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Wednesday, 13 December 2017

Homebrew SDR HF Transceiver - RF Amp

The RF amplifier is the first component of the receive chain. It is used to amplify the incoming signals and this circuit provides about 7dB of gain, a little over multiplying the incoming signal by 5.

The amplifier is called a Termination Insensitive Amplifier (TIA) and is taken directly from the 2009 paper describing TIA's by W7ZOI & K3NHI. In the paper this amp is the one in Figure 6. The benefit of a TIA is that it is not affected by the termination impedance of the preceding or following stages so we have one less potential issue to think about. In the final transceiver I will build a bidirectional TIA but for now and to get things moving on the receive side I settled on this design. KK9JEF has a good video showing how TIA's work.

Directly copied from W7ZOI & K3NHI's paper the circuit is:
Credit: W7ZOI & K3NHI

In my version I used ceramic capacitors for the 0.1uF bypass capacitors rather than the electrolytics shown in the circuit. The 4.3K ohm resistor is not a standard value so I used a 1K ohm and a 3.3K in series (1K + 3.3K = 4.3K)

The input to the amp is the connector shown on the bottom left of the circuit and the output is the one on the right hand side. Below is a photograph of my build.

You can see that I have used an SMA connector on the output port. Since I took the picture I have also added an SMA on the input. I've decided that I like using these as it means I can disconnect and reconnect modules easily. I bought a bunch of sockets and crimp plugs (for use with RG174 coax) on ebay. However, there is no reason as far as I know why you shouldn't solder the coax centre and braid to the board directly as I have done many times before.

As an FYI here is a close up of the SMA connector on the board. I just scraped the copper away with a knife and checked with a multimeter that the copper the centre pin is soldered to was not connected to the rest of the board.

Finally I connected a signal source to the input (AD9850 and Arduino, see here) and using my oscilloscope measured the gain at several different frequencies. Yellow line is the input and blue line is the amplified output. At 10 MHz the gain is about 8dB and you can see that at 14 MHz the gain has started to drop.

3 MHz

7 MHz

10 MHz

14 MHz

Next post: Band pass filter.

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