Transmitter Blocks

 

Like any transmitter, this one is basically a bunch of gain stages. The interesting parts are the keyed waveform shaping circuit, and maybe the sequencer and PIN diode T/R switch. We’ll look at each block in turn.

When you suddenly turn an RF signal on or off, you generate a wide-bandwidth transient. We usually call these key clicks. By slowing down the rise and fall times during keying, bandwidth (and clicks) are reduced. A mathematically optimal wave shape was derived by Kevin Schmidt, W9CF. That form is an error function. Such a function is easy to generate in real-time using DSP, but since I was doing an analog system, I used the fact that erfc() is approximately equal to the hyperbolic tangent function tanh(). That’s something that we can generate with a differential pair of transistors.


When the circuit is keyed, a ramp generator drives the tanh() converter, which produces the gentle transition. An LMH6505 linear variable-gain amplifier (VGA) shapes the RF signal from the exciter. It has a nice dynamic range, in excess of 60 dB up to at least 50 MHz, with very low distortion up to +10 dBm output. All I had to to was adjust the ramp rate to suit they keying rate, and carefully adjust the min/max voltages of the tanh() output to perfectly match the VGA dynamic range


To provide complete muting and freedom from backwave, I used a small RF relay I had laying around. An analog switch would be better; just didn’t have one handy. In the board photo, there’s a blob of grey clay covering the relay to quiet it... Too much clatter when keying.

An Optimal Waveform Shaping Circuit

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The shaper is followed by a pre-driver stage which is based on a high-performance DSL driver amplifier, a TSH6012. I wrote about its performance in a report, IQPRo_Performance_and_Wideband_Opamps.pdf. It’s capable of well over +20 dBm output with very low distortion out to 100 MHz. You can see it in its little copper shield in the photo at right. The LMH6505 is on an evaluation board and the tanh() generator is built on the main perfboard

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