06 aprilie 2017

IF tap for external SDR on a Yaesu FT-840

So, a fellow ham (YO6FPW) has send me his radio to repair and for a very interesting mod!
He want to be able to use his RTL-SDR receiver with this radio.
The repair was nothing interesting. The Rx/Tx relay was fried and the RS signal was not present on the first batch of switching diodes.
The relay was changed with a similar one and the fried PCB was repaired with a wire directly from the Toshiba high current driver.
On the other hand, the asked mod was a real challenge!
The first IF on this radio is on 47.055 MHz so it's suitable to use it with a low cost RTL SDR but the signal is very low before the crystal filter. It's better to have the signal tapped before the filter. Why? Well, the answer it's obvious: to have a whole 2 MHz to enjoy!

So, first, I tried to use the G4HUP schematics (PAT)  but it didn;t work as expected. The signal was very low, maybe because I tapped the IF signal from a different point or maybe because my J310 are fake :-).

This is where I put the IF tap:

It's a low impedance, low signal point so I needed a good RF preamplifier.
Listening with a RTL-SDR produced usable signals only on strong signals so I was looking in my little boxes after some suitable components. I remembered that I have somewhere some Analog Devices AD8008 for a "never started project" and I decided that this is my path!

So, I quickly draw a schematic, I also drawed on a PCB and put it to work.

Here is the schematic, based on the application note from Analog:

Everything was in SMD because I was looking for a small footprint, but the result was nice (at least for what I needed:

I also found some metal sheet to made a custom shield case.

I didn't save the results from the IFR sweep analyser; the amplification is around 12 db at 5V and is very flat, which is what I was looking for! At 9 V, the amplification is around 17 db! Good to know!

The 5V for powering the amplifier is taken directly from the Q1034 on the SMD side of the "RF" PCB board. I didn't took it from L 1012 as is shown on the G4HUP page because there was some problems with that path and I didn't want to stress the PCB. So I took the 5V directly from the TD6278P SMD circuit (Q1034), on pin 12 (RS).
This 5V is present when the radio is on receive only and when transmitting is 0V so the preamp is protected from accidental high RF at input.

As for the output, I had a nice BNC connector with a small RG316 cable, just asking to be on the rear side of the Yaesu FT-840!

This is the final work:

And here is a small youtube movie with the SDR at work:

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