FD. The receiver then sends a response, addressed to the controller, with a value in the command number field indicating the success (FB) or failure (FA) of the command. Thus, a successful command would return Fig.1: this buffer circuit was used to isolate the AGC line from the receiver and thereby avoid any loading effects from the PC. the string FE FE E0 08 FB FD. The complete process of stepping to a new frequency thus requires a total of 17 bytes. At 1200 baud, this takes approximately 120ms, plus the response time of the receiver CPU. The minimum step size of the synthesiser is 100Hz. Actually, the digital synthesis is done in 1kHz increments. The 100Hz steps are generated within the Icom with a D/A converter driving a VFO. However, for our present purposes this is of no consequence. The maximum step size can be anything you like and in fact the receiver could be hopped about in frequency in a completely random manner, though it’s hard to imagine why anyone would want to do so. Normally, one would choose a step Fig.2: this spectrum is the electromagnetic interference from a 33MHz 386 PC taken over the range from 50 to 100MHz. Radiation below 50MHz was found to be negligible. The 99MHz peak is most likely the third harmonic of the 33MHz internal CPU clock & the others are probably harmonics of the bus clock. April 1994  71 the same way as for the frequency commands. Extracting the AGC voltage To extract the AGC voltage, it is necessary to open the receiver, identify the relevant circuit connection and bring it out to a suitable socket. We chose to add a buffer amplifier to avoid any possibility of disturbing normal operation of the receiver. As shown in Fig.1, the buffer circuit uses an OP90 op amp as it has low power consumption, low offset voltage and operates from a single supply. The circuit has a gain of 2, to bring AGC voltage swing up to the full input range of the analog/digital converter. Icom are even kind enough to supply the R7000 receiver with a spare RCA jack on the rear panel, so no chassis work is re­quired. Measuring the AGC voltage Fig.3: this is the radiated spectrum of ABC channel 2 as received at Kensington, on the UNSW campus. This plot shows the structure of a television signal in the vicinity of the vision carrier (& should convince any doubters, if they still exist, of the reality of sidebands). The video signal is amplitude modulated onto the carrier & the dominant frequency component of this modulation is at the linescan frequency of 15.625kHz. The sidebands are thus located 15.625 kHz apart and extend symmetrically either side of the carrier. (Close in to the carrier the sidebands are expected to be equal. If the scan covered a wider frequency range the vestigial sideband character of the modulation would become apparent). The scan was performed with the “SSB” filter (FWHM of 2.8kHz), in 1.4kHz steps. size equal to half the filter bandwidth. This “Nyquist sampling” ensures the maximum amount of information is extracted from the spectrum. The settling time of the receiver tuning circuit is very fast. However, the AGC amplifier incorporates a time constant which is different for each of the receiver modes. In a moment we will show how we use the different modes to give the different filter resolutions. For the AM filter, the AGC time constant has been measured and found to be approximately 200ms. The AGC set­tling occurs concurrently with the transmission of the “acknowl­edge” signal, so it is not all “dead” time. Including all over­heads and settling time, reliable tuning of the receiver (to a signal stable within 0.5%) is 72  Silicon Chip achieved in approximately 500ms. Slight speed improvements could be made by increasing the trans­mission speed, or sacrificing stability in the signal. The resolution of the scan can be selected by choosing the R7000 receive mode which then selects the IF filter bandwidth. The filters have bandwidths of 2.8kHz (SSB), 6kHz (AM/FM narrow), 15kHz (AM/FM), 150kHz (FM wide). The receive mode (and hence filter bandwidth) is selected with command number 06 prior to beginning a scan, in a similar way to the frequency stepping mode. For example, selection of the AM filter (data field 02) would be done by the command string FE FE 08 E0 06 02 FD. The receiver responds in exactly In order for the computer to be able to read the AGC vol­tage, an analogto-digital (A/D) converter is of course re­quired. Almost any A/D would be suitable here, as the application is quite undemanding. We used a 12bit PC ADDA-12 card from ESIS in Sydney. This inexpensive unit works particularly well and has a conversion time of only 60 microseconds but care must be taken when using a fast computer. The card uses a monolithic successive-approximation con­verter which is clocked by strobing a register on the card. If this happens too quickly, the converter becomes confused and the accuracy drops dramatically. On the 386-33 PC that we used, we had to add delays in the code to prevent this from happening. The AGC voltage output of the receiver is highly non-linear with signal strength, as one might expect. Conversion of the raw AGC voltage to signal strength is achieved in the computer using a simple look-up table with linear interpo­lation between points. Creating the look-up table requires the use of either a calibrated signal generator or a signal genera­tor plus calibrated attenuator. The Icom R7000 receiver conveniently includes a 20dB switchable attenuator in the front end which could be used to get the calibration procedure off to a good start. Separate tables are needed for each of the monotonicly decreasing part of the curve, where there is only one signal value for a given AGC voltage. At zero and 10dB on the FM wide filter curve, there are two signal strength values for the one AGC value). Thus the system currently has a (software limited) sensitivity of 3 microvolts in the most sensitive bands. Software Fig.4: this is a spectrum taken of the FM band from 88 to 108MHz, taken with the FM WIDE filter (150kHz) in 75kHz steps. The anten­na was just a length of wire. The large peak at 107.3MHz is 2SER which is located on the University of Technology building in Ultimo and has a line of sight view to the Physics building at UNSW. The peak at 102.5MHz is 2MBS, with a transmitter located on the AMP building in Sydney, again very close and line of site to Kensington. Although the stations with powerful transmitters (2DAY 104.1, 2MMM 104.9, 2JJJ 105.7 and ABC 92.9) appear very weak, it should be noted that the vertical scale is linear and these stations are only a few dB below the most powerful. receiver filters and (at least in principle) for each of the four “front-ends” which the receiv­ e r automatically switches between as it changes bands. However, this variation of calibration with frequency is probably only a small effect and in many applications the system is only required to operate over a narrow frequency band. The data gathered by the calibration procedure is used to convert AGC voltage to signal strength, using a look-up table with linear interpolation. The discontinuities in the graph are an artefact of the signal generator used and occur at points where it switches circuits to alter range. Note the very wide dynamic range achievable, 100dB in the case of the AM filter. The soft­ware used only the The Icom R7100 Communications Receiver The Icom communications receiver pictured here is the R7100 model. This supersedes the R7000 model referred to in this article but it can be used for spectrum analysis in exactly the same way. Among its many features, the R7100 continuously covers the frequency spectrum from 25MHz to 2000MHz, has all-mode capability, 900 memory channels, and either direct keyboard entry or manual frequency selection. For further information, contact Emtronics, 92-94 Wentworth Ave, Sydney. Phone 211 0988. 74  Silicon Chip Depending on capabilities of the PC and on what your fa­vourite programming language is, the software can range from simple to complex. We wrote our program in C, with the following modules: (1) R7000LIB to handle the communications to the receiver, covering commands to set and read the frequency and receiving mode and interpret the responses for the receiver; (2) ADDALIB to perform the A/D conversions; and (3) AGCTOSIG to convert AGC voltages to signal strength using the data files generated from known signal strengths. Using these libraries, we built programs to perform auto­ mated scans, interactively scan, draw graphs and so on. The system can be used as a spectral analyser for virtually any application within the tuning range of the receiver (25-999MHz and 1025-1999MHz in the case of the Icom R7000). The main limitation is one of speed. Because of the time taken to scan across the spectrum, the result will only be meaningful if the spectrum is effectively unchanging during this time. The accompa­ nying spectrum plots demonstrate the capabilities of the system. Conclusion Computer interfacing to the Icom R-7000 receiver is straightforward and gives satisfying results. In fact, under computer control the extraordinarily good performance of these receivers in terms of versatility, stability, sensitivity and low spurious response levels becomes apparent. The spectrum analyser described here is just one example of what can be done once a PC is given control of the receiver and is able to monitor signal strength. Another interesting application for the avid SWL or DX’er would be to log the signal from various HF stations from around the globe. Why not become your own ionospheric SC prediction service?