Design

From the work I've described in previous logs, I now have a sense of what I'll need from the IF amplifier stage of my receiver, so I set out to design something around the following specs:

• Variable gain — Manual adjustment is fine. The maximum gain should be around 55 dB so that the complete receiver has the minimum 70 dB of gain I'm looking for. (The LNA I have in mind has ~26 dB of gain, but I loose ~11 dB through the mixer stage.)
• DC offset adjustment — This is to manually trim out the DC offset generated by the mixer. I previously measured this to be ~4 mV.
• A bandwidth of at least 135 kHz — Since I'm aiming to receive broadcast FM.
• 50 ohm input impedance — To terminate the mixer output.
• DC coupled — Since this is a zero-IF receiver.

Here's my first pass at such a design:

The overall design is split across two separate op-amp gain stages.

The first stage is a non-inverting amplifier, with a fixed gain of around 25 dB.

The second stage is an inverting amplifier, with gain which can be varied between around 6 and 30 dB. The adjustment is made manually using a variable resistor. The summing configuration is used to introduce a small, manually adjusted, DC offset to compensate for any DC offset generated by the mixer. Any such offset will have been amplified by the first stage, meaning I might see an offset of about 70 mV here (assuming a 4 mV offset out of the mixer), which seems tolerable. With this arrangement the DC offset and gain adjustments operate independently.

I have a 50 ohm resistor on the input to give a broadband match. The high input impedance of the first stage means the overall input impedance is set by this resistor.

I also use a resistor to set the output impedance to 50 ohms. The idea here is that this will provide termination when driving e.g. a high impedance load over a length of coax. The op-amp probably can't supply enough current to drive a 50 ohm load at anything other than low amplitudes, so such a coax won't typically be terminated by the load.

For the op-amps I elected to use NE5532s since they are (a) readily available in small quantities, (b) inexpensive, (c) available in an easy to work with DIP package, and (d) have just about enough bandwidth.

Build

A built a prototype of this circuit, which looks like this:

Testing

I wanted to measure the gain and bandwidth of the circuit to check it meets the spec. I attempted this on my LiteVNA, but the results I got at these low frequencies didn't look great, and besides, it only goes down to 50 kHz. So instead, I made the measurements manually by driving the amplifier with a home brew signal generator and measuring the output on my scope.

This plot shows gain vs frequency at the circuit's minimum and maximum gain settings:

This looks pretty good to me. The gain is rolling off faster than predicted by the simulations I did in LTSpice, which I don't understand. (Inaccuracy in the op-amp model I used perhaps?) However, the 3dB bandwidth at maximum gain is roughly 125 kHz, which I'll call good enough.

I also measured S11 to check that the input impedance will provide a good 50 ohm match for the mixer output. The unused mixer product at twice the local oscillator frequency will be at 216 MHz at the upper end of the broadcast FM band, so I made the measurement up to 300 MHz. I had a 20 dB attenuator between the VNA and the amplifier, which I calibrated out, and I set the VNA to its minimum output power. (Though I'm not sure either of these steps were necessary since the amplifier doesn't have any gain at the swept frequencies.) Here's the result:

Again, this looks good.

Finally, I note that I've also noodled with the DC offset adjustment which seems to work as expected.