using an RTL-SDR connected to the first I.F. mixer…
© 2018 by KV5R. All Rights Reserved. Rev. 2/8/2019.
The Icom IC-7100 is a nifty radio. Like the old 706MkIIG, it covers all amateur bands (except 220) from 160 meters through 70 centimeters, in all modes. It's small, and has a detached control head. But that's where the similarity ends. The 7100 adds DStar digital voice, I.F. DSP filters (3 per mode, each tunable from 50Hz to 3.6kHz CW/SSB, to 10kHz AM, and to 15kHz FM), a DATA mode that used with SSB provides another set of three filter bandwidths for computer digital modes, and a built-in USB soundcard and serial interface. It connects directly to a computer with the supplied USB cable, providing USB audio, CI-V CAT control (virtual RS-232 serial via USB). In other words, it needs no interface box — it's built-in! It also has an intuitive touch-screen and plain-text menus.
But one thing it doesn't have is an I.F. output for a panadapter. Of course, I just had to add one, because a panadapter is just too useful to be without. Gone are the days of slowing dialing through the bands to hear what's there. With a panadapter, you see the whole band at once, and just click on interesting signals to instantly tune to them. Dialing through a band is like living in a house with one small window. Running a panadapter is like knocking out a whole wall!
I hear many old hams still do not understand the benefit of a panadaper. But once they start using one they see how useful it is and how much it adds to the enjoyment of ham radio. You instantly see band activity, adjacent QSOs, who’s splattering, and when another favorite frequency becomes active. You are no longer limited to that 3kHz window for band information.
What’s a Panadapter?
A panadapter is a spectrum scope. It displays a section of radio frequency spectrum as a real-time graph of signal strength (Y) and frequency (X). It also displays a recent history of signals as a moving “waterfall” graphic. This allows the user to see the whole band at once, as opposed to having to slowly tune through a band to find signals. It also allows the user to visually check the spectral purity of signals. So it’s like a spectrum analyzer, but thousands of dollars cheaper.
How it Works
There are several ways to implement a panadapter with a ham radio. One way is to connect an HF-capable SDR (software-defined radio) to the station’s antenna, using the receive antenna loop on many radios, or an external automatic transmit/receive (T/R) switch. In this implementation, the SDR acts as a second receiver, and since it is implemented in software it can not only demodulate signals, but also provide a panadapter display. The disadvantage to this method is that SDRs that will receive HF are rather expensive, starting at around $130. And there is always the possibility of blowing the SDR when transmitting, if switching doesn’t work just right.
Another way to implement a panadapter is to have the SDR connected to the first I.F. (intermediate frequency) mixer of the radio, before the buffer amp and roofing filter. This method has several advantages: (1) no antenna switching is needed; (2) the radio’s front-end and band-pass filters greatly improve the signal before it gets to the SDR; and (3) since most first I.F. mixers are above 25 MHz, an inexpensive ($23) RTL-SDR dongle is sufficient.
The 7100's first I.F. mixer is located way up at 124.487 MHz, and that happens to be well into the range of inexpensive RTL-SDR USB dongles, without a down-converter. The dongle digitizes the 124 MHz I.F. signal and pumps the stream into the computer, where SDR software like HDSDR displays it as a spectrum graph and waterfall, up to 2.56 MHz bandwidth (1 MHz is much more practical to use), and zooming in a little to 500 kHz covers the 80-meter band. It also allows you select any audio bandwidth and displays the received demodulated audio on a second spectrum and waterfall. For rig control it can use Afreet OmniRig or Ham Radio Deluxe. And that's not all: by disabling rig control you can tune it independently and have a second receiver. So for a mere $30 (RTL-SDR and a little cable) you add both panadapter and second receiver to the IC-7100, and it's just as useful as the latest radios with built-in spectrum/waterfall and second receiver, for $thousands less! And your panadapter is on a big computer screen, not the tiny one used in expensive radios.
How I did It
Disclaimer: The author is not responsible for your mistakes. If you are not skilled, and equipped, to solder tiny SMT (surface-mount technology), you should not even consider attempting any modification of a modern radio. This article is intended for people that know how to look up specs, solder, and test delicate electronic circuits. There is always some risk in modifying any electronics. Proceed only at your own risk! Also, modifications will void your warranty.
Following the instructions of a Youtube video, I added a tiny 5pf capacitor to the mixer output, then brought out a little RG-316 Teflon coax with an SMA connector. That connects to the NESDR SMArt, an advanced, purpose-built RTL-SDR dongle with aluminum case, SMA connector, and best of all, a 0.5ppm TCXO (high-stability oscillator). I started out with the Nooelec “Blue” dongle, but its stability just wasn't good enough for SSB listening, and I was always fiddling with the calibration. Then Nooelec came out with the SMArt, $22, and I jumped on one. The IC-7100 also has a 0.5ppm TCXO, and using WWV I can set it to within ±0.2 Hz at 15 MHz. Then I set the SDR dongle to about ±1 Hz, which is the resolution of the software.
Note: Web articles age, so be sure to research get the best RTL-SDR available. In 2018 the two best are the Nooelec NESDR-SMArt, and the RTL-SDR.COM V.3, which also has a TCXO and adds bias-tee and direct sampling mods. Do not even think of buying the cheapo $15 DVB sticks—they have no shielding, no input protection, and unstable oscillators. A really good RTL-SDR is $23, and a really bad one is $15, so spend wisely and get one you’ll be happy with. The next step up from a good RTL-SDR dongle is the SDRPlay RSP-1a, at around $130, from HRO, the only USA source. It has 14 bits of resolution, better dynamic range, wider bandwidth, and bandpass filtering, so is an excellent choice as a general wide-band radio receiver, but is quite a bit of overkill for use just as an I.F. panadapter.
Story in Pictures
A few days later, I got out the tools and decided to follow the YT video and add an I.F. tap and RTL-SDR dongle.
I used the Nooelec Blue that I already had. I cut the cord off its little (useless) antenna,
and connected it here, with a 5pf ceramic disc capacitor. Not pretty! But it worked. Yes, it probably voided the warranty.
After getting tired of its temperature/frequency instability, I ordered this Nooelec NESDR SMArt, and a little 12-inch cable. I got one with a female SMA on one end and a right-angle male SMA on the other end, so I could use either one and cut the other off.
The hardest part was deciding where to bring the cable out. The 7100 isn't like a larger desktop radio where you plenty of extra rear-panel space. I really wanted to use the chassis-mount female, and not have a pigtail dangling out of the radio. Here? Nope.
There's room for a hole only here, but alas,
I'm not gonna run that coax across high-power RF components—on transmit the induced RF would surely blow the dongle, and probably the I.F. mixer and buffer, too. Don't run the coax anywhere near high-power RF components!
Maybe file a mouse-hole in the cover and come out the side?
But there's a vent there, and I plan to Velcro the dongle to the radio, so it won't be just flopping around.
Maybe come out the front! Drill an eighth-inch hole in the side of the front plastic cover, and loop it around like that! But I couldn't see a place to solder the shield on that side of the tap capacitor. In retrospect, I should have done it this way, because coming out the back results in a rather strong overload of the dongle, enough that I can't use the spectrum/waterfall to look at my own transmit signal. So, if you do this mod, consider doing it like this picture. The further the dongle and cable is away from the RF deck, the better. I may eventually change mine.
So I went back to plan A and came out here, as before.
At least it puts the coax on the opposite side of the heatsink from the transmit finals and filter coils.
Bring it out with small needle-nose. The RG-316 is rather tight in the hole I used, so you have to push a little, then pull a little. Be careful to not skin the coax.
Set the dongle on top (or where ever you will place it) and adjust the SMA pigtail length.
Tie off the cable here so it can't be pulled. The only notch in the radio's cast aluminum chassis from top to bottom is where the 2 ribbon cables go.
Measure the exact length to the capacitor lead and cut.
Dress out the end, trim the shield and tin both leads. Don't work above the PCB!
Here, from the first dongle install, is the only picture I got of the actual capacitor connection point. It's all very tiny, and you need a headband magnifier. Form and tin the cap's leads, then quickly tack-solder it by heating the lead, not the PCB. Note that this is the proper place to tap. It's after the 1st IF mixer, but before its buffer amp (which puts out way to much signal for the RTL-SDR), and is before the 15MHz roofing filter. So the dongle gets several MHz bandwidth, and benefits from the radio's bandpass filters, pre-amp, and attenuator. But from this point on, it's two separate radios. So, for example, adjusting a filter on the 7100 will not affect the SDR, and vice-versa.
Here's the connection of the new coax. Solder the tinned shield in the middle of that case-ground, and tack the tinned center conductor to the tinned capacitor lead. Don't go adding solder above the PCB! One drip and some tiny SMT component is toast. If you absolutely do need to add a bit to get it to tack properly, at least lay some damp paper over the PCB.
Test it, then put on the covers, and stick the dongle down with Velcro.
Connect the USB-A extension. Note the FT-140-77 toroid I put on the USB cable, to keep stray RF out of both the dongle and the computer. It's a good idea to use one on each end of the USB extension cable. You can push a USB-A connector 2-3 times through an FT-140, and RF choking goes up by the square of the turns count. So, 3 turns is better than 9 snap-on chokes.
And there it is, running HDSDR, now with acceptable frequency stability.
IC-7100 and SDR Frequency Calibration
Calibrate the 7100 (using Set → Functions → REF Adjust) by tuning to WWV (or another frequency standard). Switch the radio back and forth from CW to CW-R, and carefully adjust REF Adjust until the CW offset tone equalizes. For greater accuracy, use the USB audio output to a computer and read the audio beat frequency on software that will do so, such as MixW in PSK mode, on the AFC reading in the status bar. You'll notice there are actually about 3 encoder pulses (of the main tuning dial) per percent reading in REF Adjust screen. By moving the dial the smallest possible amount, you can get to within +- 0.2 Hz, when switching CW to CW-R.
Calibrate the SDR as follows: First make sure HDSDR and the 7100 are controlling each other via CI-V. Set HDSDR (and thus, the radio) to exactly 10 MHz, AM mode, and receive WWV on both. Click the ExtIO button to open the ExtIO control box and set the PPM as close as possible. Then right-click the ExtIO button to open the RF Front-end Frequency Options control box. Use the Global Offset field to fine-tune the calibration. With that box still open, click the AFC button. HDSDR will then adjust the frequency to synchronize with the WWV carrier. Note the difference (offset from 10MHz) and adjust the Global Offset by that number of Hertz, in the opposite direction.
Now both the 7100 and the RTL-SDR are synchronized to WWV (or another frequency standard) as closely as possible. I have found that my 7100 is much more stable that the spec’d 0.5ppm, and almost never needs recalibration. But the little $22 dongle, also spec’d at 0.5ppm, is considerably less temperature stable. When the room temperature changes a few degrees, it will go off by several cycles. No problem — when you need it to be exact, just go to AM, AFC, and adjust Global Offset again. It's very easy once you've done it a few times.
Better audio with a boom mic and equalizer!
73, — KV5R