Adding SDR to the FT-920
It is not difficult to add a second receive channel to the FT-920, to provide SDR and bandscope capability. There are a few different ways to achieve this end. What I have done here is the easy way, but not the cheap way.
The first step, however you may change things afterwards, is to obtain the Z10000a buffer amp from Clifton Labs. It is a small board that has a high impedance input, and can connect to the FT-920 first I.F. mixer output without loading. That I.F. is at 68.985 MHz, so input Z is a crucial concern here. There are instructions from K7VRE on how he installed his. The main connection point on the FT-920 circuit board is at J1007, which couples to the first mixer output through a 1 pF capacitor. (This jack is for alignment purposes, and is otherwise not used.) I thought this seemed a bit small, but he assured me that the Z10000a worked fine through it, even adjusted for unity gain. This makes for a no mods approach to tapping into the FT-920.
In my case it was a little more complex, as I already had the Inrad roofing filter mod installed. This puts J1007 on the wrong side of the filter, so I had to go back, remove the main board, and move the coupling capacitor from J1007 to a different point, ahead of the filter at T1013. I decided to go with a small 30pF cap to see what the effects of loading might be, and it was on hand. Also, the Inrad mod uses some long runs of coax to go back and forth to their board, so the loading at the mixer was already much more than normal. Inrad uses a post filter buffer amp to make up the loss. I figured that the input Z of the Z10000a would reduce the load on the mixer appreciably, even with a higher value of C to J1007. After I installed the preamp, I made some tests to determine at what gain to set the Z10000a. The -6dB (R7=2200 ohms) was about right, which would indicate the gain through the mixer is now higher than before, due to reduced load. I ran the coax to the roofing filter through a series 50 ohm resistor, so that its variable loading would not affect the flatness of the spectral display. Otherwise, it creates an objectionable dip and peak in the passband. The coax to the IF-2000 board (see below) is taken directly from the Z10000a output. I also reduced the protection resistor at the output of the Z10000a from 50 to 22 ohms to reduce the output Z, which also helps minimize the above mentioned passband ripple. After the amp is running, it is worthwhile to align transformers T1013 and T1014, and to balance the mixer for minimum noise out with VR1001.
The next step is to convert the high first I.F. down to a low value suitable for detection by any of the increasing number HF SDR boards that are available. I went with the IF-2000 by RFSpace . It is made for the FT-950, which has an I.F. of 69.45 MHz. Talking to Pieter at RFSpace, I was assured that the tuned circuits of the IF-2000 were sufficiently broad to accomodate the shift in input frequency required for the FT-920. The I.F. output is now at a nominal 11.015 MHz.
The somewhat expensive IF-2000 is a very straightforward design, with a Minicircuits GALI-51+ MMIC preamp, which feeds a Minicircuits UHF diode balanced mixer, and an output diplexer circuit. The local oscillator is a CTS chip at 80.000 MHz. The input has a 50 ohm attenuator, to account for the gain available in the transcievers that were designed with the IF tap already available. Perhaps it also functions to terminate the diode ring over a wide range of frequencies. I choose to remove the attenuator and feed directly, adjusting the overal gain with the Z10000a board so as to maximize MDS levels. I was not concerned about overloading, as the FT-920 has its own attenuation if needed, and the Z10000A has an appropriate output impedance to drive the mixer.
The above photo shows the IF-2000 mounted to the side of the FT-920 case. I drilled two holes for mounting screws, and two more holes for routing the coax in and out. For power, I connected to the 13.8VDC line at the adjacent Inrad filter, and dropped it through a 50 ohm resistor to make 9VDC at the IF-2000. The I.F. output coax goes to the Softrock Lite II SDR from KB9YIG , which is mounted next to the mixer. I used the kit for 30m, and changed the xtal to 44.000 MHz, but left the low Q input circuit as is. The actual center of the passband is 11.015 MHz, but it is good practice to offset the Softrock a little to avoid the possibilty of the LO making its way back into the FT-920. A better crystal frequency would be something like 44.050 MHz, but that wasn't in the junk box.
The above shows the Z10000a mounting, and its connection to J1007 and to the IF-2000. It also receives a connection to the 13.8VDC supply. The only concern in all this was to keep the inside of the radio clean, and to carefully mount the Z10000a clear of any components. The coax from the IF-2000 is visible along the edge of the chassis. I snaked the coax through the drilled holes and soldered directly to the circuit board. The audio connection to the SDR goes through a couple of holes and out. Someday I will put a miniature stereo socket on the back of the case for the audio.
Another possibility for achieving the same functionality as the above components, at low cost but with more hands on experimentation, would be to use the kits available from W8DIZ. For instance his diode ring mixer would be quite suitable. Partsandkits.com. There is also the possibility to use two of these mixers with quadrature LO signals, to make a direct IQ conversion to baseband in one step. Details on a VHF direct conversion receiver can be seen at the sprawling web pages of SM5BSZ. If I had better test gear, I would go this route.
There are several choices to consider as to which SDR circuit to use, other than the Softrock. One would do well to look at the document by AC0C on the choices of software and hardware available. For this project the Softrock is ideal, giving very good performance and balance.
The final picture shows the shack, with Linrad running, and the EMU-0202 on the desk.
There are a few good options for SDR software in Linrad. Here are some screenshots and brief introductions: SDR for Linux. This list is by no means complete, it is just those which I have been able to actually get working ;-].