So, about a year ago, I decided I wanted to tinker with SDRs. Many of these, such as the Flex radios, were way too expensive – so I looked into softrock type radios. It didn’t take long for me to get my hands on a softrock 80M/40M/20M kit (pick a band, put the appropriate parts in). I modified it, based on info I found on the web) for 455 kHz. This allowed me to hook it onto the IF chain for my BC780XLT Scanner (added an IF tap to that radio), a MacKay Dymek DR44, or any other radio I felt like modifying (unless it had a 455 kHz IF output).
As time progressed, I decided I REALLY wanted some form of panadaptor display on my FT1000MP Mk V Field. The newer radios have an output for it, the older radios (FT101) have an output for it… but why not this one??
I eventually came across this site: IC8POF’s “La Braciola” Panadaptor concept
In the site, IC8POF added a CiaoRadio SDR to the 455 kHz IF of the radio – his rationale was that there was not an affordable solution for a 70 MHz SDR.
In theory, a tap on the 8 MHz second IF would provide a wider bandwidth than the 455 kHz IF, so I purchased a Soft66AD SDR (from Japan) – at under a hundred dollars, it is an interesting HF SDR to play with – however, the software and support are a little “iffy”. For these reasons, among others, the whole project got shelved for a while.
Around May/June of 2012, I found a 64-1700 MHz SDR for under $30 (see my SDR page – accessible from the main radio page). This sparked some real interest, since it was now possible to connect to the first IF for cheaper than a softrock, and much more reliably so, as well. This prompted me to reconsider punching some holes in the back of the radio – and it just so happened that KC0ALC was placing a digikey order… those 3 SMA jacks just fell into his shopping cart.
The night they arrived, I began scoping out the best location for 3 SMA jacks – as it turns out, there’s a nice open spot both inside and outside the chassis next to the CAT jack – which internally is next to the I.F. board. The drilling then began (my finger still hurts… in retrospect, there is likely a better way to make sure the drill doesn’t punch through and damage boards within the radio)
(little problem with the images – they’ll be back soon, if i can find them)
The hole for the center jack was a little larger than I had expected, so there is some wiggle room for that one, but overall, the holes came out fine. The next challenge was finding IF tap points, and proper blocking caps. The caps I used for each IF are as follows:
|
I.F. Order |
Frequency |
Capacitor Value |
Capacitor Impedance
|
|
1st |
70.2 MHz |
0.001 uF |
2.27 ohms
|
|
2nd |
8.2 MHz |
0.022 uF |
0.9 ohms
|
|
3rd |
455 kHz |
0.1 uF |
3.49 ohms
|
These values were chosen for low impedance (<10 ohms), and convenience, since they were what Radio Shack had on-hand.
I found that tapping onto the input to the Inrad roofing filter was the best point to pull off the 70 MHz IF – as I showed on my SDR page. I used good quality coax for this cable, since the 70 MHz IF had the most potential for loss if I used lower quality coax.
The 8.2 MHz IF was difficult to find an acceptable tap point, since some of the components in the schematic are harder to find on the actual PCB. It turns out there’s an accessible via near the 8.2 MHz crystal on the IF board, which has a nice strong signal for the desired IF. I found that, if one doesn’t mind a sideband reversal, one of the pins used for the 8.2 MHz IF filter can be used – but I was trying to avoid this. It seems that the mode selection still affects the actual bandwidths and frequency shifts used by the radio.
For the 455 kHz IF, I used the same point IC8POF used – a test point on the edge of the board. For both this, and the 8 MHz IF, I used some coaxial audio cable (form an RCA cable) – so far, I have not noticed any issues with it, since I connected the ground well at one end. I am currently experimenting with grounds connected at both ends, ensuring that I do not cause ground loops.
Once the connections were made, I began looking at the time domain output, since my SDR was on my desk at work. I soon noticed that, in addition to my received signal, I could see my transmit audio, as well. As long as I was not using an external amplifier, it was even proportional to the power output (and on the same scale as my receive signal) – making my 5 MHz oscilloscope that much more useful for monitoring station operation! If I plan to use an amplifier, this scope has had one channel modified with a crude zener diode detector, so I can monitor the envelope of my transmitted signal through a 100:1 oscilloscope probe. It requires changing scales, but it works well.
The images below illustrate receive signal (AM 600 WMT – Coast to Coast AM intro), and transmit signal (part of my callsign, into a dummy load – 28.3 MHz USB) on my 5 MHz oscilloscope. Obviously, a cell phone makes a poor quality scope-camera… but you get the idea. The real advantage here? This thing works better than the old SB-610, and it doesn’t make RF hash on my radio as it sweeps!
One item of note – the output on the scope and SDR from the IF taps occur AFTER the AGC circuit – this has several implications…
1) Since the S meter is questionable when the AGC is off (maybe this is only my radio? – either way, it works MUCH better when AGC is on) – if I want to use it as a tuning indicator, the AGC needs to be off – this results in static being a low voltage signal, and station strength being relative to the S meter. If I’m really worried about an S meter, i can just tune the sub VFO to the same frequency, and look at the S meter there.
2) if i turn the AGC ON, then the static and station S readings are equalized, making the scope less useful as a tuning indicator.
3) Turning the SDR AGC on in addition to the radio’s AGC is not a bad idea – this allows the SDR to self correct so that there’s not issues with the signal popping in and overloading the SDR causing popping noises to come otu of the PC sound card (if used).