While working on the oscillator section I looked into ceramic filters a little more and found an alternate which I think will work better than the 2 kHz device we chose previously. The LTC455GW is also a 455 kHz filter but has a bandwidth of 4.5 kHz which will allow more frequencies through for clearer speech. There's also a more complete datasheet available so design will be easier. It's more than twice the price of the old device but it's still only $3 a piece in small quantities so it should be ok.

The Si5351A modules outputs a square wave with an unloaded 3.2v peak-to-peak (pp). Having used this device I know the output into 50Ω is closer to 1.6v pp. You can also google for this information. The mixers need an input of 200-300mV pp so we need to attenuate the output. The mixer input has an impedance of approximately 1.5kΩ. It's complex impedance but we'll deal with just the resistive portion for now. Calculating the attenuation using one of the many available tools usually requires an attenuation in power dB. Converting the voltages to power we get:

RMS voltage of a square wave is half the pp voltage

This shows we need about a 31 dB attenuator pad. I used this helpful tool to calculate the values for a pi attenuator. We need to use either a pi or tee as a bridged tee requires the input and output impedances to match. I usually start a little lower when there's a large attenuation needed as I've found actual attenuation tends to go up as does VSWR. I entered the input and output impedances and an attenuation of 24dB giving the following ideal values:

• Input shunt: 50.769 Ω
• Series: 2161.565 Ω
• Output shunt: 4776.276 Ω
• Minimum attenuation required for match: 20.719 dB

I went back to Micro-Cap and created a simple circuit with a 14.2 MHz square wave at 50 Ωdriving a 1.5 kΩ load. (The files are in github) A transient simulation confirmed the values with an output pp voltage of 276 mV. Micro-Cap allows you to double-click a resistor and select either Single or Combinations to use standard resistors instead of the idea value. It tells you the one or multiple resistors you can use and the percentage error from the original value. I did this for all three resistors in the pad and ended up with 51.1 Ω, 2.15 kΩ, and 4.75 kΩ values for the resistors. Each is within 0.6% of the ideal values. The resulting pp output is 278 mV which is in the allowed range. The final attenuation is 29.97 dB which is close to our estimated 31 dB. Input and output VSWR are 1.013 and 1.012 which is not going to cause a problem.

I mentioned that the impedance at the mixer is actually 1.5k || 3pF. This is a fairly small imaginary impedance value and we don't care about an exact match since loss of a bit more power isn't a problem and some minor distortion to the waveform shouldn't have a big effect on switching either.

Finally, we need to match the 50 Ω ports on the BGA2818 amplifiers to the 1500 Ω ports on the IF filter. We're interested in a fairly small bandwidth so an LC network should work well. The max Q of the match is 445k/7k or about 63.  This tool calculates L-match, Pi-Match, and T-Match circuits. I used a Q of 45 to calculate a pi-match then implemented it in Micro-Cap.