NASWA Journal Columns · Equipment Reviews, December 1998

Equipment Reviews, December 1998

Last Updated on December 8, 2005 by Ralph Brandi

The Ten-Tec Model RX-320 PC Radio

This has been one of the most fun-to-use receivers I have ever tested — I think that any SWL with a computer would love to have one of these radios! Since the radio’s control functions and most of the signal processing are done via software, the radio has a fantastic price to performance ratio, with a price of $295 plus shipping. The Ten-Tec RX-320 is a 3 x 6.25 x 6.5 inch black metal box that plugs into a serial port of a personal computer running Windows 3.1 or 95. The computer must be a 386 or better and have at least one megabyte of free hard disk space available. The DSP functions are handled by a chip in the receiver, so processor speed is not important for receiver performance. I used a 200 MHz Pentium and response to commands was almost instantaneous. The receiver is supplied with a 9 pin (DB9) serial cable, AC adapter, a patch cord for connecting the receiver’s audio output to the computer’s sound card line input, control software on a 3.5 inch diskette and a telescoping whip antenna which screws into the radio’s circuit board via a hole in the top cover. Provision for an external antenna (RCA phono connector) and an external 4 ohm speaker (1/8 inch jack) are available on the radio’s rear panel.

The RX-320 is a general coverage receiver with a frequency range of 100 kHz to 30 MHz. Demodulation, filtering and AGC are all done by digital signal processing. Reception modes of AM, USB/LSB and CW are supported. There is no synchronous AM mode. The receiver’s IF bandwidths are 8, 5, 2.5, 1.8 and 0.5 kHz (shape factor 1.5:1 or better) on the latest models. The receiver was originally equipped with 6 and 3 kHz values instead of the 8 and 5 kHz and the advertising I have seen as late as the November 1998 QST still shows values of 6 and 3 kHz. The folks at TenTec told me that the change was made for better SWL performance. AM sensitivity is rated as 0.64 uV with the wide filter (80% modulation, one kHz tone, 12 dB S+N/N). SSB sensitivity is 0.3 uV with the 2.5 kHz filter. Third order intercept is specified as +10 dBm with a 90 dB dynamic range with the 2.5 kHz filter with 50 kHz signal spacing. The radio is a triple conversion design with intermediate frequencies of 45 MHz, 455 kHz and 12 kHz-the final conversion being for the DSP circuitry.

Installation

I really appreciate that TenTec supplies all the necessary cables for connecting the RX-320 to a computer. They even include a RCA connector for the external antenna jack. The radio does require a free serial port to be connected to, but it can be configured to use COM 1-4. The serial cable and the audio patch cable are six feet in length, to allow placing the receiver away from the computer and monitor. The control software is loaded from a 3.5 inch diskette and takes up less than a megabyte of hard disk space. The software version I received with the radio was Version 1.25, but Version 1.26, which repairs a couple of glitches, is now available on the TenTec Website. I initially had no audio output from the sound card, but this was a problem with my computer. I had to activate the line input via the sound card’s mixer program. It took longer to figure this out than it did to install the software and start using the radio! If your computer doesn’t have a sound card, the RX-320 provides a one watt output to drive an external 4 to 8 ohm speaker.

Gee, it looks like a radio!

At this point, I strongly advise you to go to TenTec’s Website and download the RX-320 control program software, since a picture is worth 10,000 words. The software will operate without the radio. Once the control program is started, the main screen comes up with three windows: the radio panel, the memory window and the spectrum scan display. These windows can be opened or closed by clicking on buttons on the program’s toolbar or via the “View” option on the Menu bar. The radio panel window is the primary one that most operators will use. It looks just like the front panel of a typical communications receiver with frequency display, tuning knob (with spinner indent!), pushbuttons for mode, filter and tuning step and a slider for volume control. The computer’s pointing device (I prefer a trackball to a mouse, due to limited real estate on the desk) is used to select the desired function. The top portion of this window has the frequency display, with readout to 1 hertz. Along with the frequency the current filter, mode and tuning step are displayed. There is a clock with both local and UTC times displayed. The radio’s clock is slaved to the computer’s clock. Offset between UTC and local time is set via the “Options” button on the toolbar or via “Setup” on the Menu bar. Just below the frequency display is the log scale. The log scale is calibrated in 2 kHz steps and covers a span of approximately 260 kHz. A red index line in the center of the log scale indicates the currently tuned frequency.

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Where’s the tuning knob?

The log scale can be “dragged” to retune the receiver — very useful for quick frequency excursions, but not very precise. At the right end of the log scale are right and left arrowheads. When these are clicked the radio will tune in steps based on the currently selected step size in the direction of the selected arrowhead. Available step sizes are 10, 5 or 1 kHz or 100 or 10 hertz. When a specific mode is selected, a default tuning step is selected: 5 kHz in AM and 100 hertz for SSB/CW. These defaults cannot be reset. At the left end of the log scale are double left and right arrowheads. When these are clicked, the radio tunes in steps equal to 10 times the current step size.

Yes, there is a (virtual) tuning knob. By pointing to the tuning knob on the panel display the radio can be tuned by clicking. Point to the upper half of the knob and a clockwise arrow appears. The receiver then tunes up in frequency by the selected step size for each click — point to the lower half of the knob to tune downward. The receiver tunes continuously if the left mouse button is held depressed. For those who refuse to use a mouse, the computer’s Up and Down arrow keys will also tune the radio.

Direct frequency entry is also possible via the numeric keypad (Num Lock must be enabled). I found it easiest to enter frequencies in MHz format, i.e. with a decimal point. Kilohertz entry can be forced by typing “k” instead of “Enter” after the desired numerals. The instruction manual states that the program will attempt to determine the format based on the numbers entered. This didn’t work too well — entering frequencies less than “1000” tuned the radio in kHz, but numbers greater than 1000 resulted in the radio tuning to hertz. For example, entering “7,8,0,Enter” tuned to my local talk station on 780 kHz, but typing “1,2,9,0,Enter” tuned the radio to 1.290 kHz — below the reception range of the radio! I found it easier the use the “k” method to enter medium and longwave frequencies and the decimal point and “Enter” for shortwave — zeroes after the decimal point do not have to be entered. By clicking on the frequency display, a numeric entry window is opened and the desired frequency can be typed in, but I don’t see much point in this, as it requires an additional mouse click compared to the keypad entry method.

Other controls

To the left of the tuning knob are the buttons for bandwidth selection: 8, 5, 2.5, 1.8 and 0.5 kHz. Any of these values can be selected for any mode, but the defaults when a specific mode is selected is 8 kHz for AM, 2.5 kHz for LSB/USB and 1.8 kHz for CW. Further to the left of the tuning knob are the memory controls. At the top of this column is the memory display — this shows the station identifier (i.e. “BBC”) when the receiver is tuned to a frequency that has been saved in memory. Below this window are the memory save and recall buttons. When the “Save” button is clicked, the “Station Information Sheet” pops open. This permits entering the station callsign or identifier, frequency, mode, bandwidth, country, language and notes. Pressing the “Recall” button pulls up the memory window — which I will describe later. The bottom button in this column is “World Time” — clicking this button tunes the radio to 10 MHz in AM mode. The default frequency for the “World Time” button cannot be altered. Unfortunately, there’s no way to go back to the frequency the radio was tuned to, other than using the keypad or the other tuning methods — it would be nice if clicking the “World Time” button a second time would return the receiver to the previously tuned frequency.

To the right of the tuning knob are the mode buttons: AM, LSB, USB and CW. The AM button is labelled “AM/SW”, I presume for those North Americans who equate “AM” with the medium wave band. Contrary to what is shown in the ads for the RX-320, there is no “SAM” button for synchronous AM detection (hopefully a future development, but “not at this time” according to TenTec). Further to the right are the tuning step buttons: 10K, 5K, 1K, 100 and 10 hertz. There are no provisions for 9 kHz steps for overseas MW listeners. The minimum tuning step of 10 Hz is adequate for SSB/CW signals, but is still too coarse for really good ECSS tuning of AM signals with music content, although it is adequate for voice programs. The other obstruction to ECSS tuning is outlined below. Any tuning step can be selected for any mode, but switching modes resets the receiver for the default step for that mode; 5 kHz for AM and 100 hertz for the other modes. In addition, switching modes changes the resolution of the tuning display to the default tuning step for that mode.

Zero-beating, Not!

This can present a problem when attempting to “zero-beat” a station to mesure the exact frequency or for ECSS tuning of AM signals. An example is in order to illustrate this. If the radio is tuned to a signal at 9.755 MHz in AM mode, then switched to USB and 10 Hz tuning step selected, the signal is zerobeat at 9.75486 (the master oscillator is off slightly — TenTec says that the capability to adjust this oscillator will be incoporated into future software revisions), then switch to LSB to compare the sidebands. At this point, the tuning step switches to 100 Hz, which is OK, because it can be changed, but the receiver retunes to 9.75480 MHz (note that the tuning goes to the lower 100 Hz increment, not the nearest). Therefore, one sideband cannot be compared to the other without resetting the tuning step size and retuning. If the AM mode is then re-selected the receiver retunes to 9.755 MHz, since the tuning step defaults back to 5 kHz and the display reverts to 1 kHz resolution. Going back to the zero-beat frequency requires resetting the tuning step and retuning the radio. The solution would be to have “sticky” defaults where the software remembers the last step used for a particular mode or being able to specify the defaults via an options menu.

Unique S-meter

A non-standard “S” meter can be displayed by checking a box in the User Options brought up by the “Options” button on the toolbar or via the “Setup” choice on the Menu bar. It is non-standard in two ways: it has a 0 to 80 scale rather than “S” units and it has TWO needles! The lower needle indicates the current relative received signal strength while the upper is a “peak hold” indicator with a decay time of about one second.

Other features

So far, what I have described is a radio with a virtual front panel. However, the RX-320 software has two additional components that make it more than that. Although these features are available through software control programs for other radios, TenTec’s implementation of them make them very functional and easy to use.

The first of these features is the memory window. When this option is selected, a window opens with a list of stations stored in memory displayed. The memories can be reviewed by scrolling through the list. The memories can be displayed in order of callsign/station identifier, country of origin or frequency. Clicking on a memory highlights the listing and it can be tuned, deleted or edited. The edit command pulls up the “Station Information Sheet” which contains fields for station, frequency, mode, bandwidth, country, language and notes. There is no “Copy” command for making duplicate entries where only one field such as the frequency needs to be changed. On the main memory screen is a radio button for “Auto Tune” — when this option is selected, the radio will automatically tune to the desired memory channel with a single mouse click.

I found the memory list to be very useful and quick to use. The number of available memory channels is only limited by the computer’s available RAM, according to the manual, although surely what is meant is hard disk space, since the receiver’s memories are non-volatile. The manual states that any PC will store 1000’s of memories. The only drawback I can foresee to this memory capacity is that there is no way to search the list of memories. I think it would be nice if there was a search function, especially if it was the type of search that Microsoft uses in some of their programs where the search results automatically narrow as more characters of the desired search target are typed in. Unfortunately, based on my correspondence with TenTec, there are no plans to implement such a feature at this time.

Is the band open?

The other feature of the RX-320 that I found quite exciting was the spectrum window. This window is a spectrum analyzer display, which shows a plot of signal strength vs. frequency. The sweep widths available are 1.5 MHz, 750, 300, 150, 75, 30 or 3 kHz. The center frequency can be the frequency that the receiver is currently tuned to or can be input by the user. The sweep time is 25 seconds and the receiver mutes during the scan. Once the scan is completed, the user can point to the spikes on the display and tune to that frequency by clicking. I found that the spikes lagged the actual frequency by one or two kilohertz, so some fine-tuning may be required. This was no big deal, since it just involves moving the mouse pointer and clicking a time or two. The spectrum analyzer works quite well — it does a good job of distinguishing signals from noise. I really enjoyed this capability, especially for band-scanning and searching for new stations.

Is it a good radio?

Since the RX-320 has no front panel controls and signal detection and processing are handled by a couple of integrated circuits, the overall cost of the radio is low. I would comfortably state that this is the best radio available for $300. Due to the inherent cost savings in the design, overall performance is approximately equivalent to “normal” radios costing up to twice as much. Of course, one must factor in the cost of the required computer system, but most people that would be interested in the RX-320 have a computer already. I don’t have any experience with the Icom PCR-1000 or the Winradio, so I can’t compare how the TenTec performs relative to these other PC radios. The PCR-1000 and Winradio do have VHF/UHF coverage, unlike the RX-320, which only goes up to 30 MHz.

Due to its lack of advanced features such as passband tuning and notch filter, the RX-320 isn’t a replacement for kilobuck DX machines, but it does a creditable job for both program listening and DX’ing. Sensitivity is more that adequate, especially with an outdoor antenna. I could routinely copy hams on ten meters and got plenty of staions on the tropical bands. The filtering, due to the tight skirt selectivity inherent with DSP filters, is outstanding. The 8 kHz bandwidth was excellent for program listening and the 5 kHz value works well for rejecting adjacent channel interference. The 5 kHz filter sounds somewhat “pinched” (consistent with the tight skirts), but the receiver could be off-tuned by one kHz or so to “brighten up” the sound while still rejecting interference. Both the 2.5 and 1.8 kHz filters were very useful for listening to hams. The AGC characteristics were good overall, although there was some “thumping” on weak signals. I couldn’t detect any difference in the AGC when switching from AM to SSB modes. I was unable to detect any spurious signals from local MW broadcasters, either above or below the standard AM broadcast band, even with a half wavelength 90 meter dipole. TenTec rates the receiver’s dynamic range at 90 dB (2.5 kHz bandwidth at 50 kHz spacing) and the third order intercept at +10 dBm. I thought that the audio quality was excellent, but this will be dependent on the quality of the sound card and speakers that are attached to the host computer. The built-in audio amplifier had more than enough power to drive a Drake MS-4 speaker to room-filling volume. The external speaker output can be used for headphones, but a mono to stereo adapter is required for stereo headphones.

Hints and quirks

With my particular computer and monitor, I was unable to use the RX-320’s whip antenna for anything more than a quick test due to computer noise. Most of the noise was from the monitor. Based on my experience, I would recommend locating the antenna for the radio as far from the computer as possible and using good quality coaxial cable to connect the antenna to the RX-320. Initially, I still had a noise problem when using an outdoor antenna with a coax lead-in — I found that center conductor of the coax had a poor solder joint at one of the connectors. Resoldering the joint almost completely removed the noise. The connecting cables for the radio are six feet in length, so the radio can be placed away from the computer and closer to the antenna. A good earth ground can be useful for reducing noise, as well.

When I used the control program with my monitor set for large fonts, the bottom 1/5 of the clock digits were cut off. Switching the monitor preferences for small fonts restored the full clock display.

I did notice some quirks with the memory display window on the radio panel. I had saved the frequencies for RFPI (6,975 and 15,049 kHz) in memory. I was quite surprised to see “RFPI” pop up in the memory window as I was tuning the 160 meter ham band one evening, at 1942 kHz. Subsequently, “WLW” (normally at 700 kHz) appeared in the window as I tuned past 13,807 kHz while bandscanning. Note that the radio didn’t tune to these frequencies — just that the stored station identifier appeared in the display window. I’m not sure what causes these anomalies.

Occasionally, the control program wouldn’t start — a “floating point error” message was displayed. This is a bug that occurs when the S meter display option is chosen. Version 1.26 of the control software supposedly fixed this bug, but I still had the problem intermittently. While cycling the power switch in frustation during one occurrence of this problem, I managed to break the switch. Fortunately, the power switch is the only component that is not mounted on one of the RX-320’s two circuit boards, so a quick trip to Radio Shack and some soldering was all that was needed. The power switch is somewhat awkward to reach, but the receiver can be left powered up at all times.

The fun factor

I really enjoyed using the TenTec RX-320. My usual radios are beyond an arm’s length from my computer, so it was really nice to be able to open the receiver’s window and retune the radio while using the computer. I had no conflicts while using the radio with any software. While I would like to see some changes, primarily being able to specify defaults, adjust the master oscillator and adding a search function to the memory page, the RX-320 is a solid performer and is really fun to use. Selectable sideband synchronous AM detection would also be a nice addition, but I am unsure of the feasibility of this modification. A programmer’s reference guide is available at TenTec’s website, so it will be interesting to see if any third-party software becomes available.

I think that the highest compliment that I can pay to a piece of review equipment is that I end up buying it instead of sending it back. That’s my plan for the RX-320. I would also like to compliment TenTec’s factory service — I managed to break the potentiometers on my 1254 kit receiver — they sent me replacements at no charge. If you would like to see another review of the TenTec RX-320, check out Tom Sundstrom’s review in the Receiver Shopping List at the Radio Nederlands website (www.rnw.nl) — he gives it five stars and I concur. The price for the receiver is $295 plus shipping direct from TenTec. The radio has a 30 day money back guarantee (minus shipping charges) and a one year warranty. TenTec’s address is 1185 Dolly Parton Parkway, Sevierville, TN 37862; (800) 833-7373; E-mail: sales@tentec.com ; Website: www.tentec.com.

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