SDRs at the first IF of IC-7410 TRX as a panadator: More detail

Note: I take no responsibility for any attempts at doing what is described here. I am not an expert with these radios and am following my own interpretation of how this is done. It is shared on the basis of the philosophy of amateur radio.

The main advantage of a 1st IF tap is avoiding the problem of sharing one antenna with a RTX and SDR; not simple.

As mentioned in an earlier post, I have installed a tap into the 1st IF of my IC-7410. The tap is made into the TRX’s first IF (64,455 kHz), above the roofing filters (the key to a good RX). The ICOM even have a socketed test point to do it (some hesitation playing inside a new $2000 TRX! Much reading of circuit diagrams). With the plug (hard to get but only $1.50 and are used on all main brands of RTX), isolation amplifier- http://www.cliftonlaboratories.com/z10000_buffer_amp.htm (gives details of how it is done), preferably through a bandpass filter (obtained but not installed) then to Funcube. I have it running, but not permanently installed.

The Taiko Denki TMP-K01X-A1 coax connectors are available from https://rfconnection.com/ for US$1.50 each or try eBay.com.

The isolation amplifiers and fitting kits are also available from https://www.sdr-kits.net/Panoramic-Adaptor-Tap-Boards

 

 

An SDR at the first IF is really neat. It can be used as a panadator for the RX, although the SDRs display gets a bit woozy as I tune. However, you can see a large section of the band with the SDR and all the little signals that are swamped just using audio. The SDR can be used independently as a rx and tune within the first IF; basically using all the good front end of the RTX that the SDR doesn’t have.

The address for the 1st IF isolation amplifier I used is http://www.cliftonlaboratories.com/z10000_buffer_amp.htm. It gives details for fitting the amplifier for other TRX including FT-1000MP. When purchasing, You need to specify the gain of the amplifier, usually unity, as the signal should be fairly strong once it gets to the first IF. Delivery from the USA to Australia can take 3 weeks, ask how much extra for faster delivery.

Another way is at http://www.w1ghz.org/small_proj/small_proj.htm, about halfway down. It includes a filter to remove the local oscillator, and a further article that uses an optoisolator to turn the SDR off during transmit. I had been looking for this site as I wanted to add both the filter and switch, having bought the parts to do it. The Mini-Circuits parts are available on special order from http://www.minikits.com.au/, as are many other interesting bits; he makes up patch cables to order too. There is another article on 1st IF taps that uses an in-line Mini-Circuits filter, which is easier to do, but I haven’t been able to find it.

The cheapest SDR to cover the 1st IF is just an RTLSDR dongle from eBay. You can get them on eBay for less than $20. Try to get one from an Australian store, otherwise you will be waiting weeks for delivery. I could only find one this morning: http://www.ebay.com.au/itm/USB-Digital-TV-Radio-for-PC-DVB-T-DAB-FM-SDR-Receiver-Dongle-RTL2832U-/331059172700?pt=AU_Components&hash=item4d14aa155c. Just search eBay for “RTL2832”.

These dongles go down to about 60 MHz, just enough for the 1st IF.

The other one you can use is a Funcube Pro+, but they are from the UK and more expensive, ~AU$200 although are plug and play. I use one of them.

For software, there is SDR Console, HDSDR, SDR#, and CuteSDR among others. There is a site devoted to RTLSDRs that cover all of these and more: http://www.rtl-sdr.com/.

For a panadapter, SDR# is probably enough. See http://www.rtl-sdr.com/rtl-sdr-quick-start-guide/. Use the latest version of zadig: http://zadig.akeo.ie/. If using Windows 8, as I do, there is a special start-up mode for installing non-signed drivers. Windows 7 just gives a warning.

It can be tricky getting the RTL dongles driver and associated files installed.

Added 9 Jan 2014

I have snipped parts of the block and circuit diagrams to make it clearer where the 1st IF tap is and how it seems a reasonably safe place to put it.

However, a word of caution to others wanting to modify different TRX for a panadapter. The IC-7410 is a very different design to other radios as much of the circuits for Rx and Tx are not shared. The best place for a panadapter is before the roofing filters if they are used. Unlike older superhet designs, the 7410 generates SSB modulation in the DSP, not through a sharp skirt filter (typically 9 MHz or 455 kHz). As such, taking an IF tap near the main SSB filter of a conventional TRX is very different as the RX and TX paths are shared, unlike what is done in the 7410.

 The tapping point is most clear in the circuit diagram of the 1st Rx mixer and the last Tx mixer, as they are clearly separate, plus there is a test connector to plug into. There is possibly no need to switch the isolating amplifier out when in Tx, but the bandpass filter could still help remove the local oscillator, but it should be minimal as a balanced mixer is used. It may be possible to just run coax out of the TRX and have the isolating amplifier outside.

Similarly, the tapping point can be seen approximately in the block diagram that clearly shows the different Rx and Tx paths. The green path is Rx.

It has been useful to re-visit the 1st IF tap on the IC-7410 to reassure myself that it seems a safe place to do it.

Further, re-reading the Rx and Tx design descriptions and circuits makes it clearer that the IC-7410 is a hybrid software-defined radio (SDR). All the Rx and Tx is done in the firmware programmable digital signal processor (DSP). The analogue parts are mixers, amplifiers and band-pass filters etc. The main signal filtering is done in the DSP as is the modulation/demodulation. Most SDRs could do with the bandpass filtering of the 7410.

Coming back to one of the points of a 1st IF tap is the difficulty of using an SDR and a TRX sharing a single antenna. Reading the circuit diagram, it should be possible to find a tap point where the Rx is separated from the Tx. With that, the SDR and TRX could safely share the same antenna with a splitter and 3 db loss of signal. A job for another day... Comments are welcomed.