Choosing a mixer for HF transceiver

Hello guys,
I'm trying to design an analog portable single side band receiver (ham radio transceiver in future).

I am looking for a good matching components.
Initially I started with SA612 mixers but I was disappointed by their noise level.
So I tried AD831 which had almost no gain while dissipating tonnes of heat. I understood I was foolish underestimating their power consumption and made many mistakes.

I have drawn this block diagram:

It uses Si5351 as VFO. First mixer VFO is set above receive frequency. RF BPF is up to 2MHz wide. IF 8MHz BPF is 3 KHz wide. LO can move up & down to be able to receive USB or LSB.

Now I am trying to build something which will be power efficient, compact.
So I picked LT5526 as double balanced mixers.
But I am looking at LT5560 and LTC5562.
LTC5562 seems having better characteristics and claims to have ESD protecting which sounds important for HF applications having an external antenna. The only caveat it has some note about external matching for lower frequencies which is my case.

For IF amplifier I picked AD8338. It has attracted me by presence of AGC and 80dB of dynamic range. But I struggle a little bit with proper configuration for this chip. I had to manually set AGC setpoint. Also I think I made poor decisions regarding my PCB design, I guess it starts self excitation. Many people recommend 4 layers board but it is an overkill for ham radio. I think it is doable with 2 layers. Also I was thinking to tap to AGC loop and use it for S-meter.

Next, I am thinking about some LNA after RF bandpass filters but unsure about it. In different designs I see LNAs and attenuators there. If signal will be stong it will overload LNA and small signal might be lost. From the other side, ham radio signals are usually very weak unlike broadcast radio stations so it worth to have it.

For audio amplifier I used SSM2211 and LM358 as preamp/filter. It is the only one which has option to have single ended output which is one my requirements.

TBH, I am not RF engineer and radio is my hobby so I am very welcome to any advice.

typo fix
[edited by: zoonman at 4:21 AM (GMT 0) on 21 Jan 2020]
  • Hello,

    I think nice design, well described. Did you check your frequency plan? I am not sure, but if I understand the solution correctly, 1st IF (8MHz) is in the input RF band. Or is there a gap in the RF received band? If no, you should revise the frequency plan and use e.g. 1st IF above the RF band. You have to decide about the noise figure. Usually the noise figure is requested ~3dB (or less) for the VHF, UHF bands but I don't have experience with the HF band, I assume the noise figure can be higher because of higher noise background. To reach better NF, LNA shall be placed in front of mixer of course at the expense of linearity. Define what you want to reach (NF, IIP3) and calculated level diagram gives you answers whether use or not to use gain blocks somewhere in the RF chain. 

  • So frequency plan is relatively easy - amateur bands are usually very narrow except 10 meter band which is large. Others within 500 kHz. Theoretically I can do upconversion to 35 MHz but there are less options from VGA perspective. It could be complicated for me to design a proper AGC loop.
    I think NF is more important than IIP3. This is where it gets tricky for me. Should I be looking at mixers NF too? Also I slightly confused about conversion gain. I don't completely understand what does it mean. Maybe I should slow down and read some theory around mixers.

  • I read SA612 datasheet and noticed that it has just 5dB noise figure which is smaller than all ADI mixers. Now I am very confused.

  • Ok, understand, so 1st IF is positioned in the unused RF band-gap. 

    I think NF is more important than IIP3.

    I can not agree with you. Both parameters are important and specially nowadays the IIP3 is probably more important than NF. Everywhere are different sources of interferers which increase the noise background. I want to say if you design RX with extremely low NF, the receiver will receive and amplify useless noise from different SMPS, industrial instruments, cabling, which generate EMI spectrum mainly in the HF band. As you know, the NF is above all defined by RX front-end and IIP3 by 2nd IF. 

    4 layers board but it is an overkill for ham radio

    From my experience, the price difference is not significant between 2L and 4L boards. Especially if you don't have special requirement and you use any pool-service.

  • I was looking at LNAs and I cannot find anything in a reasonable price range. Probably something like BGA2866 will suit my needs.

    Regarding 4 layers PCB here are the quotes:

    4 layers

    Almost $60.

    2 layers$27.

    I think 2x price difference is huge. Even $27 for a hobby project is a bit expensive.

    I will give a shot for LT5526 as a mixer. Also I recently learned about critical path so if my tracks are short enough impedance mismatch will be negligible.

    Also I thought about RX/TX switch and I think HMC544A should get the job done.