Part 2: Further Adventures with the IC7700 and a second receiver
Having made a hybrid coupler and tried it out with the Pro 3, I decided that the Pro3 was to much for a second receiver and have moved it off my operating position and into the hands of Tim VE9XA (I hope you get as much enjoyment from the Pro 3 as I did , Tim).
Coupler Issues: The hybrid coupler, reported in Part 1, works fine for delivering a second signal to any second receiver device. The 7700 allows a simple button push to engage the external receiver loop. On the down side, I have noticed the 3 or 4 dB loss on weak signals, especially on 20 M and above. It isn’t much, and I doubt I will find it a problem unless I am concentrating on working some especially weak station. I did add a switch and a second RCA jack on the box to allow switching to the BOG antenna, when needed. So there is now an out put for a second receiver and an input for occasional use of another receive only antenna. I didn’t want to switch any of the 4 ports on the 7700 as I have a perfectly useable setup to switch my main antennas and amplifier on those ports. Further, the input grounding relay isn’t required unless I am actually using another transceiver as a second receiver. If not, I can just pull the plug from the ACC socket on the 7700 and let the coupler sit as a static device. If you are just feeding a second receiver only, one could just hook up a ‘Y’ in this external receive loop and feed the second device directly. The impedance mismatch isn’t likely to produce much of a signal drop anyway, and might even be less than the hybrid couplers. (I will try that at some point)
706 MK2G as second receiver: I have one of these nifty little rigs around my shack and it is very easy to use it as a second receiver. I tried this out on a few split CW contacts. Worked fine. The 706 has full featured controls, mine has a CW filter, and the DSP functions work great. It’s fine for the occasional second receiver option, but not as useful as having a nice big spectrum display on a second receiver. A dedicated second receiver, with spectrum display, is what I desire.
SDR-IQ : I work in the field of DSP and software receivers ( not for RF signals) and have followed the drastic reduction in price and huge increase in capability of DSP chip sets over the last few years. (These developments have been largely driven by the mass market needs of the telephone data industry). A full software HF radio is indeed a reality these days. I had looked at moving to a Flex 5000 radio as a new main radio, but decided against it. I prefer the front end and ‘GUI’ of a well designed analog radio, and ICOM still makes one of the best, to my mind. I have no complaints with the 7700. It is a wonderful radio to operate. I have used it in two short CW contests and it lives up to its reputation. But, I had decided to get a software radio as well, and needing a second receiver, I used this as an opportunity to jump into the arena.
I looked at the Flex, RF Space products, and the Perseus. The Flex is a nice machine, but I don’t want the transmit features, and the Flex (and Power SDR software) need a high end computer and sound card. I want to use this with my station laptop without performance degradation, and run the software at same time as the full suite of DXLab. The Perseus is a highly reputed device, but fairly expensive. The RF Space receivers (SDR-IQ and SDR-14) are interesting because they use a USB interface and bypass the sound card as an input device. This is a very good system, and really simple to implement. They have two gadgets.. an SDR-14 which does 0 to 30 MHz with a huge bandwidth ( over 1.6 MHz !) and an SDR-IQ with a bandwidth of 190 kHz. (This unit is a bargain at $500 US). See http://www.rfspace.com/SDR-IQ.html for a description. (The SpectraVue software is free. WinRad also works with this unit and is free. ) Further, these units are powered from the USB port. You can’t get much simpler than that. In fact, take this little box with your laptop and you have a complete high end receiver… no other equipment needed.
I ordered one up from Universal Radio and it arrived promptly. It’s a tiny box and has little space requirements. I will come back to its operation later.
Probing potential IF outputs on the 7700. There are two ways to use the SDR with your main radio. One way is to just hook the thing to your antenna and have at it. This does work fine, but you are missing the filtering features of your main transceiver. I wanted to explore this area as well as direct antenna hookup.
Time to look at the 7700 superhet scheme. The main receiver takes the 0 to 30 MHz signal to a first IF of 60 MHz (and up). The second IF conversion is to 36 kHz and up. This is where the DSP and filtering operates. The problem with taking the 36 kHz signal out ( very doable) is the roofing filters in the 7700. They are restricting the IF pass band to one of several selected bandwidths from 15 kHz to 3 kHz. I really wanted to look around with the SDR outside of this pass band. The first IF is also too high for the analog to digital converter in the SDR.
However, there is a second receiver in the 7700 which is built around the spectrum scope. The first receiver IF is tapped and sent to a second receiver on the scope board. This unit has a second IF conversion to 45 MHz. This is still too high for the SDR, but there is a third conversion to 200 kHz to allow processing for the spectrum display DSP. This one showed promise. I flipped the 7700 over (carefully.. it’s a big radio) and exposed the ‘scope board’, as ICOM calls it. I used the input of the SDR with a scope probe ( another neat feature of this unit… it’s a fully functional spectrum analyzer for a lot of shop use !) and hunted around the 200 KHz for a signal. There is one, and it does work, but it is timed to sweep in conjunction with the display sweep of the 7700 and doesn’t seem to be all that useable. I may come back to probing this spot a little more in the future, but for the present, this doesn’t seem like a workable idea.
One could add an external conversion for the higher IF, but really, what’s the point ? Might as well use the SDR right on the antenna for now and see how it performs, without the radios built in filtering.
Talking to the SDR-IQ: There is a com port as part of the SDR-IQ. The internal commands are available to programmers. There is also a suite of radio command sets included for each of the major manufacturers, including ICOM. If you can get your transceiver and logging program to talk to each other, you should be able to get the SDR to follow the receiver frequency. In my case, I wanted the transceiver frequency to be centered in the SDR’s bandwidth. I use a homebrew CAT interface between my laptop and the transceiver and run Dave Bernstein’s DXLAb suite on the lap top. This software is absolutely the tops for dx hunting, log management and even simple contesting. It works perfectly with the ICOM radios. It’s completely free, BTW, and support is via a Yahoo group. It is rare to have more than one day for a response to a question or a problem, and upgrades to the software are really effortless. Further, third party developers have made some great modules to work with it. (Like DX Atlas maps) I have tried DX4WIN and it can’t hold a candle to DX Lab. I don’t understand why anyone would pay money for it, frankly. (My opinion) To get the SDR to participate in the CAT radio net, it turned out to require another CAT device on the SDR port with the devices radio line plugged into the common radio line used between the existing transceiver and the computer. I found this less than clear and a tip from Dave’s website helped out, although I don’t think the question responder had actually tried it out.. too many devices specified! Another minor problem… the com port DB9 on the SDR doesn’t have an RTS or DTR line which you can use to power a CAT device. An external power supply is needed. This isn’t surprising as the SDR-IQ is self powered from the USB port, and the designers may be conserving limited power. Although, there is an irritating LED on the front panel that is flashing continuously and could easily be unhooked. That would donate 10 mA or so for DTR or RTS use. I might give that a try, actually.
I wanted the frequency to travel around the devices. And this did indeed happen but it took a while. The instructions with the SDR are not terribly clear. And I had to put quite a bit of effort into getting the CATs to talk to each other (Sounds like real cats, doesn’t it?) I use homebrew CATs recommended by Dave Bernstein (the 4 transistor, self powered ones) and it was very fussy getting this all to work. In the end, the order of power up was critical. Fine now that I know what to do, but this could be easier. I found I had to power up the SDR’s CAT last, after everything else was working. But work it does and the SDR can be set to follow the transceiver or DXLab Commander frequency. You might not actually need this feature that often, so think about whether you really need it before you start fussing with it. It could be very handy if you jump around the bands a lot. Saves continually setting the SDR frequency.
Postscript: (The auto baud rate function seems to create havoc with the three units.. at 9600 baud everything seems to be happy)
Postscript: A better way to arrange communications was suggested by Dave, N7DRK. The com port on the IC7700 is hooked directly to a com port on the computer, and only one CT17 is used to go between the SDR-IQ and the IC7700 remote CI-V jack. I tried this and it works better than the two CT-17 system. Everything seems to run smoother. Thanks Dave !
SDR-IQ SectraVue operation: In simple terms.. this thing works great! You can make it do almost anything. It can serve as a pan adapter with any radio’s IF output… no adapters needed. Or, just hang it on the antenna and use it as a second receiver. I haven’t seen the need to add any additional filtering yet… although it could REALLY use an internal auto correlation FFT function (That’s ‘NR’ or noise reduction, to you math challenged folks). The built in demodulators are great. The filter pass band is live adjustable to anything you like. You can set the units scan bandwidth to anything you want up to 190 kHz. The gain, FFT functions, graph and display functions are fully adjustable. And, this thing is FAST! It runs circles around the built in spectrum scope of the 7700 (which is quite a good one) and the resolution bandwidth can go down to .05 Hz if you want. In practice, I find 2 to 5 Hz is fine. The display is very adjustable with colours and features. It takes a bit of play time to see what they can do. You can make some gorgeous displays. There are all kinds of display options as well, including waterfalls, 3D displays, continuums, etc. Also, there is a built in recording function and the ability to output directly to spreadsheet files, and other formats. ( I will probably be using this gadget in my work too, after seeing this). There are two decoders available and you can output them to different channels (left and right) or mix them together. When you lock a decoder on a signal… it stays there even if you change the bandwidth center frequency with the radio or computer log software. A very useful feature. The only thing that is really needed (hint.. hint… RF Space..), is a built in noise reduction processor for SpectraVue.
As an aside, for those of you that have toyed with idea of having a spectrum analyzer to play with radio circuits… this gadget and your laptop is really all you need. It is a full featured analyzer.
WinRad comparison: I downloaded and tried WinRad which is a freebie for those interested. It has a beautiful GUI and works very well with the SDR-IQ. There is no noise reduction processor here either (needed) and one small problem with the window… you can’t re-size it. You are forced to use it full screen. That’s fine, if you have lots of monitor real estate, but it would be nice to be able to resize the window. This is not a commercial product, so that is only going to happen if someone volunteers with the owner to do it. It is hard to decide which software package is better. WinRad works well but has fewer adjustable features and doesn’t allow coms with the transceiver CAT bus. So, for me, I will keep both programs available and play with them both.
PostScript: Pieter from RF Space suggested I look at WinRad again and try the ‘N Red’ for noise reduction. It works great ! I need to spend more time with this lovely piece of software. As a general purpose receiver control and GUI, it does a wonderful job with the SDR-IQ.
Radio reception performance: In a word.. it’s pretty good ! I can hear just about everything that I can hear on the transceiver and the demodulators work beautifully. If are a serious SWL or broadcast band DXer, this little gadget and your computer might be all you need for serious effort. I understand there is an issue of some birdies that appear in one or two of the higher broadcast bands, but I haven’t heard them yet. My primary use is for ham radio anyway.
Complaints: Three small ones alluded to above. The auto correlation function would really add to the utility of the software. Adding power to the RTS and DTR pins would be useful for the CAT users. And.. the irritating blinking LED on the front panel. It shows data processing, something like the hard drive LED on PC’s. It isn’t needed, and is very distracting. Solved this problem with a piece of black tape, but I might unhook it, and see about diverting some power to the RTS and DTR pins on the DB9 connector on the rear panel.
Postscript: ( I removed the yellow LED leaving the front panel a little less busy. Also put the USB 5 volts on Pin 4 and 7 (DTR and RTS) of the DB9 connector, which now powers up the CT-17 device attached to it.)
Second receiver analysis: This unit fills the bill and does everything I could have hoped for, and a lot more. For a real bargain of a price too! I recommend this gadget for a second receiver. In fact, as a receiver by itself, this is a pretty neat gadget. If you want to spend a little more money, almost any of the SDR’s are going to be great for a second receiver or a fully featured panoramic spectrum display.
Here is a screen shot of the unit running with DXLab, on a Sony VIAO B100B and extended desktop on a second monitor.